A. Glyn Bengough

Life on Earth is sustained by a small volume of soil surrounding roots, called the rhizosphere. The soil is where most of the biodiversity on Earth exists, and the rhizosphere probably represents the most dynamic habitat on Earth; and certainly is the most important zone in terms of defining the quality and quantity of the Human terrestrial food resource. Despite its central importance to all life, we know very little about rhizosphere functioning, and have an extraordinary ignorance about how best we can manipulate it to our advantage. A major issue in research on rhizosphere processes is the intimate connection between the biology, physics and chemistry of the system which exhibits astonishing spatial and temporal heterogeneities. This review considers the unique biophysical and biogeochemical properties of the rhizosphere and draws some connections between them. Particular emphasis is put on how underlying processes affect rhizosphere ecology, to generate highly heterogeneous microenvironments. Rhizosphere ecology is driven by a combination of the physical architecture of the soil matrix, coupled with the spatial and temporal distribution of rhizodeposits, protons, gases, and the role of roots as sinks for water and nutrients. Consequences for plant growth and whole-system ecology are considered. The first sections address the physical architecture and soil strength of the rhizosphere, drawing their relationship with key functions such as the movement and storage of elements and water as well as the ability of roots to explore the soil and the definition of diverse habitats for soil microorganisms. The distribution of water and its accessibility in the rhizosphere is considered in detail, with a special emphasis on spatial and temporal dynamics and heterogeneities. The physical architecture and water content play a key role in determining the biogeochemical ambience of the rhizosphere, via their effect on partial pressures of O2 and CO2, and thereby on redox potential and pH of the rhizosphere, respectively. We address the various mechanisms by which roots and associated microorganisms alter these major drivers of soil biogeochemistry. Finally, we consider the distribution of nutrients, their accessibility in the rhizosphere, and their functional relevance for plant and microbial ecology. Gradients of nutrients in the rhizosphere, and their spatial patterns or temporal dynamics are discussed in the light of current knowledge of rhizosphere biophysics and biogeochemistry. Priorities for future research are identified as well as new methodological developments which might help to advance a comprehensive understanding of the co-occurring processes in the rhizosphere.

Root elongation in drying soil is generally limited by a combination of mechanical impedance and water stress. Relationships between root elongation rate, water stress (matric potential), and mechanical impedance (penetration resistance) are reviewed, detailing the interactions between these closely related stresses. Root elongation is typically halved in repacked soils with penetrometer resistances >0.8-2 MPa, in the absence of water stress. Root elongation is halved by matric potentials drier than about -0.5 MPa in the absence of mechanical impedance. The likelihood of each stress limiting root elongation is discussed in relation to the soil strength characteristics of arable soils. A survey of 19 soils, with textures ranging from loamy sand to silty clay loam, found that ∼10% of penetration resistances were >2 MPa at a matric potential of -10 kPa, rising to nearly 50% >2 MPa at - 200 kPa. This suggests that mechanical impedance is often a major limitation to root elongation in these soils even under moderately wet conditions, and is important to consider in breeding programmes for drought-resistant crops. Root tip traits that may improve root penetration are considered with respect to overcoming the external (soil) and internal (cell wall) pressures resisting elongation. The potential role of root hairs in mechanically anchoring root tips is considered theoretically, and is judged particularly relevant to roots growing in biopores or from a loose seed bed into a compacted layer of soil.

SUMMARY Mechanical impedance to root growth is one of the most important factors determining root elongation and proliferation within a soil profile. Penetrometers overestimate resistance to root growth in soil by a factor of between two and eight and, although they remain the most convenient method for predicting root resistance, careful interpretation of results and choice of penetrometer design are essential if improved estimates of soil resistance to root elongation are to be obtained. Resistance to root growth through pressurized cells containing ballotini considerably exceeds the confining pressure applied externally to these cells. Results from this work are reappraised. Existing models of soil penetration by roots and penetrometers are reviewed together with the factors influencing penetration resistance. The interpretation of results from mechanical impedance experiments is examined in some detail and root responses, including possible mechanisms of response, are discussed.

Root growth in the field is often slowed by a combination of soil physical stresses, including mechanical impedance, water stress, and oxygen deficiency. The stresses operating may vary continually, depending on the location of the root in the soil profile, the prevailing soil water conditions, and the degree to which the soil has been compacted. The dynamics of root growth responses are considered in this paper, together with the cellular responses that underlie them. Certain root responses facilitate elongation in hard soil, for example, increased sloughing of border cells and exudation from the root cap decreases friction; and thickening of the root relieves stress in front of the root apex and decreases buckling. Whole root systems may also grow preferentially in loose versus dense soil, but this response depends on genotype and the spatial arrangement of loose and compact soil with respect to the main root axes. Decreased root elongation is often accompanied by a decrease in both cell flux and axial cell extension, and recent computer-based models are increasing our understanding of these processes. In the case of mechanical impedance, large changes in cell shape occur, giving rise to shorter fatter cells. There is still uncertainty about many aspects of this response, including the changes in cell walls that control axial versus radial extension, and the degree to which the epidermis, cortex, and stele control root elongation. Optical flow techniques enable tracking of root surfaces with time to yield estimates of two-dimensional velocity fields. It is demonstrated that these techniques can be applied successfully to time-lapse sequences of confocal microscope images of living roots, in order to determine velocity fields and strain rates of groups of cells. In combination with new molecular approaches this provides a promising way of investigating and modelling the mechanisms controlling growth perturbations in response to environmental stresses.

Summary The production of exudates by plant roots and microbes in the rhizosphere, together with intense wetting and drying cycles due to evapotranspiration, stimulate changes in soil structure. We have attempted to separate these two processes using an experimental model with bacterial exopolysaccharides (dextran and xanthan) and root mucilage analogues (polygalacturonic acid, PGA), and up to 10 cycles of wetting and drying. To characterize the soil structure, tensile strength, water sorptivity and ethanol sorptivity of the amended soils were measured, and thin sections were made. Xanthan and PGA induced greater tensile strength of the amended soil, suggesting that they increased the bond energy between particles. Porosity increased with each cycle of wetting and drying, and this increase was less pronounced for the PGA 2 g l −1 than for the xanthan and dextran. This suggests that PGA stabilized the soil against the disruptive effect caused by the wetting and drying. The PGA was the only polysaccharide that influenced water sorptivity and repellency, resulting in slower wetting of the treated soil. Wetting and drying led to an increase of the sorptivity and a decrease of the repellency for all treatments with the exception of the PGA‐amended soils. The PGA may therefore stabilize the soil structure in the rhizosphere by increasing the strength of bonds between particles and decreasing the wetting rate. Influence de mucilages racinaire et microbiens modèles sur la structure du sol et le transport d'eau Résumé La production d'exsudats par les plantes et les microbes de la rhizosphère ainsi que les cycles d'humectation–dessiccation très intense due à l'évapotranspiration, entraînent des modifications de la structure du sol. Notre objectif a été de séparer ces deux processus en utilisant un modèle expérimental avec des polysaccharides bactériens (dextran et xanthan) et un analogue d'exsudat racinaire (acide polygalacturonique, APG), et jusqu'à dix cycles d'humectation et dessiccation. Afin de caractériser la structure du sol, la résistance en traction ainsi que l'infiltration de l'eau et de l'éthanol dans le sol amendé par les différents polymères ont été mesurés, et des lames minces ont été réalisées. Le xanthan et l'APG ont provoqué la plus forte augmentation de la résistance en traction, ce qui serait attribuable à une plus grande énergie de liaison entre les particules de sol. La porosité a augmenté avec chaque cycle d'humectation–dessiccation pour tous les traitements et cette augmentation a été moins prononcée pour l'APG 2 g l −1 par rapport au xanthan et au dextran. Cela suggère que l'APG a stabilisé le sol contre la déstructuration provoquée par les cycles d'humectation–dessiccation. L'APG a été le seul polysaccharide qui a influencé– dans le sens d'une diminution – l'infiltration de l'eau dans le sol amendé. Les cycles d'humectation–dessiccation ont entraîné une augmentation de l'infiltration de l'eau dans le sol amendé par les différents polymères à l'exception de l'APG. Ce dernier stabiliserait donc la structure du sol dans la rhizosphère en augmentant la force de liaison entre les particules et en diminuant la vitesse d'humectation du sol.

The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis-based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

Plants form the base of the terrestrial food chain and provide medicines, fuel, fibre and industrial materials to humans. Vascular land plants rely on their roots to acquire the water and mineral elements necessary for their survival in nature or their yield and nutritional quality in agriculture. Major biogeochemical fluxes of all elements occur through plant roots, and the roots of agricultural crops have a significant role to play in soil sustainability, carbon sequestration, reducing emissions of greenhouse gasses, and in preventing the eutrophication of water bodies associated with the application of mineral fertilizers. This article provides the context for a Special Issue of Annals of Botany on ‘Matching Roots to Their Environment’. It first examines how land plants and their roots evolved, describes how the ecology of roots and their rhizospheres contributes to the acquisition of soil resources, and discusses the influence of plant roots on biogeochemical cycles. It then describes the role of roots in overcoming the constraints to crop production imposed by hostile or infertile soils, illustrates root phenotypes that improve the acquisition of mineral elements and water, and discusses high-throughput methods to screen for these traits in the laboratory, glasshouse and field. Finally, it considers whether knowledge of adaptations improving the acquisition of resources in natural environments can be used to develop root systems for sustainable agriculture in the future.

Root hairs are a key trait for improving the acquisition of phosphorus (P) by plants. However, it is not known whether root hairs provide significant advantage for plant growth under combined soil stresses, particularly under conditions that are known to restrict root hair initiation or elongation (e.g. compacted or high-strength soils). To investigate this, the root growth and P uptake of root hair genotypes of barley, Hordeum vulgare L. (i.e. genotypes with and without root hairs), were assessed under combinations of P deficiency and high soil strength. Genotypes with root hairs were found to have an advantage for root penetration into high-strength layers relative to root hairless genotypes. In P-deficient soils, despite a 20% reduction in root hair length under high-strength conditions, genotypes with root hairs were also found to have an advantage for P uptake. However, in fertilized soils, root hairs conferred an advantage for P uptake in low-strength soil but not in high-strength soil. Improved root-soil contact, coupled with an increased supply of P to the root, may decrease the value of root hairs for P acquisition in high-strength, high-P soils. Nevertheless, this work demonstrates that root hairs are a valuable trait for plant growth and nutrient acquisition under combined soil stresses. Selecting plants with superior root hair traits is important for improving P uptake efficiency and hence the sustainability of agricultural systems.

Summary We hypothesized that plant exudates could either gel or disperse soil depending on their chemical characteristics. Barley ( Hordeum vulgare L. cv. Optic) and maize ( Zea mays L. cv. Freya) root exudates were collected using an aerated hydroponic method and compared with chia ( Salvia hispanica L.) seed exudate, a commonly used root exudate analogue. Sandy loam soil was passed through a 500 ‐ μm mesh and treated with each exudate at a concentration of 4.6 mg exudate g −1 dry soil. Two sets of soil samples were prepared. One set of treated soil samples was maintained at 4°C to suppress microbial processes. To characterize the effect of decomposition, the second set of samples was incubated at 16°C for 2 weeks at −30 kPa matric potential. Gas chromatography–mass spectrometry ( GC – MS ) analysis of the exudates showed that barley had the largest organic acid content and chia the largest content of sugars (polysaccharide‐derived or free), and maize was in between barley and chia. Yield stress of amended soil samples was measured by an oscillatory strain sweep test with a cone plate rheometer. When microbial decomposition was suppressed at 4°C, yield stress increased 20‐fold for chia seed exudate and twofold for maize root exudate compared with the control, whereas for barley root exudate decreased to half. The yield stress after 2 weeks of incubation compared with soil with suppressed microbial decomposition increased by 85% for barley root exudate, but for chia and maize it decreased by 87 and 54%, respectively. Barley root exudation might therefore disperse soil and this could facilitate nutrient release. The maize root and chia seed exudates gelled soil, which could create a more stable soil structure around roots or seeds. Highlights Rheological measurements quantified physical behaviour of plant exudates and effect on soil stabilization. Barley root exudates dispersed soil, which could release nutrients and carbon. Maize root and chia seed exudates had a stabilizing effect on soil. Physical engineering of soil in contact with plant roots depends on the nature and origin of exudates.

New PhytologistVolume 157, Issue 2 p. 315-326 Free Access Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil D. B. Read, Corresponding Author D. B. Read Department of Soil Science, The University of Reading, Whiteknights, PO Box 233, Reading, RG6 6DW;Author for correspondence: Dr Derek Read Tel: +44 118931 6557 Fax: +44 118931 6660 Email: [email protected]Search for more papers by this authorA. G. Bengough, A. G. Bengough Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorP. J. Gregory, P. J. Gregory Department of Soil Science, The University of Reading, Whiteknights, PO Box 233, Reading, RG6 6DW;Search for more papers by this authorJ. W. Crawford, J. W. Crawford SIMBIOS Centre, School of Science and Engineering, University of Abertay Dundee, Bell St., Dundee DD1 1HG;Search for more papers by this authorD. Robinson, D. Robinson Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UUSearch for more papers by this authorC. M. Scrimgeour, C. M. Scrimgeour Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorI. M. Young, I. M. Young SIMBIOS Centre, School of Science and Engineering, University of Abertay Dundee, Bell St., Dundee DD1 1HG;Search for more papers by this authorK. Zhang, K. Zhang Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorX. Zhang, X. Zhang Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this author D. B. Read, Corresponding Author D. B. Read Department of Soil Science, The University of Reading, Whiteknights, PO Box 233, Reading, RG6 6DW;Author for correspondence: Dr Derek Read Tel: +44 118931 6557 Fax: +44 118931 6660 Email: [email protected]Search for more papers by this authorA. G. Bengough, A. G. Bengough Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorP. J. Gregory, P. J. Gregory Department of Soil Science, The University of Reading, Whiteknights, PO Box 233, Reading, RG6 6DW;Search for more papers by this authorJ. W. Crawford, J. W. Crawford SIMBIOS Centre, School of Science and Engineering, University of Abertay Dundee, Bell St., Dundee DD1 1HG;Search for more papers by this authorD. Robinson, D. Robinson Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UUSearch for more papers by this authorC. M. Scrimgeour, C. M. Scrimgeour Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorI. M. Young, I. M. Young SIMBIOS Centre, School of Science and Engineering, University of Abertay Dundee, Bell St., Dundee DD1 1HG;Search for more papers by this authorK. Zhang, K. Zhang Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this authorX. Zhang, X. Zhang Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Dundee, DD2 5DA;Search for more papers by this author First published: 24 January 2003 https://doi.org/10.1046/j.1469-8137.2003.00665.xCitations: 209AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Summary • Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes. • The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties. • The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil. • These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere. Introduction Root-derived mucilage is a crucial component of the rhizosphere, contributing to many fundamental plant–soil interactions, such as root penetration, soil aggregate formation, microbial dynamics and nutrient turnover (McCully, 1999). Many functions have been suggested for mucilage, but little is known about its influence on specific physical properties of the rhizosphere. Passioura (1988) noted that soil contains surface active materials of biological origin, which modify the surface tension of soil solution. He suggested that, if roots exuded similar surfactants (or stimulated their production by microorganisms), they would reduce the tension of water in the soil at a given water content. This would influence the ability of the roots to take up water, especially at higher tensions. In a previous study, Read & Gregory (1997) measured the surface tension of mucilage collected from the roots of 3–4-d-old, axenically grown maize (Zea mays L. cv. Freya) and lupin (Lupinus angustifolius L. cv. Merrit) seedlings. The surface tension of both maize and lupin mucilage was reduced to c. 48 mN m−1 at total solute concentrations > 0.7 mg ml−1, indicating the presence of powerful surfactants. Also, during collection of mucilage with Pasteur pipettes, small bubbles and planar membranes frequently formed. Stable membrane formation is a sensitive indicator of the presence of a surfactant (Ballard et al., 1986). The similar reductions of surface tension measured in both maize and lupin mucilage suggested that the type of surfactant present was the same. The major components of mucilages are sugars. Monomeric neutral sugars increase the surface tension of water and are not surface active (Shaw, 1980). During analysis of the neutral sugar composition of maize mucilage by gas chromatography of peracetylated derivatives (Osborn et al., 1999), several compounds more volatile than the observed sugars were also detected. Subsequent investigation by gas chromatography-mass spectrometry (GC-MS) indicated that some of these compounds possessed hydrocarbon chains, up to 18 carbon atoms in length. It has been suggested that polar glycolipids are synthesised as intermediates during the production of mucilage at the root tip (Green & Northcote, 1979). These glycolipids and other phospholipids associated with plant cell membranes would be expected to show marked surface activity (Ballard et al., 1986). Early studies of the root epidermis using electron microscopy reported the presence of oily droplets in mucilage of onion (Allium cepa) (Scott et al., 1958). The droplets stained red with Sudan III, indicating the presence of saturated fatty substances. Similarly, Dawes & Bowler (1959) conducted a microscopic study of the root hair structure of radish (Raphanus sativus). The root hairs were covered with a mucilaginous layer and staining with Sudan III indicated the presence of fatty substances in the root hair mucilage and in the mucilage of the root epidermal cells. In the light of these observations, it seemed probable that the reductions in surface tension observed in experiments with maize and lupin mucilage were attributable to the presence of lipids. The presence of lipid surfactants may have substantial effects on the water retention and hydraulic conductivity of the rhizosphere, on chemical adsorption, and on microbial processes. Matric potential is one of the most important components of the water potential in soil/plant systems (Campbell, 1985), governing the amount and distribution of water in the soil pore space and mediating many other soil processes, directly and indirectly (Young & Ritz, 2000). The water potential under a curved air–water interface, in idealised pore space, is given by the capillary rise equation: Ψm=−2γ/rρw where r is the radius of curvature of the interface, γ is the surface tension and ρw is the density of water. Therefore, lowering the surface tension of the soil solution should produce a proportional increase in matric potential, allowing the plant to extract more available water from the soil. The effect of surfactants on the hydraulic conductivity of soil has been investigated in bioremediation studies, where surfactants have been applied in bulk to soil to flush out hydrophobic organic contaminants. Saturated hydraulic conductivities and unsaturated diffusivities of loams were reduced by up to two orders of magnitude in the presence of surfactants such as sodium dodecylsulphate (Allred & Brown, 1994; Liu & Roy, 1995; Tumeo, 1997), seriously decreasing the effectiveness of surfactant-based remediation techniques. Suggested mechanisms for the effect include expansion and sodium dispersion of the clays, fine particle mobilisation and precipitation of divalent salts of the surfactants. In addition to effects on physical properties, surfactants may also modify the chemical properties of the rhizosphere. For example, adsorption of root mucilage and polygalacturonic acid decreased subsequent P adsorption (Gaume et al., 2000). However, the effect of root mucilage simply as a source of surfactants has not been previously considered. It is possible that root-produced surfactants may affect phosphate adsorption by competing directly for adsorption sites, or by altering the energetics of the interaction between the phosphate ion and the adsorption site. Root mucilage has been found to affect rates of nitrogen immobilisation and mineralisation (Mary et al., 1993), but these results were considered in the context of mucilage as a carbon source for heterotrophic microbes. Because mucilage affects soil physical properties in the rhizosphere and thereby modifies the environment in which soil microbes function, there may well be consequences for microbial viability and activity. The aims of this study were: first to detect and identify any fatty acids in root mucilage and to identify the parent lipid type; and second to evaluate the likely effects of root-released phospholipid surfactants on soil matric potential, phosphate adsorption and soil N dynamics in the rhizosphere. Lipids present in root mucilage were analysed by GC-MS and thin layer chromatography (TLC), and alterations in soil properties were assessed using a commercially available phospholipid surfactant (lecithin), which is chemically similar to the surfactants found in root mucilages. Materials and Methods Analysis of surfactants in root mucilages Germination of seeds and collection of mucilage Seeds of maize (Zea mays L. cv. Freya), lupin (Lupinus angustifolius L. cv. Merrit) and wheat (Triticum aestivum L. cv. Charger) were surface-sterilised in sodium hypochlorite solution (2% for 10 min), then rinsed thoroughly in sterile deionised water. The seeds were then germinated on moist filter papers in Petri dishes, in the dark at 26°C. After 4–5 d, mucilage was collected from the tips of the germinating roots using a drawn glass Pasteur pipette, in a laminar flow cabinet. The mucilage was centrifuged at 12 000 r.p.m. for 30 min, decanted and filtered (0.2 µm nylon syringe filter) to remove all the insoluble plant material. The harvested mucilages were freeze-dried immediately (10−1 mbar at −40°C). For comparison, the root tissues of each species were also processed to extract lipids, using the method described by Christie (1989). Excised roots from the seedlings were homogenised in isopropanol, filtered and the residue re-extracted with fresh isopropanol. The filtrates were combined and the solvent was removed on a rotary evaporator. The residue was dissolved in 2 : 1 chloroform/methanol and washed with 0.88% aqueous potassium chloride solution in a modified ‘Folch’ procedure (Ways & Hanahan, 1964; Christie, 1989). The resulting organic layer containing the purified lipid was separated, filtered and the solvent removed on a rotary evaporator. Analysis of fatty acids Derivatisation Lipids were transesterified with sulphuric acid in methanol according to the method described by Christie (1989). This produces methyl ester derivatives of the component fatty acids. The freeze-dried mucilage was dissolved in toluene to which a 1% solution of sulphuric acid in methanol was added. The mixture was left for 18 h at 50°C in a stoppered tube. Water containing 5% sodium chloride was then added and the esters were extracted twice with hexane. The hexane layer was washed with 2% aqueous potassium bicarbonate solution and dried over anhydrous sodium sulphate. The solution was filtered and the solvent volume reduced in a stream of nitrogen, prior to analysis. The purified lipid extracts from root tissues were dissolved in toluene and transesterified in the same manner. Analysis by GC-MS Analyses were carried out on a Hewlett Packard 5890 GC fitted with a Restek Rtx-50 column (15 m long, 0.25 mm i.d., 0.1 µm coating) with helium as the carrier gas at an inlet pressure of 5 psi, coupled to a Hewlett Packard 5970 Mass Selective Detector. The initial temperature of the column was 50°C for 3 min, then increasing at 15°C min−1 to 250°C, held for 5 min. The fatty acid methyl esters were identified from their mass spectra (Christie, 1989) and by comparison of retention times to standards. Results are reported using standard fatty acid nomenclature. The number in front of the colon indicates the number of carbon atoms in the fatty acid; the number after the colon indicates the number of double bonds in the carbon chain. Where the configuration/position of the double bond is known, ‘c’ (cis) or ‘t’ (trans) is shown followed by a number representing the position relative to the carboxyl end of the molecule. Analysis of parent lipids The freeze-dried sample was transferred to a test tube with acidified brine. Methanol was added to the tube and vortex mixed, then twice the volume of chloroform was added and also vortex mixed. The lower layer was separated and the solvent removed. The residue was taken up in chloroform and spotted onto a TLC plate that was developed with 65 : 25 : 4 chloroform/methanol/water. The plate was dried then sprayed with Phospray reagent which stains phosphorus-containing spots blue. Relative retentions were compared with phospholipid standards with phosphatidylethanolamine migrating ahead of phosphatidylcholine. Measurement of surface tension Surface tension was measured by the capillary rise method (Nelkon & Ogborn, 1978) using small precision-bore capillaries (radius = 0.315 mm), thoroughly cleaned in sodium hydroxide solution. Capillary rise (typically ranging between 25 and 45 mm) was measured at 20°C with a travelling microscope using the equilibrium position of the receding meniscus (Read & Gregory, 1997). Individual measurements are accurate to ± 1 mN m−1. Effect of phospholipid on soil properties Selection of lipid The use of root-derived mucilage as a source of surfactant was impractical for these experiments because large quantities were required, and because it would have been impossible to separate effects of the surfactant from those of other components of the mucilage. Therefore a commercially available phosphatidylcholine surfactant (soybean lecithin (Sigma Chemical Co., St. Louis, MO, USA) was used. Although lecithin probably contains a higher proportion of unsaturated fatty acids than the surfactant lipids in mucilages, measurements showed that the surface tension of a 500-mg l−1 solution was about 50 mN m−1, which is comparable with the surface tension observed for root mucilages. Selection of soils Bullionfield soil, collected from Scottish Crop Research Institute, Dundee, was used throughout the experiments. This is a dark brown, sandy loam formed over sandstone and is slightly acidic. For comparison, when it was appropriate, Sonning soil and acid-washed sand were also used in the experiments. Sonning soil was collected from Lamyard field at The University of Reading Farm, Sonning, Berkshire, UK. This soil is a freely draining, sandy loam, formed on fluvial valley gravels and is neutral to slightly acidic. Water release properties The water release properties of Bullionfield soil and acid-washed sand were measured on tension tables and pressure plates. Half the soil and sand samples were treated with deionised water and half with lecithin solution at a concentration of 500 mg l−1. The soil was air-dried and sieved to < 2 mm, the sand was dry sieved to < 250 µm. The soil was packed into Perspex rings (40 mm internal diameter, 10 mm deep, sealed at the base with 20 µm nylon mesh) to give a dry bulk density of 1.1 Mg m−3. Acid-washed sand cores were packed in the same way, but to a dry bulk density of 1.7 Mg m−3. The samples were saturated from the bottom with deionised water or lecithin solution (500 mg l−1), then placed on tension tables held at tensions ranging from 0.5 to 17 kPa or pressure plates at tensions of 250, 900 and 1500 kPa. Three replicates were used for each treatment, at each tension. Samples were weighed periodically until equilibrium water content had been achieved, which was usually within 24 h on the tension tables, although, for the pressure plates, longer equilibration times were required. Phosphate adsorption Phosphate adsorption/desorption was measured using the method described by Rowell (1994). Three replicate soil samples (2.5 g air-dry, < 2 mm) were shaken for 24 h with six standard phosphate solutions in 10 mM calcium chloride. Bullionfield soil is a strong phosphate adsorber, so standard solutions of higher concentration than usual (containing 0, 5, 10, 20, 30 and 50 µg P ml−1) were required. The experiment was repeated with Sonning soil, which is more weakly phosphate-adsorbing, for comparison. Half the soil samples were treated normally, while half were shaken with standard solutions also containing 500 mg l−1 lecithin. At the end of the shaking period, the suspensions were filtered through Whatman no. 41 filter papers and P concentration was determined by the phosphomolybdate method, measuring the absorbance at 880 nm using a Perkin Elmer Lambda 2 UV/Visible spectrometer. The absorbance measurements ranged between 0.003 and 0.699 and the standard error within each set of three replicates was never greater than ±0.003. The presence of lecithin had no effect on the development of the phosphomolybdate complex or the calibration of the method. In both soils, the amount of P initially present was small compared with the amount in the standard solutions. Also, because the lecithin was added at the same time as the phosphate in the standard solutions, it was possible that any effects were due to the lecithin adsorbing to the soil surface before the phosphate, that is blocking P-adsorption, rather than desorbing P from soil particle surfaces. To test this, a further experiment was conducted with Bullionfield soil. Soil samples (2.5 g) were shaken for 24 h with 10 ml of 100 µg P ml−1 solution (in 10 mM calcium chloride). Then, 15 ml of 10 mM calcium chloride solution was added to half the samples and 15 ml of lecithin solution (1000 mg l−1, in 10 mM calcium chloride) was added to the rest. The samples were shaken again for 24 h, filtered, and solution P concentration determined by the phosphomolybdate method. Soil N dynamics The effect of lecithin on net N mineralisation rate in Bullionfield soil was investigated using a 15N pool dilution technique (Gibbs & Barraclough, 1998). In this technique, a small quantity of 15NH4+ is added to the soil. Provided the 15N label mixes with the indigenous soil NH4+, then the decline over time in 15N abundance in the NH4+ pool is a direct indicator of the rate at which mineralisation introduces unlabelled NH4+ into the pool. The soil was sieved < 6 mm while moist. Incubations were carried out using 40 g samples of wet soil (water content 0.24 g g−1 dry soil), packed into 20 mm high plastic rings, 63 mm diameter, covered at the base with 280 µm nylon mesh. The rings were packed to a density of 1.1 Mg dry soil m−3 and sealed with parafilm. Four pin holes allowed gaseous exchange, but restricted water loss. The soil samples were incubated at 20°C for 14 d prior to treatment, periodically watered back to their original weight and re-sealed with parafilm. Three replicates were used for each treatment. After 14 d equilibration, the soil was amended with 10 µg N g−1 dry soil as (15NH4)2SO4, containing 5 atom%15N, in 1 ml of deionised water. Lecithin was added to the ammonium sulphate solution at three concentrations 0, 250 and 500 mg l−1, so the surfactant was added to the soil simultaneously with the labelled ammonium. The 15N-labelled solution was added dropwise from a pipette over the whole surface area of each ring. The samples were then covered and returned to a constant temperature room at 20°C. Half the samples were extracted by shaking the soil with 200 ml of 1 M KCl for 1 h, 1 d after amendment (T1); with the rest extracted 3 d later (T4). NH4+ and NO3− concentrations were determined colorimetrically using a Tecator FIAstar 5010 flow injection auto analyser (Foss Tecator AB, Höganäs, Sweden). Then, NH4+ and NO3− were concentrated by diffusion (in the presence of magnesium oxide for NH4+ and magnesium oxide and Devarda's alloy for NO3−) onto separate Whatman GF/D glassfibre discs acidified with 10 µl 2.5 M KHSO4. 15N: 14N ratios of the discs were determined using a VG 622 mass spectrometer coupled to a Europa Scientific Roboprep combustion analyser (Europa Scientifia, Crewe, UK). Results Analysis of surfactants in root mucilages Fig. 1 shows the fatty acid methyl ester profile of lipids extracted from the root tissues of each plant species. The chromatograms are dominated by three fatty acids: 16 : 0 (11.46 min), 18 : 2 c9, c12 (12.78 min) and 18 : 3 c9, c12, c15 (12.94 min). The most intense peak was different for each of the three species: maize, 18 : 2; lupin, 18 : 3; wheat 16 : 0. Peaks assigned to 18 : 0 (12.70 min) and 18 : 1 c9 (12.67 min) were easily detected, overlapping on the chromatograms slightly, but both were relatively minor components. Very small peaks due to 14 : 0 (10.09 min) and 12 : 0 (8.58 min) could sometimes be detected above the baseline noise. The relative amounts of the major fatty acid components of root tissues and mucilages, estimated from GC peak areas, are summarised in Table 1 . Figure 1Open in figure viewerPowerPoint Gas chromatograms showing the fatty acid composition of the root tissues of: (a) maize (b) lupin and (c) wheat, following extraction, hydrolysis and methyl esterification. Five fatty acids were typically present: saturated (16 : 0 and 18 : 0) and unsaturated (18 : 1, 18 : 2 and 18 : 3). The peak labelled ‘S’ is due to a 19 : 0 standard which was occasionally added prior to derivatisation. Table 1. Fatty acid methyl ester analysis of the root tissues and mucilage of maize, lupin and wheat (concentrations in mol%, based on relative GC peak areas from 1, 2 ) Fatty acid Maize: Lupin: Wheat: Root tissue Unfiltered mucilage Filtered mucilage Root tissue Filtered mucilage Root tissue Filtered mucilage 16 : 0 33.2 42.9 72.6 29.5 58.2 45.2 87.6 18 : 1 2.6 0 0 5.5 0 2.1 0 18 : 0 2.0 9.7 21.5 1.2 27.1 3.7 11.4 18 : 2 55.3 47.4 5.9 18.9 14.7 36.4 1.0 18 : 3 6.9 0 0 44.9 0 12.6 0 In contrast with root tissues, the filtered mucilages consisted almost entirely of saturated fatty acids, especially 16 : 0 with some 18 : 0, along with some unsaturated 18 : 2 (Fig. 2; Table 1). The intensity of these chromatograms was very low, particularly with lupin mucilage. All the chromatograms shown in the figures were obtained using the mass spectrometer in total ion detection mode. Selected ion detection, monitoring just five or six major fatty acid fragment ions, simplified the analysis and was useful for confirmation purposes. However, the selected ion mode distorts relative peak areas, depending on the choice of ions used, and therefore was less useful for quantitative analysis. Figure 2Open in figure viewerPowerPoint Gas chromatograms showing the fatty acid composition of: (a) unfiltered maize mucilage; (b) maize mucilage after filtration through a 0.45-µm syringe filter to remove all insoluble plant material; (c) filtered lupin mucilage; (d) filtered wheat mucilage; following extraction, hydrolysis and methyl esterification. In the filtered mucilages, the two saturated fatty acids (16 : 0 and 18 : 0) predominate. The peak labelled ‘S’ is due to a 19 : 0 standard which was occasionally added prior to derivatisation. Analysis of unfiltered maize mucilage also produced a chromatogram of low intensity. The resulting fatty acid profile was intermediate between those of maize root tissue (Fig. 1) and filtered maize mucilage (Fig. 2). The unsaturated 18 : 2 fatty acid was relatively more abundant, with the saturated acids, especially 16 : 0, accounting for the rest (Table 1). TLC of the filtered mucilages showed that phosphatidylcholine was the main phospholipid class present, although traces of phosphatidylethanolamine were also detected. Fig. 3(a) shows the surface tension of filtered wheat mucilage over a range of total solute concentrations. The wheat mucilage was harvested at a solute concentration (determined by evaporation to constant weight) of c. 2.5 mg ml −1 and subsequently diluted for the surface tension measurements. As concentration increased, the surface tension decreased to < 50 mN m −1 at c. 0.5 mg ml −1 , after which it remained about 48 mN m −1 to concentrations up to 2.4 mg ml −1 . For comparison, the surface tension of soybean-derived lecithin (Sigma Chemical Co. 99%), a typical phosphatidylcholine, was also measured. Surface tension decreased in a similar manner to the wheat mucilage, reaching a value of c. 49 mN m −1 at a concentration of 1000 mg l −1 ( Fig. 3b ). Although surface tension was still decreasing slightly, this concentration was close to the solubility limit of the soybean lecithin used. Figure 3Open in figure viewerPowerPoint Variation of surface tension (at 20°C) of (a) filtered wheat mucilage, and (b) soybean lecithin (a phosphatidylcholine), in aqueous solution. Despite sterilisation, some batches of seeds developed signs of bacterial contamination. Fatty acid analysis of these mucilage samples produced a different profile (Fig. 4), consisting of odd-numbered carbon chain lengths and branched chains, in addition to the even-numbered, unbranched fatty acids characteristic of plants. Hydroxyl-substituted fatty acids were also present (e.g. 2-hydroxydodecanoic acid, 12 : 0 2-OH). The amount of fatty acids in the contaminated samples was much greater, producing relatively intense gas chromatograms. Similarly, TLC showed much higher lipid concentrations where bacterial contamination was present, and the predominant lipid was phosphatidylethanolamine instead of phosphatidylcholine. Figure 4Open in figure viewerPowerPoint Fatty acid analysis of a wheat mucilage sample showing clear signs of bacterial contamination. The profile is intense relative to the mucilage chromatograms (i.e. the quantity of fatty acids in the sample is much greater) and consists of fatty acids with odd-numbered carbon chain lengths, branched chains (‘br’) and occasionally hydroxyl-substitution (12 : 0 2-OH), in addition to the even-numbered, unbranched fatty acids characteristic of plants. Effect of phospholipid on soil properties Water release properties Fig. 5 shows the moisture release of Bullionfield soil at a density of 1.1 Mg m −3 , with and without lecithin. The presence of the surfactant always reduced the equilibrium water content at any given tension. The effect was greatest at low tension where water content was reduced by as much as 10%, at the same potential, by the lecithin. The surface tension of the lecithin solution is about 65% that of pure water so it was possible to calculate the expected moisture release curve for comparison with the measured values ( Fig. 5 ). At low tension, the agreement between this calculated line and the measured data was good, but at higher tensions (> 4 kPa) the lines diverge and the effect of the surfactant was smaller than predicted. Figure 5Open in figure viewerPowerPoint Water release curves for Bullionfield soil at a density of 1.1 Mg m −3 with pure water (circles, solid line) and with lecithin solution at a concentration of 500 mg l −1 (squares, solid line), measured on tension tables. Error bars, offset from the data points for clarity, represent one standard error. For c

Reliable techniques for screening large numbers of plants for root traits are still being developed, but include aeroponic, hydroponic and agar plate systems. Coupled with digital cameras and image analysis software, these systems permit the rapid measurement of root numbers, length and diameter in moderate (typically &lt;1000) numbers of plants. Usually such systems are employed with relatively small seedlings, and information is recorded in 2D. Recent developments in X-ray microtomography have facilitated 3D non-invasive measurement of small root systems grown in solid media, allowing angular distributions to be obtained in addition to numbers and length. However, because of the time taken to scan samples, only a small number can be screened (typically &lt;10 per day, not including analysis time of the large spatial datasets generated) and, depending on sample size, limited resolution may mean that fine roots remain unresolved. Although agar plates allow differences between lines and genotypes to be discerned in young seedlings, the rank order may not be the same when the same materials are grown in solid media. For example, root length of dwarfing wheat (Triticum aestivum L.) lines grown on agar plates was increased by ~40% relative to wild-type and semi-dwarfing lines, but in a sandy loam soil under well watered conditions it was decreased by 24–33%. Such differences in ranking suggest that significant soil environment–genotype interactions are occurring. Developments in instruments and software mean that a combination of high-throughput simple screens and more in-depth examination of root–soil interactions is becoming viable.

Reinforcement of soil by fibrous roots is crucial for preventing soil erosion and degradation, yet the underlying mechanisms are poorly understood. We investigated soil reinforcement by roots of barley (Hordeum vulgare) planted at different densities in a controlled glasshouse and a separate field study. Soil shear strength increased with planting density (0–950 m−2) at 5 weeks with an average 6.7 ± 1.40 kPa increase in strength over the fallow (7.5 ± 0.47 kPa). At 20 weeks, planting density had less of an effect, with on average a 29% increase in strength contributed by roots. In the glasshouse study, roots increased shear strength by an average of 53%, with a positive effect found for the eight planting densities tested ranging from 0 to 1130 plants/m2. Detailed measures of root tensile strength, and diameter distributions at the shear plane, allowed us to apply and test two existing root reinforcement models of Wu et al. [Wu, T.H., Mckinnell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince-Of-Wales-Island, Alaska. Canadian Geotechnical Journal 16, 19–33] and Pollen and Simon [Pollen, N., Simon, A., 2005. Estimating the mechanical effects of riparian vegetation on stream bank stability using a fiber bundle model. Water Resources Research, 41]. A progressive failure Fibre Bundle Model, developed by Pollen and Simon [Pollen, N., Simon, A., 2005. Estimating the mechanical effects of riparian vegetation on stream bank stability using a fiber bundle model. Water Resources Research, 41], predicted reinforcement better than the catastrophic failure model by Wu et al. [Wu, T.H., Mckinnell, W.P., Swanston, D.N., 1979. Strength of tree roots and landslides on Prince-Of-Wales-Island, Alaska. Canadian Geotechnical Journal 16, 19–33], but neither described reinforcement well for field-grown plants near maturity at 20 weeks.

Simple indicators of crop and cultivar performance across a range of soil types and management are needed for designing and testing sustainable cropping practices. This paper determined the extent to which soil chemical and physical properties, particularly soil strength and pore-size distribution influences root elongation in a wide range of agricultural top soils, using a seedling-based indicator. Intact soil cores were sampled from the topsoil of 59 agricultural fields in Scotland, representing a wide geographic spread, range of textures and management practices. Water release characteristics, dry bulk density and needle penetrometer resistance were measured on three cores from each field. Soil samples from the same locations were sieved, analysed for chemical characteristics, and packed to dry bulk density of 1·0 g cm−3 to minimize physical constraints. Root elongation rates were determined for barley seedlings planted in both intact field and packed soil cores at a water content close to field capacity (–20 kPa matric potential). Root elongation in field soil was typically less than half of that in packed soils. Penetrometer resistance was typically between 1 and 3 MPa for field soils, indicating the soils were relatively hard, despite their moderately wet condition (compared with <0·2 MPa for packed soil). Root elongation was strongly linked to differences in physical rather than chemical properties. In field soil root elongation was related most closely to the volume of soil pores between 60 µm and 300 µm equivalent diameter, as estimated from water-release characteristics, accounting for 65·7 % of the variation in the elongation rates. Root elongation rate in the majority of field soils was slower than half of the unimpeded (packed) rate. Such major reductions in root elongation rates will decrease rooting volumes and limit crop growth in soils where nutrients and water are scarce.

Plant roots have considerable impact on the mechanical stability of soil, but to date the underlying mechanisms have been poorly quantified. In this study, controlled laboratory studies of soil reinforced with willow trees ( Salix viminalis cv Tora) found a strong correlation between the cross‐sectional area of soil covered by roots and shear reinforcement. We separated broken versus pulled‐out roots and measured individual root diameters crossing the shear‐plane. The shear strength of planted specimens compared with non‐planted specimens increased eight‐fold at 0.10‐m shear depth, more than four‐fold at 0.25‐m depth, and more than doubled at 0.40‐m depth. These data were used to evaluate several models of root‐reinforcement. Models based on catastrophic and simultaneous failure of all roots overpredicted reinforcement by 33% on average. Better agreement between experimental and model results was found for a stress‐based fiber‐bundle‐model, in which roots break progressively from weakest to strongest, with the load shared on the remaining roots at each step. Roots have a great capacity to reinforce soils, with existing models providing reasonable predictions of increased shear strength. However, deterministic understanding and modeling of the processes involved needs to consider root failure mechanisms. In particular, the role of root stiffness and root–soil adhesion is not considered in existing models of soil reinforcement by plant roots.

Models of root system growth emerged in the early 1970s, and were based on mathematical representations of root length distribution in soil. The last decade has seen the development of more complex architectural models and the use of computer-intensive approaches to study developmental and environmental processes in greater detail. There is a pressing need for predictive technologies that can integrate root system knowledge, scaling from molecular to ensembles of plants. This paper makes the case for more widespread use of simpler models of root systems based on continuous descriptions of their structure. A new theoretical framework is presented that describes the dynamics of root density distributions as a function of individual root developmental parameters such as rates of lateral root initiation, elongation, mortality, and gravitropsm. The simulations resulting from such equations can be performed most efficiently in discretized domains that deform as a result of growth, and that can be used to model the growth of many interacting root systems. The modelling principles described help to bridge the gap between continuum and architectural approaches, and enhance our understanding of the spatial development of root systems. Our simulations suggest that root systems develop in travelling wave patterns of meristems, revealing order in otherwise spatially complex and heterogeneous systems. Such knowledge should assist physiologists and geneticists to appreciate how meristem dynamics contribute to the pattern of growth and functioning of root systems in the field.

In this paper, we provide direct evidence of the importance of root hairs on pore structure development at the root-soil interface during the early stage of crop establishment. This was achieved by use of high-resolution (c. 5 μm) synchrotron radiation computed tomography (SRCT) to visualise both the structure of root hairs and the soil pore structure in plant-soil microcosms. Two contrasting genotypes of barley (Hordeum vulgare), with and without root hairs, were grown for 8 d in microcosms packed with sandy loam soil at 1.2 g cm-3 dry bulk density. Root hairs were visualised within air-filled pore spaces, but not in the fine-textured soil regions. We found that the genotype with root hairs significantly altered the porosity and connectivity of the detectable pore space (> 5 μm) in the rhizosphere, as compared with the no-hair mutants. Both genotypes showed decreasing pore space between 0.8 and 0.1 mm from the root surface. Interestingly the root-hair-bearing genotype had a significantly greater soil pore volume-fraction at the root-soil interface. Effects of pore structure on diffusion and permeability were estimated to be functionally insignificant under saturated conditions when simulated using image-based modelling.

Roots are important to plants for a wide variety of processes, including nutrient and water uptake, anchoring and mechanical support, storage functions, and as the major interface between the plant and various biotic and abiotic factors in the soil environment. Therefore, understanding the development and architecture of roots holds potential for the manipulation of root traits to improve the productivity and sustainability of agricultural systems and to better understand and manage natural ecosystems. While lateral root development is a traceable process along the primary root and different stages can be found along this longitudinal axis of time and development, root system architecture is complex and difficult to quantify. Here, we comment on assays to describe lateral root phenotypes and propose ways to move forward regarding the description of root system architecture, also considering crops and the environment.

Approximately 40% of total terrestrial precipitation transits the tiny volume of rhizosphere soil around plant roots before being transpired, making it one of the most hydrologically active regions of the biosphere. This study considers several findings at the root–soil interface that affect our understanding of water retention and flow in the root zone and hence the water relations of all vegetated soil profiles. Imaging methods, including neutron radiography and light transmission, are illuminating the dynamics of water content around plant root systems. These methods, together with studies on samples of mucilage and mucilage‐compound‐amended soils, have provided increasing evidence that rhizosphere hydraulic properties differ from those of bulk soil. Changes in soil structure due to root growth, rhizodeposition, and repeated drying cycles change the pore size distribution and coat soil particles with organic compounds. Some of these compounds exhibit hydrophobic or hydrophilic behavior, depending on the soil water content, giving rise to the hysteretic‐like behavior in the rhizosphere that has been observed in dynamic image sequences. Data from studies that consider the water retention properties of maize ( Zea mays L.) mucilage from primary and nodal roots together with polymer gels are compared to consider the likely impact of mucilage on soil water release. Roots often generate and are intimately associated with flow paths for water and solutes in the soil, and vegetation is well known to exert a major influence on catchment hydrology. The potential use of vegetation to manage hydrologic processes at field scale is considered briefly as a way of influencing water outflow rates and engineering soils for particular purposes.

Summary There is an urgent need for simple rapid screens of root traits that improve the acquisition of nutrients and water. Temperate cereals produce rhizosheaths of variable weight, a trait first noted on desert species sampled by T ansley over 100 yr ago. This trait is almost certainly important in tolerance to abiotic stress. Here, we screened association genetics populations of barley for rhizosheath weight and derived quantitative trait loci ( QTL s) and candidate genes. We assessed whether rhizosheath weight was correlated with plant performance and phosphate uptake under combined drought and phosphorus deficiency. Rhizosheath weight was investigated in relation to root hair length, and under both laboratory and field conditions. Our data demonstrated that rhizosheath weight was correlated with phosphate uptake under dry conditions and that the differences in rhizosheath weight between genotypes were maintained in the field. Rhizosheath weight also varied significantly within barley populations, was correlated with root hair length and was associated with a genetic locus ( QTL ) on chromosome 2 H . Putative candidate genes were identified. Rhizosheath weight is easy and rapid to measure, and is associated with relatively high heritability. The breeding of cereal genotypes for beneficial rhizosheath characteristics is achievable and could contribute to agricultural sustainability in nutrient‐ and water‐stressed environments.

The physical role of root hairs in anchoring the root tip during soil penetration was examined. Experiments using a hairless maize mutant (Zea mays: rth3-3) and its wild-type counterpart measured the anchorage force between the primary root of maize and the soil to determine whether root hairs enabled seedling roots in artificial biopores to penetrate sandy loam soil (dry bulk density 1.0-1.5g cm(-3)). Time-lapse imaging was used to analyse root and seedling displacements in soil adjacent to a transparent Perspex interface. Peak anchorage forces were up to five times greater (2.5N cf. 0.5N) for wild-type roots than for hairless mutants in 1.2g cm(-3) soil. Root hair anchorage enabled better soil penetration for 1.0 or 1.2g cm(-3) soil, but there was no significant advantage of root hairs in the densest soil (1.5g cm(-3)). The anchorage force was insufficient to allow root penetration of the denser soil, probably because of less root hair penetration into pore walls and, consequently, poorer adhesion between the root hairs and the pore walls. Hairless seedlings took 33h to anchor themselves compared with 16h for wild-type roots in 1.2g cm(-3) soil. Caryopses were often pushed several millimetres out of the soil before the roots became anchored and hairless roots often never became anchored securely.The physical role of root hairs in anchoring the root tip may be important in loose seed beds above more compact soil layers and may also assist root tips to emerge from biopores and penetrate the bulk soil.

New PhytologistVolume 157, Issue 3 p. 597-603 Free Access Plant influence on rhizosphere hydraulic properties: direct measurements using a miniaturized infiltrometer P. D. Hallett, Corresponding Author P. D. Hallett Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKAuthor for correspondence: Paul Hallett Tel: +44 (0)1382 62731 Fax: +44 (0)1382 62426 Email: [email protected]Search for more papers by this authorD. C. Gordon, D. C. Gordon Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKSearch for more papers by this authorA. G. Bengough, A. G. Bengough Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKSearch for more papers by this author P. D. Hallett, Corresponding Author P. D. Hallett Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKAuthor for correspondence: Paul Hallett Tel: +44 (0)1382 62731 Fax: +44 (0)1382 62426 Email: [email protected]Search for more papers by this authorD. C. Gordon, D. C. Gordon Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKSearch for more papers by this authorA. G. Bengough, A. G. Bengough Plant–Soil Interface Programme, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UKSearch for more papers by this author First published: 03 March 2003 https://doi.org/10.1046/j.1469-8137.2003.00690.xCitations: 89AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Summary • An infiltrometer device, 0.4 mm in radius was designed specifically to measure the hydraulic characteristics of rhizosphere soil. Its testing and application to the rhizosphere of four plant species–barley (Hordeum vulgare), oil-seed rape (Brassica napus), potato (Solanum tuberosum) and grass (Lolium multiflorum) – was described. • In excavated blocks of field soil, there was a significant influence of plant species on sorptivity and water repellency in the rhizosphere. • Further controlled laboratory tests on young plants in moist, sieved soil showed reduced water sorptivity owing to increased repellency in the rhizosphere compared with bulk soil for barley but not oil-seed rape. • Root exudates may clog pores or become hydrophobic on soil particle surfaces. The slightly higher water repellency measured in rhizosphere soil would have minimal influence on plant water uptake. However, it may provide a buffer against desiccation at lower water contents and reduce structural degradation of rhizosphere soil by slaking. Introduction The ability of a plant to extract water from soil is influenced by the hydraulic transport properties of rhizosphere soil close to the root (Stirzaker & Passioura, 1996; Sperry et al., 1998). Considerable research, ranging from visual observations of root-adherent soil to highly descriptive studies of pore structural properties, have shown the rhizosphere to have very different physical properties from the bulk soil (Amellal et al., 1999; McCully, 1999). This arises because enhanced levels of biological exudates and cycles of wetting and drying near to the root increase aggregation by binding together soil particles (Watt et al., 1994; Traoréet al., 2000). Hydraulic transport in the rhizosphere may also be influenced by surfactants produced by plant roots (Read et al., 2002) and the coating of soil particles by complex organic compounds (DeBano, 2000). Both of these factors should make rhizosphere soil more hydrophilic, enhancing soil wetting. However, the organic compounds produced by plant roots and some microbes are complex, and under certain physical conditions or when in contact with soil particles, the hydrophilic ligands may bind and produce an exposed hydrophobic surface that is water repellent (Czarnes et al., 2000; Lichner et al., 2002). The degree of hydrophobicity (repellency) may therefore depend on the quantity and type of organic compounds present, the soil matric potential, and the number of wetting and drying cycles undergone. Plant genotype will have an important effect on the nature of plant and microbial exudates present in the rhizosphere and potentially on soil hydraulic properties. Direct measurements of rhizosphere hydraulic properties are needed to determine the net effect of the cocktail of exudates released by roots and rhizosphere microbes. Direct studies of rhizosphere hydraulic properties are limited. Noninvasive X-ray computed tomography (CT) scans (Grose et al., 1996) and NMR imaging (Brown et al., 1990) have shown a zone of water depletion close to the root. Wetter soil conditions in the rhizosphere have also been identified and attributed to the presence of a mucigel (Young, 1995). Alami et al. (2000) measured the pore structure properties of rhizosphere soil to very fine resolution using mercury porosimetry and from these results suggested that hydraulic transport would be enhanced compared with bulk soil. No study has measured the hydraulic characteristics of rhizospheric soil directly, however, because it has not been possible using conventional equipment. In this paper, a method is presented whereby hydraulic transport properties of the rhizosphere soil can be measured directly using a miniaturized infiltrometer device. The infiltrometer has a contact radius 0.4 mm so that its zone of influence is largely confined to the rhizosphere (Amellal et al., 1999). Field and laboratory studies were conducted to assess the influence of plant genotype on the transport properties of rhizosphere and bulk soil. Materials and Methods Measuring fine resolution hydraulic properties Three different hydraulic properties were measured in the rhizosphere and bulk soil: (1) water sorptivity, (2) ethanol sorptivity and (3) water repellency. Water sorptivity is the rate at which soil imbibes water, much like a sponge. It influences the movement of water to and within the rhizosphere, particularly during rainfall when the soil is dry. The soil matric potential, pore structure, and particle hydrophobicity all influence water sorptivity. The influence of hydrophobicity was isolated by measuring the ethanol sorptivity, which because of its nonpolar nature and contact angle with hydrophobic surfaces provides a transport measurement not influenced by repellency. An index of water repellency was evaluated by comparing the water and ethanol sorptivity values. This parameter describes the extent that water sorptivity may be altered through the coating of soil particles with hydrophobic organic compounds. Hence, we can separate out effects of changes in pore structure associated with roots from changes in repellency caused by the presence of root and microbial exudates. To obtain direct measurements of rhizosphere hydraulic properties, a miniaturized infiltrometer device (Fig. 1) was constructed based roughly on the design of Leeds-Harrison et al. (1994). The infiltrometer had a contact radius of 0.4 mm, which produced a small area of influence comparable to the size of the rhizosphere. The set-up consisted of a standard 200 µl pipette tip attached via flexible tubing to a liquid reservoir on a recording balance. Loose, cleaned fibres from the exhausted ink reservoir of a marker pen were inserted into the pipette tip to improve soil contact and allow for the establishment of a negative hydraulic head. These fibres were chosen after an exhaustive test of different sponges and fibres showed that they were the best at not impeding liquid transport and restricting air-entry. A manometer was attached to the main tube so that an accurate measurement of hydraulic head could be obtained close to the sample. All measurements were conducted at −40 mm hydraulic head by adjusting the height of the tip in relation to the top of the liquid reservoir and taking into account the influence of capillarity forces contributed by the infiltrometer tip. A more detailed description of the different components of the device is listed in the legend of Fig. 1. Figure 1Open in figure viewerPowerPoint The device used to measure soil hydraulic characteristics at very small scale. It consists of (a) an infiltrometer tip, (b) manometer tube for an accurate measurement of hydraulic head and (c) a liquid reservoir on an electronic balance. The inset image is a close-up of the infiltrometer tip. Contact between the infiltrometer tip and the soil sample was observed at ×8 magnification using a stereo microscope. This allowed for an accurate selection of the measurement location and adjustment of the ‘lab–jack’ supporting the soil specimen so that compressive deformation caused by the probe was minimized. It was also possible to visually check that no bubbles were present within the infiltrometer that might affect its conductance. Liquid uptake by the soil from the infiltrometer reservoir was logged from the balance at 2 s intervals for 130 s. After about 30 s, the water flow rate, Q, was steady and used to evaluate sorptivity, S using the equation (Eqn 1) which accounts for the geometry of the wetting front and the influence of soil properties. The parameter b depends on the soil-water diffusivity function, r is the radius of the infiltrometer tip, and f is the fillable air-porosity (Leeds-Harrison et al., 1994). The value of b can range from 0.5 ≤ b ≤ π/4 with 0.55 being an ‘average’ value used here. The water repellency index (R) was determined from the sorptivity measurements of water, SW and ethanol, SE (Tilman et al., 1989) at −40 mm pressure head. Accounting for differences in surface tension and viscosity of the two liquids provides an index R given by (Eqn 2) with R = 1.0 signifying a totally nonrepellent soil (at 20°C, Tillman et al., 1989). The value of R is directly proportional to the influence of repellent substances on reducing S. If, for example, R = 5, S is 1/5 the level it would be in the absence of repellency. Testing the miniaturized infiltrometer Before any tests on soil, the miniaturized infiltrometer was checked to ensure that its liquid conductance was large enough to not impede transport measurements. The first test was to lower the tip until a liquid drop formed under positive pressure. This pressure is caused by capillary forces within the tube and was subsequently added to any negative pressure potential created during testing by adjusting the tip height in relation to the reservoir surface. If the infiltrometer tip dripped water under positive pressure, any impedance to measuring sorptivity on real soil was first determined by conducting a few measurements on 0.1 mm diameter spherical glass ballotini, loosely packed into a 1 cm3 cube tray, which provides a homogeneous porous material with a sorptivity several fold greater than soil. These tests were done at −40 mm head so that they would be under the same conditions as the subsequent soil tests. Field study Large intact blocks (350 × 250 × 80 mm) of soil were taken from the rooting profile of mature crops of (1) Hordeum vulgare L. cv. Optic (barley) fertilized with 30 kg N and 5 kg P ha−1, (2) Brassica napus L. cv. Mascot (oilseed rape) fertilized with 90 kg N and 15 kg P ha−1, (3) Solanum tuberosum cv. Maris Piper (potato) fertilized with 120 kg N and 150 kg P ha−1 and (4) Lolium multiflorum cv. Meribel (grass) plants fertilized with 50 kg N and 50 kg P ha−1, grown on neighbouring plots within the same arable field. Applied N was ammonium nitrate. The soil was derived from undifferentiated sandstone (Carpow series) comprising 71% sand, 19% silt, and 10% clay with a pH(H2O) of 6.2, 1.9% C and 0.07% N. To minimize damage to the soil, a 400-mm deep trench was dug in front of the plant and the edges of the soil face were cut to the dimensions of a plastic box used to transport the soil. The box was then inserted into the cut face and then the box and soil were cut away. Sampling was conducted in mid-September, a few days before harvesting. The soil was stored at 4°C for at least 72 h before testing to allow for the water to equilibrate and to reduce any biological activity that may influence the hydraulic properties on the newly exposed soil surfaces. Measurements of SW and SE were taken within 1 mm of a root (rhizosphere) and at least 20 mm from roots (bulk soil). The number of replicates and the water content at testing are listed in the Results. All measurements were in the top 100 mm depth of the sample. Laboratory study A laboratory study was conducted on soil planted with the same cultivars of (1) barley, (2) oil-seed rape or not planted to serve as a (3) control. Soil from the same site used for the field study was sampled in January, dried to 18 g 100 g−1, and passed through a 2-mm sieve. Germinated seeds were planted in the top 5 mm of a ‘rhizotrunk’ packed with this soil to a density of 1200 kg m−3. The rhizotrunks were constructed from 22 mm deep × 47 mm wide plastic conduit trunk cut to a length of 300 mm. Easy access to the rooting system was facilitated by a removable surface along the length of the trunk. the bottom end of the rhizotrunk was sealed. The soil was packed tight against the plastic walls to minimize evaporation from these surfaces. The plants were grown for 10 d in a controlled environment chamber set to 15°C and with maximum light intensity for 16 h d−1. The rhizotrunks were inclined at 10° to the vertical so that the roots would grow preferentially along the removable surface. After removal of this surface, measurements of SW and SE were taken within 1 mm of the roots (rhizosphere) and at least 20 mm away from roots exposed on the surface of the soil (bulk soil). This approach was preferred over excavating the soil to expose roots because of the potential damage to the soil pore structure and abrasion of organic compounds from particle surfaces. Potential artefacts at the soil–plastic interface in the rhizotrunks include a lower surface contact area between soil particles and roots, and lower mechanical impedance. The packing of the soil against the rhizotrunk walls and the dominance of closer lateral gas diffusion from the surrounding soil, as opposed to further vertical gas diffusion down the soil–plastic interface, will probably cause minimal differences in soil atmosphere compared with pores away from the removable plastic surface. Recent work has also shown that in many soils up to 80% of wheat roots are located within the 1-mm thick sheath of soil surrounding macropores (Pierret et al., 1999), so even if preferential gas transport and drying occurs at this surface, using rhizotrunks is extremely relevant to the situation of roots in the field. Statistical analyses Water and ethanol sorptivity data were analysed using anova to test for differences between plant type and the location in the soil (rhizosphere vs bulk soil). Differences between these values for individual plant species was assessed using a t-test. The Fisher-Behrens test was used to determine differences in repellency between rhizosphere and bulk soil. Results Testing the miniaturised infiltrometer The selected infiltrometer tips provided reproducible results and a sufficient level of hydraulic conductance to not impede sorptivity measurements on soil (Fig. 2). It was possible to evaluate the infiltration rate, Q, from the extremely linear (r2 > 0.99) water uptake between 30 s and 120 s. The sorptivity of the ballotini was (mean ± SE) 2.50 ± 0.07 mm s−1/2 with a 9.3% coefficient of variation when measured with 0.4 mm radius tips. Larger infiltrometer tips with a radius of 0.5 mm and 0.6 mm had a more rapid infiltration rate because of the contact radii with the ballotini. However, the calculated sorptivity for these larger tips was similar to the smaller 0.4 mm radius tips (P > 0.10, df = 9; P indicates the level of significance determined using anova). Figure 2Open in figure viewerPowerPoint Water uptake by ballotini over time. The results are for two different probes indicated by open and filled symbols. These two probes were used for all of the measurements that are reported. Water uptake rates, Q, were evaluated for a selected period indicated by the fitted regression line. Field study Water sorptivity was significantly different between plant genotypes (P < 0.001, df = 101) but it was not different between rhizosphere and bulk soil (P > 0.10, df = 101) (Fig. 3a). Parallel measurements of ethanol sorptivity isolate pore structure influences on hydraulic transport. Ethanol sorptivity was influenced significantly by the plant genotype (P < 0.001, df = 121) and was higher in the rhizosphere compared with bulk soil (P < 0.01, df = 121) (Fig. 3b). On further statistical analysis, only oil seed rape had a significantly different ethanol sorptivity between the rhizosphere and bulk soil (P < 0.001, df = 24). There are apparent differences in the level of repellency between plant treatments, with potato showing the greatest water repellency and oil-seed rape the least (Fig. 3c). Figure 3Open in figure viewerPowerPoint Influence of plant genotype on hydraulic characteristics for the field experiment. The closed bars indicate the rhizosphere and the open bars are for the bulk soil. Significant differences between rhizosphere and bulk soil values are indicated by ***, P < 0.001. Laboratory study For all of the laboratory tests there was no influence of sampling depth on the measured transport properties of the soil (R2 < 0.5, P > 0.10, df = 135). All data from each treatment were grouped before subsequent statistical analyses. The laboratory study eliminated the influence of different cultivation practices and long-term structure dynamics on the porosity properties of the soil (Fig. 4). This is verified by the similar ethanol sorptivity measurements found for the different plant treatments (P > 0.10, df = 135) (Fig. 4b). Rhizosphere and bulk soil also had similar ethanol sorptivities (P > 0.10, df = 135). Water sorptivity was lower in the rhizosphere than the bulk soil (P < 0.001, df = 120) and affected significantly by the plant type (P < 0.01, df = 120). However, only barley showed a significant difference in water sorptivity between rhizosphere and bulk soil when individual plant species were analysed (Fig. 4a). Repellency followed a similar trend, with only the rhizosphere soil of barley having a significantly higher level than the bulk soil or the control. Figure 4Open in figure viewerPowerPoint Influence of plant genotype on hydraulic characteristics for the laboratory experiment with the soil at its ambient water content. The closed bars indicate the rhizosphere and the open bars are for the bulk soil. Significant differences between rhizosphere and bulk soil values are indicated by **, P < 0.05, and ***, P < 0.001. Discussion Rhizosphere hydraulic transport and the influence of genotype There were some differences in hydraulic transport between the rhizosphere and bulk soil, with water sorptivity generally reduced in the rhizosphere. Plant root exudates and secondary microbial metabolites are almost certainly responsible for these differences, since they may coat soil particles and induce the slightly higher water repellency measured in the rhizosphere (Czarnes et al., 2000). Deciphering the effect of genotype from the results of the field samples is complicated by the different cultivation practices used before planting. All of the cultivated fields except grass, however, had relatively similar ethanol sorptivity values in the bulk soil, suggesting that the pore structure was not markedly different. The slightly higher ethanol sorptivity values found under grass supports previous research by Hallett et al. (2001a) and is likely a result of the high density of grass roots. Both of these factors will increase pore continuity and therefore the ability of soil to imbibe liquids and roots to extract water. Only the rhizosphere soil of oil-seed rape had a higher ethanol sorptivity than the bulk soil, suggesting that this genotype induced changes in rhizosphere soil aggregation, which would also improve the capacity of the plant to extract water from the soil. Increased aggregation and reduced water repellency levels may have been responsible for the greater water sorptivity found for oil-seed rape and grass compared with barley and potato in the field experiment. The laboratory study showed that oil-seed rape had a smaller influence on rhizosphere soil water repellency than barley, which is reflected in the differences in water sorptivity between the rhizosphere and bulk soil. Pore structure differences caused by aggregation probably did not influence water sorptivity since the ethanol sorptivity values were very similar between treatments. Aggregation is generally induced by a cycle of wetting and drying, so these similar results for ethanol sorptivity were expected. The differences in rhizosphere water transport properties between plant genotypes may be due to the amount and chemical composition of root exudates. Isothiocyanates released to soil by Brassicas (which include oil seed rape) are known to suppress fungal pathogens (Kirkegaard et al., 1996) and these chemicals may also reduce other fungi that are known to induce water repellency (Bond, 1964; Hallett et al., 2001b). This may explain the higher water sorptivity found for oil-seed rape in the field and laboratory studies on moist soil. Barley roots exude large quantities of readily decomposable compounds (Fan et al., 2001) that may stimulate microbial processes that induce repellency (Hallett & Young, 1999). The repellency measurements may also be an artefact of ethanol transport properties. Young (1995) speculated that mucigel may alter the water holding capacity in the rhizosphere. When the mucigel is in a hydrated state it may clog pores and reduce water transport. Mucigel may be solubilized by ethanol, producing higher ethanol sorptivity values and therefore higher calculated water repellencies in the hydrated state. This problem could potentially be overcome by replacing ethanol with water containing a surfactant or another liquid that will not solubilize hydrophobic organic compounds. Potential and limitations of the miniature infiltrometer technique Hydraulic transport properties in the rhizosphere were isolated for the first time from the bulk soil by direct infiltration measurements. The miniaturized infiltrometer worked well: the water sorptivity values reported here were fairly typical of those measured using much larger infiltrometers (Tillman et al., 1989). The accuracy of the miniaturized infiltrometer, tested using ballotini (Fig. 2), was similar to standard one-dimensional infiltration tests conducted at much larger scales (Leeds-Harrison et al., 1994). Error in infiltration measurements can be caused by air-entrapment, the contact between the infiltrometer tip and soil, and the inherent heterogeneity of the tested porous media (Youngs, 1995). Moreover, the error will be smaller when soil is tested, as it has a far lower water sorptivity. When soil was tested, the wetting front in advance of the infiltrometer tip after 120 s of testing had a 2–3 mm zone of influence. This is based on a range of S between 0.3 mm s−1/2 and 0.6 mm s−1/2, the infiltration I per unit area of wetting front evaluated from the standard sorptivity relationship I = St1/2 (Leeds-Harrison et al., 1994) and an assumed spherical shape to the wetting front. The zone of influence is roughly the largest size of the sieved soil before packing and the rhizosphere (McCully, 1999). Even if the zone of influence was larger than the rhizosphere, as would be the case for sandier soils, water transport is limited by the area of lowest sorptivity (Koorevaar et al., 1983), which would likely be the contact point of the infiltrometer tip near to the root (i.e. rhizosphere soil). Thus, we are confident that the hydraulic properties measured are applicable to the rhizosphere scale and the technique will be useful for a wide range of soils. Implications for plant physiology There is sufficient evidence from the direct measurements of hydraulic transport in the rhizosphere to suggest that it has physical properties that differ from the bulk soil. This corresponds with other research where rhizosphere soil has been collected and measured for a range of physical properties (Watt et al., 1994; Amellal et al., 1999; Traoréet al., 2000). Our results provide a direct measurement on intact soil so it is relevant to understanding the influence of plant roots on small-scale transport properties in field conditions. These data will assist in the development of hydraulic transport models of water distribution and uptake around roots (Clausnitzer & Hopmans, 1994; Stirzaker & Passioura, 1996), and in the understanding of water infiltration around roots following rainfall. The measured levels of water repellency, however, would have minimal influence on the ability of plants to extract water from soil. If the cause of repellency is mucigel that clogs soil pores the rhizosphere may be buffered from desiccation stresses and may improve the conductivity of water under unsaturated conditions. This could be confirmed by testing rhizosphere soil over a range of water contents. Our tests were conducted at optimal water contents for plant growth. Another implication may be the stabilization of the soil pore structure against wetting stresses (Czarnes et al., 2000), which may produce a superior physical environment for root growth and gas transport. In conclusion, we demonstrated that a miniature infiltrometer can be built to function at the rhizosphere scale. The technique offers a way to quantify rhizosphere hydraulic properties directly, for the first time, both for field and laboratory samples. Plant genotype and the condition of the soil both influence water transport, potentially due to alteration in soil structure and the coating of soil particles with organic compounds in the rhizosphere. Acknowledgements The Scottish Crop Research Institute receives Grant-in-Aid support from the Scottish Executive Environment and Rural Affairs Department. References Alami Y, Achouak W, Marol C, Heulin T. 2000. Rhizosphere soil aggregation and plant growth promotion of sunflowers by an exopolysaccharide- producing Rhizobium sp. strain isolated from sunflowers roots. Applied and Environmental Microbiology 66: 3393 – 3398. Amellal N, Bartoli F, Villemin G, Talouizte A, Heulin T. 1999. Effects of inoculation of EPS-producing Pantoea agglomerans on wheat rhizosphere aggregation. Plant and Soil 211: 93 – 101. Bond RD. 1964. the influence of the microflora on the physical properties of soils. I. Effects associated with filamentous algae and fungi. Australian Journal of Soil Research 2: 111 – 122. Brown JM, Kramer PJ, Cofer GP, Johnson GA. 1990. Use of nuclear- magnetic-resonance microscopy for noninvasive observations of root-soil water relations. Theoretical and Applied Climatology 42: 229 – 236. Clausnitzer V, Hopmans JW. 1994. Simultaneous modeling of transient three-dimensional root growth and soil water flow. Plant and Soil 164: 299 – 314. Czarnes S, Hallett PD, Bengough AG, Young IM. 2000. Root- and microbial-derived mucilages affect soil structure and water transport. European Journal of Soil Science 51: 435 – 443. DeBano LF. 2000. Water repellency in soils: a historical overview. Journal of Hydrology 231–232: 4 – 32. Fan TWM, Lane AN, Shenker M, Bartley JP, Crowley D, Higashi RM. 2001. Comprehensive chemical profiling of gramineous plant root exudates using high-resolution NMR and MS. Phytochemistry 57: 209 – 221. Grose MJ, Gilligan CA, Spencer D, Goddard BVD. 1996. Spatial heterogeneity of soil water around single roots: Use of CT-scanning to predict fungal growth in the rhizosphere. New Phytologist 133: 261 – 272. Hallett PD, Baumgartl T, Young IM. 2001a. Sub-critical water repellency of aggregates under a range of soil management practices. Soil Science Society of America Journal 65: 184 – 190. Hallett PD, Ritz K, Wheatley RE. 2001b. Microbial derived water repellency in golf course soil. International Turfgrass Society Research Journal 9: 518 – 524. Hallett PD, Young IM. 1999. Changes to water repellence of soil aggregates caused by substrate-induced microbial activity. European Journal of Soil Science 50: 35 – 40. Kirkegaard JA, Wong PTW, Desmarchelier JM. 1996. In vitro suppression of fungal root pathogens of cereals by Brassica tissues. Plant Pathology 45: 593 – 603. Koorevaar P, Menelik G, Dirksen C. 1983. Elements of soil physics. Amsterdam, The Netherlands: Elsevier. Leeds-Harrison PB, Youngs EG, Uddin B. 1994. A device for determining the sorptivity of soil aggregates. European Journal of Soil Science 45: 269 – 272. Lichner L, Babejová N, Dekker LW. 2002. Effects of kaolinite and drying temperature on the persistence of soil water repellency induced by humic acids. Rostilinná Výroba 48: 203 – 207. McCully ME. 1999. Roots in soil: unearthing the complexities of roots and their rhizospheres. Annual Review of Plant Physiology and Plant Molecular Biology 50: 695 – 718. Pierret A, Moran CJ, Pankhurst CE. 1999. Differentiation of soil properties related to the spatial association of wheat roots and soil macropores. Plant and Soil 211: 51 – 58. Read DB, Bengough AG, Gregory PJ, Crawford JW, Robinson D, Scringeour CM, Young IM, Zhang K, Zhang X. 2003. Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil. New Phytologist 157: 315 – 326. Sperry JS, Adler FR, Campbell GS, Comstock JP. 1998. Limitation of plant water use by rhizosphere and xylem conductance: results from a model. Plant, Cell & Environment 21: 347 – 359. Stirzaker RJ, Passioura JB. 1996. The water relations of the root–soil interface. Plant, Cell & Environment 19: 201 – 208. Tilman RW, Scotter DR, Wallis MG, Clothier BE. 1989. Water-repellency and its measurement by using intrinsic sorptivity. Australian Journal of Soil Research 27: 637 – 644. Traoré O, Groleau-Renaud V, Plantureux S, Tubeileh A, Boeuf-Tremblay V. 2000. Effect of root mucilage and modelled root exudates on soil structure. European Journal of Soil Science 51: 575 – 582. Watt M, McCully ME, Canny MJ. 1994. Formation and stabilization of rhizosheaths of Zea-mays 1 – effect of soil-water content. Plant Physiology 106: 179 – 186. Young IM. 1995. Variation in moisture contents between bulk soil and the rhizosheath of wheat (Triticum-aestivum 1 cv. Wembley). New Phytologist 130: 135 – 139. Youngs EG. 1995. Developments in the physics of infiltration. Soil Science Society of America Journal 59: 307 – 313. Citing Literature Volume157, Issue3March 2003Pages 597-603 This article also appears in:Soil microbes and plant production FiguresReferencesRelatedInformation

Summary Understanding of the detailed mechanisms of how roots anchor in and reinforce soil is complicated by the variability and complexity of both materials. This study controlled material stiffness and architecture of root analogues, by using rubber and wood, and also employed real willow root segments, to investigate the effect on pullout resistance in wet and air‐dry sand. The architecture of model roots included either no laterals (tap‐root) or a single pair at two different locations (herringbone and dichotomous). During pullout tests, data on load and displacement were recorded. These studies were combined with Particle Image Velocimetry (PIV) image analysis of the model root‐soil system at a transparent interface during pullout to increase understanding of mechanical interactions along the root. Model rubber roots with small stiffness had increasing pullout resistance as the branching and the depth of the lateral roots increased. Similarly, with the stiff wooden root models, the models with lateral roots embedded deeper showed greatest resistance. PIV showed that rubber model roots mobilized their interface shear strength progressively whilst rigid roots mobilized it equally and more rapidly over the whole root length. Soil water suction increased the pullout resistance of the roots by increasing the effective stress and soil strength. Separate pullout tests conducted on willow root samples embedded in sand showed similar behaviour to the rigid model roots. These tests also demonstrated the effect of the root curvature and rough interface on the maximum pullout resistance.

This paper presents a lattice Boltzmann model (LBM) for 2-D advection and anisotropic dispersion equation (AADE) based on the Bhatnagar, Gross and Krook (BGK) model. In the proposed model, the particle speed space is discretized using a rectangular lattice that has four speeds in nine directions, and the single relaxation time is assumed to be directionally dependent. To ensure that the collision is mass-invariant when the relaxation time is directionally dependent, the concentration is calculated from a weighted summation of the particle distribution functions. The proposed model was verified against benchmark problems and the finite difference solution of solute transport with spatially variable dispersion coefficients and non-uniform velocity field. The significant results are that it conserves mass perfectly and offers accurate and efficient solutions for both dispersion-dominated and advection-dominated problems.

Abstract. Hard‐setting soils are widespread in dry regions. Their properties are described and a physical explanation for hard‐setting behaviour is given. The limitations on soil management and physical fertility caused by hard‐setting depend on timing of rainfall or irrigation with respect to cultivations and crop development, and much research is needed to quantify reductions in crop yield imposed by these limitations.

Lack of water is a major limitation to crop production, particularly where roots of cereal crops are not able to access water stored in the subsoil. One way that roots penetrate the subsoil to access water is by following natural biopores – paths created by roots from previous crops, or as burrows from soil fauna. Burying a mesh layer horizontally in the soil can prevent root penetration to the subsoil. We used this technique with the novel modification that the mesh was punctured to create a defined number of holes per unit area; controlling access to the subsoil and to the water therein. The holes were of similar size to biopores. Five barley genotypes were late sown and grown during a dry summer. Monitoring of crop performance included plant height, leaf area and Normalized Difference Vegetation Index (NDVI). Crops grown with unrestricted access to the subsoil outperformed crops with limited or no access to the subsoil. Crops grown with controlled, limited access to the subsoil performed better than those with no access and the performance was generally related to amount of access. Changes in soil water content were in line with the amount of root access to the subsoil; confirming the association between subsoil water and crop growth and development in drought conditions. While there were no significant interactions between the genotypes and treatments used here, the method offers promise for studying some aspects of cereal ecophysiology and could be used to identify promising germplasm that may be of interest in plant breeding. Further testing is required to adapt the method for a wider range of crop types and soil conditions and testing for crops grown to maturity.

Understanding root-reinforcement of vegetated slopes is hindered by the cost and practicality of full scale tests to explore global behaviour at the slope scale, and the idealised nature of smaller-scale testing to date that has relied on model root analogues. In this study we investigated the potential to use living plant roots in small scale experiments of slope failure that would use a geotechnical centrifuge to achieve soil stress states comparable to those in the field at homologous points. Three species (Willow, Gorse and Festulolium grass), corresponding to distinct plant groups with different root architecture and ‘woodiness’ were selected and cultivated for short periods (2 months for Willow and Festulolium grass, 3 months for Gorse). The morphologies, tensile strength and Young’s modulus of these juvenile root samples and their effects on increasing soil shear strength were then measured (via tensile tests and direct shear tests) and compared with published results of more mature field grown specimens. Our test results show that when all juvenile root samples of the three species are considered, the commonly used negative power law does not fit the data for the relationship between root tensile strength and root diameter well, resulting in very low R2 values (R2 < 0.14). No significant differences in tensile strength were observed between roots with different diameter for Willow and Gorse, and the average root tensile strength for all juvenile root samples was 8.70 ± 0.60 MPa (Mean ± SE), 9.50 ± 0.40 MPa, 21.67 ± 1.29 MPa for Willow, Festulolium grass and Gorse, respectively. However, a strong linear relationship was observed between tensile strength and Young’s modulus of the roots of the juvenile plants (R2 = 0.55, 0.69, 0.50 for Willow, Festulolium grass and Gorse, respectively). From a centrifuge modelling perspective, it was shown that using juvenile plants could potentially produce prototype root systems that are highly representative of corresponding mature root systems both in terms of root mechanical properties and root morphology when a suitable growing time (2 months) and scaling factor (N = 15) are selected. However, it remains a challenge to simultaneously simulate the distribution of root biomass with depth of the corresponding mature plant. Therefore, a compromise has to made to resolve the conflicts between the scaling of rooting depth and root reinforcement, and it is suggested that 1:15 scale would represent a suitable compromise for studying slope failure in a geotechnical centrifuge.

Electrical capacitance, measured between an electrode inserted at the base of a plant and an electrode in the rooting substrate, is often linearly correlated with root mass. Electrical capacitance has often been used as an assay for root mass, and is conventionally interpreted using an electrical model in which roots behave as cylindrical capacitors wired in parallel. Recent experiments in hydroponics show that this interpretation is incorrect and a new model has been proposed. Here, the new model is tested in solid substrates.The capacitances of compost and soil were determined as a function of water content, and the capacitances of cereal plants growing in sand or potting compost in the glasshouse, or in the field, were measured under contrasting irrigation regimes.Capacitances of compost and soil increased with increasing water content. At water contents approaching field capacity, compost and soil had capacitances at least an order of magnitude greater than those of plant tissues. For plants growing in solid substrates, wetting the substrate locally around the stem base was both necessary and sufficient to record maximum capacitance, which was correlated with stem cross-sectional area: capacitance of excised stem tissue equalled that of the plant in wet soil. Capacitance measured between two electrodes could be modelled as an electrical circuit in which component capacitors (plant tissue or rooting substrate) are wired in series.The results were consistent with the new physical interpretation of plant capacitance. Substrate capacitance and plant capacitance combine according to standard physical laws. For plants growing in wet substrate, the capacitance measured is largely determined by the tissue between the surface of the substrate and the electrode attached to the plant. Whilst the measured capacitance can, in some circumstances, be correlated with root mass, it is not a direct assay of root mass.

Soil bio-engineering using vegetation is an environmentally friendly solution to stabilise soil slopes. This study investigates tensile strength, Young's modulus, and root diameter relationships for establishing woody perennials. Specimens of ten woody European shrubs and small trees were transplanted into sandy loam soil to establish for six months. Root tensile strength and Young's modulus were measured as well as the root length-diameter distribution. The effect of root water status on root diameter was evaluated for Scotch Broom. More than half of the root length for all species was thinner than 0.5 mm diameter. Typical tensile strengths were <40 MPa, with Young's modulus <600 MPa. Negative power relationships between root strength and root diameter existed only for Gorse and Spindle, whilst Blackthorn, European Box and Holly showed slight increase in tensile strength with diameter. Hawthorn, Hazel and Privet showed rapid initial increase in strength with diameter followed by strength decrease with diameter, post-peak. Young's modulus was linearly related to tensile strength for all ten species (P < 0.001; R2 values 17%–64%). Root diameter, investigated for Scotch Broom, depended strongly on root water potential and root water content by mass. Root water content could influence considerably the calculations of tensile strength. Root tensile strength-diameter relationships often do not follow a negative power law, and depends strongly on taxa. Young's modulus was strongly related to tensile strength of roots for certain species. Water status of roots strongly influences root diameter and hence strength and Young's modulus properties, and must be controlled carefully in experiments.

Infiltration rate affects slope stability by determining the rate of water transport to potential failure planes.This note considers the influences of vegetation (grass and willow) establishment and root growth dynamics on infiltration rate, as related to establishing vegetation on bioengineered slopes.Soil columns of silty sand with and without vegetation were tested by constant-head infiltration tests at 2, 4, 6 and 8 weeks after planting.Infiltration rate increased linearly with plant age and below-ground traits including root biomass and root length density.Infiltration rate for willow-rooted soil was an order of magnitude higher than for fallow soil.The plant age effect was more prominent for willow, which grew faster and with thicker roots than the grass.Illustrative seepage analysis suggests that ignoring the plant age effects could underestimate wetting front advancement to greater depths during rainfall, and underestimate suction recovery at shallow depths during internal drainage.

Summary Soil adjacent to roots has distinct structural and physical properties from bulk soil, affecting water and solute acquisition by plants. Detailed knowledge on how root activity and traits such as root hairs affect the three‐dimensional pore structure at a fine scale is scarce and often contradictory. Roots of hairless barley ( Hordeum vulgare L. cv Optic) mutant ( NRH ) and its wildtype ( WT ) parent were grown in tubes of sieved (&lt;250 μm) sandy loam soil under two different water regimes. The tubes were scanned by synchrotron‐based X‐ray computed tomography to visualise pore structure at the soil–root interface. Pore volume fraction and pore size distribution were analysed vs distance within 1 mm of the root surface. Less dense packing of particles at the root surface was hypothesised to cause the observed increased pore volume fraction immediately next to the epidermis. The pore size distribution was narrower due to a decreased fraction of larger pores. There were no statistically significant differences in pore structure between genotypes or moisture conditions. A model is proposed that describes the variation in porosity near roots taking into account soil compaction and the surface effect at the root surface.

Rhizodeposits collected from hydroponic solutions with roots of maize and barley, and seed mucilage washed from chia, were added to soil to measure their impact on water retention and hysteresis in a sandy loam soil at a range of concentrations. We test the hypothesis that the effect of plant exudates and mucilages on hydraulic properties of soils depends on their physicochemical characteristics and origin.Surface tension and viscosity of the exudate solutions were measured using the Du Noüy ring method and a cone-plate rheometer, respectively. The contact angle of water on exudate treated soil was measured with the sessile drop method. Water retention and hysteresis were measured by equilibrating soil samples, treated with exudates and mucilages at 0.46 and 4.6 mg g-1 concentration, on dialysis tubing filled with polyethylene glycol (PEG) solution of known osmotic potential.Surface tension decreased and viscosity increased with increasing concentration of the exudates and mucilage in solutions. Change in surface tension and viscosity was greatest for chia seed exudate and least for barley root exudate. Contact angle increased with increasing maize root and chia seed exudate concentration in soil, but not barley root. Chia seed mucilage and maize root rhizodeposits enhanced soil water retention and increased hysteresis index, whereas barley root rhizodeposits decreased soil water retention and the hysteresis effect. The impact of exudates and mucilages on soil water retention almost ceased when approaching wilting point at -1500 kPa matric potential.Barley rhizodeposits behaved as surfactants, drying the rhizosphere at smaller suctions. Chia seed mucilage and maize root rhizodeposits behaved as hydrogels that hold more water in the rhizosphere, but with slower rewetting and greater hysteresis.

Vegetation has been previously proposed as a method for protecting artificial and natural slopes against shallow landslides (e.g. as may be triggered by an earthquake); however, previous research has concentrated on individual root soil interaction during shear deformation rather than the global slope behaviour due to the extreme expense and difficulty involved in conducting full-scale field tests. Geotechnical centrifuge modelling offers an opportunity to investigate in detail the engineering performance of vegetated slopes, but its application has been restricted due to the lack of availability of suitable root analogues that can repeatably replicate appropriate mechanical properties (stiffness and strength) and realistic 3D geometry. This study employed 3D printing to develop a representative and repeatable 1:10 scale model of a tree root cluster (representing roots up to 1.5 m deep at prototype scale) that can be used within a geotechnical centrifuge to investigate the response of a vegetated slope subject to earthquake ground motion. The printed acrylonitrile butadiene styrene (ABS) plastic root model was identified to be highly representative of the geometry and mechanical behaviour (stiffness and strength) of real woody root systems. A programme of large direct shear tests was also performed to evaluate the additional strength provided by the root analogues within soil that is slipping and investigate the influence of various characteristics (including root area ratio (RAR), soil confining effective stress and root morphology) on this reinforcing effect. Our results show that root reinforcement is not only a function of root mechanical properties but also depends on factors including surrounding effective confining stress (resulting in depth dependency even for the same RAR), depth of the slip plane and root morphology. When subject to shear loading in soil, the tap root appeared to structurally transfer load within the root system, including to smaller and deeper roots which subsequently broke or were pulled out. Finally, the root analogues were added to model slopes subjected to earthquake ground motion in the centrifuge, where it was revealed that vegetation can substantially reduce earthquake-induced slope deformation in the soil conditions tested (76% reduction on crest permanent settlement during slippage). Both the realistic 3D geometry and highly simplified root morphologies, as characterised mechanically by the shear tests, were tested in the centrifuge which, despite exhibiting very different levels of additional strength in the shear tests, resulted in very similar responses of the slopes. This suggests that once a certain minimum level of reinforcement has been reached which will alter the deformation mechanism within the slope, further increases of root contribution (e.g. due to differences in root morphology) do not have a large further effect on improving slope stability.

Plant roots growing through soil typically encounter considerable structural heterogeneity, and local variations in soil dry bulk density. The way the in situ architecture of root systems of different species respond to such heterogeneity is poorly understood due to challenges in visualising roots growing in soil. The objective of this study was to visualise and quantify the impact of abrupt changes in soil bulk density on the roots of three cover crop species with contrasting inherent root morphologies, viz. tillage radish (Raphanus sativus), vetch (Vicia sativa) and black oat (Avena strigosa). The species were grown in soil columns containing a two-layer compaction treatment featuring a 1.2 g cm-3 (uncompacted) zone overlaying a 1.4 g cm-3 (compacted) zone. Three-dimensional visualisations of the root architecture were generated via X-ray computed tomography, and an automated root-segmentation imaging algorithm. Three classes of behaviour were manifest as a result of roots encountering the compacted interface, directly related to the species. For radish, there was switch from a single tap-root to multiple perpendicular roots which penetrated the compacted zone, whilst for vetch primary roots were diverted more horizontally with limited lateral growth at less acute angles. Black oat roots penetrated the compacted zone with no apparent deviation. Smaller root volume, surface area and lateral growth were consistently observed in the compacted zone in comparison to the uncompacted zone across all species. The rapid transition in soil bulk density had a large effect on root morphology that differed greatly between species, with major implications for how these cover crops will modify and interact with soil structure.

Abstract Background and aims Root hairs play a significant role in phosphorus (P) extraction at the pore scale. However, their importance at the field scale remains poorly understood. Methods This study uses a continuum model to explore the impact of root hairs on the large-scale uptake of P, comparing root hair influence under different agricultural scenarios. High vs low and constant vs decaying P concentrations down the soil profile are considered, along with early vs late precipitation scenarios. Results Simulation results suggest root hairs accounted for 50% of total P uptake by plants. Furthermore, a delayed initiation time of precipitation potentially limits the P uptake rate by over 50% depending on the growth period. Despite the large differences in the uptake rate, changes in the soil P concentration in the domain due to root solute uptake remains marginal when considering a single growth season. However, over the duration of 6 years, simulation results showed that noticeable differences arise over time. Conclusion Root hairs are critical to P capture, with uptake efficiency potentially enhanced by coordinating irrigation with P application during earlier growth stages of crops.

Abstract To better understand the role of root anatomy in regulating plant adaptation to soil mechanical impedance, 12 maize lines were evaluated in two soils with and without compaction treatments under field conditions. Penetrometer resistance was 1–2 MPa greater in the surface 30 cm of the compacted plots at a water content of 17–20% (v/v). Root thickening in response to compaction varied among genotypes and was negatively associated with rooting depth at one field site under non-compacted plots. Thickening was not associated with rooting depth on compacted plots. Genotypic variation in root anatomy was related to rooting depth. Deeper-rooting plants were associated with reduced cortical cell file number in combination with greater mid cortical cell area for node 3 roots. For node 4, roots with increased aerenchyma were deeper roots. A greater influence of anatomy on rooting depth was observed for the thinner root classes. We found no evidence that root thickening is related to deeper rooting in compacted soil; however, anatomical traits are important, especially for thinner root classes.

Sloughing of root cap cells and exudation of mucilage plays an important role in the penetration of compacted soils by roots. For the first time we have quantified the rate of sloughing of root cap cells in an abrasive growth medium that was compacted to create mechanical impedance to root growth. The number of maize ( Zea mays ) root cap cells sloughed into sand increased as a result of compaction, from 1930 to 3220 d −1 per primary root. This represented a 12‐fold increase in the number of cells sloughed per mm root extension (from 60 to &gt;700). We estimated that the whole of the cap surface area was covered with detached cells in compacted sand, compared with c . 7% of the surface area in loose sand. This lubricating layer of sloughed cells and mucilage probably decreases frictional resistance to soil penetration. The total carbon deposited by the root was estimated at c . 110 μg g −1 sand d −1 . Sloughed cells accounted for &lt;10% of the total carbon, the vast majority of carbon being contained in mucilage exudates.

The inherent heterogeneity of many geophysical systems often gives rise to fast and slow pathways to water and chemical movement, and one approach to model solute transport through such media is the continuous time random walk (CTRW). One special asymptotic case of the CTRW is the fractional advection‐dispersion equation (FADE), which has proven to be a promising alternative to model anomalous dispersion and has been increasingly used in hydrology to model chemical transport in both surface and subsurface water. Most practical problems in hydrology have complicated initial and boundary conditions and need to be solved numerically, but the numerical solution of the FADE is not trivial. In this paper we present a finite volume approach to solve the FADE where the spatial derivative of the dispersion term is fractional. We also give methods to solve different boundary conditions often encountered in practical applications. The linear system resulting from the temporal‐spatial discretization is solved using a semi‐implicit scheme. The numerical method is derived on the basis of mass balance, and its accuracy is tested against analytical solutions. The method is then applied to simulate tracer movement in a stream and a near‐saturated hillslope in a naturally structured upland podzol field in northeast Scotland.

The lattice Boltzmann method has proven to be a promising method to simulate flow in porous media. Its practical application often relies on parallel computation because of the demand for a large domain and fine grid resolution to adequately resolve pore heterogeneity. The existing domain-decomposition methods for parallel computation usually decompose a domain into a number of subdomains first and then recover the interfaces and perform the load balance. Normally, the interface recovery and the load balance have to be performed iteratively until an acceptable load balance is achieved; this costs time. In this paper we propose a cell-based domain-decomposition method for parallel lattice Boltzmann simulation of flow in porous media. Unlike the existing methods, the cell-based method performs the load balance first to divide the total number of fluid cells into a number of groups (or subdomains), in which the difference of fluid cells in each group is either 0 or 1, depending on if the total number of fluid cells is a multiple of the processor numbers; the interfaces between the subdomains are recovered at last. The cell-based method is to recover the interfaces rather than the load balance; it does not need iteration and gives an exact load balance. The performance of the proposed method is analyzed and compared using different computer systems; the results indicate that it reaches the theoretical parallel efficiency. The method is then applied to simulate flow in a three-dimensional porous medium obtained with microfocus x-ray computed tomography to calculate the permeability, and the result shows good agreement with the experimental data.

Fungi are thought to affect soil slaking resistance and bypass flow by inducing low levels of water repellency on soil pore surfaces. This was investigated in a laboratory study on arable field soil by selectively inhibiting fungi and/or bacteria with biocides and adding ground barley shoot substrate (2 mg g− 1 soil) to half of the samples. The fungicide Captan and the bactericide Bronopol were used alone and in combination at a concentration of 1 mg g− 1, resulting in different levels of microbial respiration and active fungal biomass (ergosterol) after 8 days of incubation. Repellency was measured on the surface and on an internal fracture of the soil cores. Wet–dry cycles were applied to some samples to investigate the persistence of water repellency. Substrate addition caused the greatest increase to water repellency after incubation, suggesting that microbial activity affects repellency. The drying surface of the soil had greater repellency than the internal fracture surface. No clear trends could be found between the level of fungal biomass and repellency; this was most evident in soils treated with bactericide and substrate. Despite having the highest fungal biomass (P < 0.01), more than 300% greater than the no-biocide control, these soils did not develop greater water repellency (P > 0.05). Wetting and drying cycles caused significant changes to repellency and structure. Ethanol sorptivity indicates pore structure and its value changed least in the soil that had the greatest fungal biomass (i.e. bactericide treated with substrate), indicating that fungal populations protected soil structures from slaking stresses. Given the poor relationship found between fungal biomass and water repellency, these results suggest that other microbial or physico-chemical processes were involved. Our findings do not support previous research that reported a close link between water repellency and fungal biomass. However, given the strong evidence of fungal-produced hydrophobins and the close links between soil structural stability, repellency and fungal biomass found in field studies, the effectiveness of the biocides used in this study or the influence of different fungal species could have influenced the results and thus requires greater investigation.

Adequate contact with the soil is essential for water and nutrient adsorption by plant roots, but the determination of root–soil contact is a challenging task because it is difficult to visualize roots in situ and quantify their interactions with the soil at the scale of micrometres. A method to determine root–soil contact using X‐ray microtomography was developed. Contact areas were determined from 3D volumetric images using segmentation and iso‐surface determination tools. The accuracy of the method was tested with physical model systems of contact between two objects (phantoms). Volumes, surface areas and contact areas calculated from the measured phantoms were compared with those estimated from image analysis. The volume was accurate to within 0.3%, the surface area to within 2–4%, and the contact area to within 2.5%. Maize and lupin roots were grown in soil (&lt;2 mm) and vermiculite at matric potentials of −0.03 and −1.6 MPa and in aggregate fractions of 4–2, 2–1, 1–0.5 and &lt; 0.5 mm at a matric potential of −0.03 MPa. The contact of the roots with their growth medium was determined from 3D volumetric images. Macroporosity (&gt;70 µm) of the soil sieved to different aggregate fractions was calculated from binarized data. Root‐soil contact was greater in soil than in vermiculite and increased with decreasing aggregate or particle size. The differences in root–soil contact could not be explained solely by the decrease in porosity with decreasing aggregate size but may also result from changes in particle and aggregate packing around the root.

The assessment of areas at risk from various soil threats is a key task within the proposed EU Soil Framework Directive. Such assessment is, however, hampered by the complex nature of the soil threats, which result from the sometimes poorly understood interaction of various soil physical properties, climatic factors and land management practices. Methodologies for risk assessment of soil threats are needed to protect the soil quality for future generations and to target resources to the areas at greatest risk. We present here a generic risk framework for the development of Bayesian Belief Networks (BBNs) to estimate the risk from soil threats. The generic BBN structure follows a standard risk assessment approach, where the risk is quantified by combining assessments of vulnerability and exposure. The soil's vulnerability to a given threat is determined from inherent soil and site characteristics as well as from climatic factors influencing soil characteristics, while the exposure estimate is based on an evaluation of the stresses inflicted by land management and climate. The generic framework is demonstrated by taking soil compaction as an example. Soil compaction is a major threat to soil function particularly in highly managed agricultural systems and is known to have many adverse effects on farming systems including decreased crop yield and soil productivity, increased management costs, increased emissions of greenhouse gases, and decreased water infiltration into the soil leading to accelerated run-off and risk of soil erosion. Existing modelling approaches to predict soil compaction risk either require data on soil mechanical behaviour that are difficult and expensive to collect, or are expert-based systems that are highly subjective and sometimes cannot accommodate the myriad of processes underlying compaction risk. Using the generic framework, a detailed BBN for assessing the risk of soil compaction is developed. The BBN allows for combining available data from standard soil surveys and land use databases with qualitative expert knowledge and explicitly accounts for uncertainties in the assessment of the risk. The BBN is applied to identify the distribution of the compaction risk across Scotland using data from the National Soils Inventory of Scotland.

Capacitance has been used as a non-destructive measure of root system size for 30 years. The equipment required is cheap and simple to apply in both field and laboratory. Good linear correlations have been reported between capacitance and root mass. A model by F. N. Dalton, predicting a linear relationship between these two variables, has become accepted widely. This model was tested for barley (Hordeum vulgare) grown hydroponically using treatments that included: raising roots out of solution, cutting roots at positions below the solution surface, and varying the distance between plant electrode and the solution surface. Although good linear correlations were found between capacitance and mass for whole root systems, when roots were raised out of solution capacitances were not linearly related to submerged root mass. Excision of roots in the solution had negligible effect on the measured capacitance. These latter observations conflict with Dalton's model. Capacitance correlated linearly with the sum of root cross-sectional areas at the solution surface and inversely with distance between plant electrode and solution surface. A new model for capacitance is proposed that is consistent with these observations.

Core Ideas We hypothesized that plant exudates gel soil particles and on drying enhance water repellency. This has been carried out using rhizosphere‐scale mechanical and hydraulic measurements. Plant exudates enhanced soil hardness and modulus of elasticity as chia seed &gt; maize root &gt; barley root. Plant exudates caused measureable decreases in soil wetting rates through water repellency. Using rhizosphere‐scale physical measurements, we tested the hypothesis that plant exudates gel together soil particles and, on drying, enhance soil water repellency. Barley ( Hordeum vulgare L. cv. Optic) and maize ( Zea mays L. cv. Freya) root exudates were compared with chia ( Salvia hispanica L.) seed exudate, a commonly used root exudate analog. Sandy loam and clay loam soils were treated with root exudates at 0.46 and 4.6 mg exudate g −1 dry soil and chia seed exudate at 0.046, 0.46, 0.92, 2.3 and 4.6 mg exudate g −1 dry soil. Soil hardness and modulus of elasticity were measured at −10 kPa matric potential using a 3‐mm‐diameter spherical indenter. The water sorptivity and repellency index of air‐dry soil were measured using a miniaturized infiltrometer device with a 1‐mm tip radius. Soil hardness increased by 28% for barley root exudate, 62% for maize root exudate, and 86% for chia seed exudate at 4.6 mg g −1 concentration in the sandy loam soil. For the clay loam soil, root exudates did not affect soil hardness, whereas chia seed exudate increased soil hardness by 48% at 4.6 mg g −1 concentration. Soil water repellency increased by 48% for chia seed exudate and 23% for maize root exudate but not for barley root exudate at 4.6 mg g −1 concentration in the sandy loam soil. For the clay loam soil, chia seed exudate increased water repellency by 45%, whereas root exudates did not affect water repellency at 4.6 mg g −1 concentration. Water sorptivity and repellency were both correlated with hardness, presumably due to the combined influence of exudates on the hydrological and mechanical properties of the soils.

Root elongation is generally limited by a combination of mechanical impedance and water stress in most arable soils. However, dynamic changes of soil penetration resistance with soil water content are rarely included in models for predicting root growth. Better modelling frameworks are needed to understand root growth interactions between plant genotype, soil management, and climate. Aim of paper is to describe a new model of root elongation in relation to soil physical characteristics like penetration resistance, matric potential, and hypoxia. A new diagrammatic framework is proposed to illustrate the interaction between root elongation, soil management, and climatic conditions. The new model was written in Matlab®, using the root architecture model RootBox and a model that solves the 1D Richards equations for water flux in soil. Inputs: root architectural parameters for Soybean; soil hydraulic properties; root water uptake function in relation to matric flux potential; root elongation rate as a function of soil physical characteristics. Simulation scenarios: (a) compact soil layer at 16 to 20 cm; (b) test against a field experiment in Brazil during contrasting drought and normal rainfall seasons. (a) Soil compaction substantially slowed root growth into and below the compact layer. (b) Simulated root length density was very similar to field measurements, which was influenced greatly by drought. The main factor slowing root elongation in the simulations was evaluated using a stress reduction function. The proposed framework offers a way to explore the interaction between soil physical properties, weather and root growth. It may be applied to most root elongation models, and offers the potential to evaluate likely factors limiting root growth in different soils and tillage regimes.

Effects of root water status on root tensile strength and Young’s modulus were studied in relation to root reinforcement of slopes. Biomechanical properties of woody roots, Ulex europaeus, were tested during progressive dehydration and after thirty-day moisture equilibration in soil with contrasting water contents. Root diameter, water content and water loss were recorded and root water potential versus water content relation was investigated. Tensile stresses induced by root contraction upon dehydration were measured. Root tensile strength and Young’s modulus increased abruptly when root water content dropped below 0.5 g g−1. The strength increase was due to root radial and axial contraction induced by root water potential drop. Diameter decrease and strength gain were the largest for thin roots because of the relatively larger evaporative surface per volume of thin roots. Largely negative water potentials in dry soil induced root drying, affecting root biomechanical properties. Root water status is a factor that can cause (inappropriately) high strength values and the large variability reported in literature for thin roots. Therefore, all root diameter classes should have consistent moisture for fair comparison. Testing fully hydrated roots should be the routine protocol, given that slope instability occurs after heavy rainfall.

Summary Roots grow thicker in compacted soil, even though it requires greater force for a large object to penetrate soil than it does for a small one. We examined the advantage of thickening in terms of the stresses around a root penetrating with constant shape, rather than the stresses around an expanding cylinder or sphere, as has been studied previously. We combined experiments and simulations of the stresses around roots growing in compacted soils. We measured the diameter of pea roots growing in sandy loam and clay loam at four different densities, and the critical‐state properties of the soils. At a penetration resistance of about 1 MPa the diameter of the roots in the sandy loam was about 40% greater than that at 0.7 MPa, and at 2 MPa it was about 60% greater. In the clay loam, there was less thickening – about 10% greater at 1 MPa and about 20% greater at 1.5 MPa. The maximum axial stresses were predicted using a critical‐state finite‐element model to be at the very tip of the root cap. When there was friction between the root and the soil, shear stresses were predicted with smaller values at the tip than just behind the tip. When the interface between the soil and the root was assumed to be frictionless, there were by definition no shear stresses. In the frictionless case the advantage of root thickening on relieving peak stress at the root tip was diminished. The axial and shear stresses were predicted to be smaller in the clay loam than in the sandy loam and may explain why the roots did not thicken in this soil although its resistance to penetration was similar. Our results suggest that the local values of axial and shear stresses experienced by the root near its tip may be as important in constraining root growth as the total penetration resistance.

SUMMARY A dry soil is generally a hard soil. Thus, the effects of water stress and mechanical impedance on plant growth are difficult to separate. To achieve this we have developed a growth cell that allows manipulation of the strength of growth media (i.e. mechanical impedance) without altering the availability of water or nutrients. We monitored leaf elongation rates of barley and wheat seedlings before and after the mechanical impedance to root growth was increased. Results show that a large and rapid reduction (within 10 min) of leaf elongation rates occurred after impedance to the roots was increased. The average reductions for barley and wheat, with associated standard errors, were 22.6 % (4.84) and 36.2% (5.48), respectively. The data are consistent with the hypothesis that mechanical impedance of roots might have a direct negative effect on leaf growth even where nutrients and water are in plentiful supply to the plant. The implications of the rate of the response are examined with respect to the underlying mechanisms controlling root‐shoot signalling.

A pore-scale modelling of soil hydraulic conductivity using the lattice Boltzmann equation method and thin-section technique is presented in this paper. Two-dimensional thin sections taken from soil samples were used to measure soil pore geometry, and three-dimensional soil structures were then reconstructed based on the thin-section measurements and an assumption that the soil structure can be fully characterised by its porosity and variogram. The hydraulic conductivity of the reconstructed three-dimensional soil structures was calculated from a lattice Boltzmann simulation of water flow in the structures, in which the water-solid interface was treated as a non-slip boundary (zero-velocity boundary) and solved by the bounce-back method. To improve the accuracy of the bounce-back method, the particle distribution functions were located at the centre of the cube that was fully occupied either by water or by solid. The simulated hydraulic conductivity was compared with measured hydraulic conductivity and the results showed good agreement. We also analysed the probability distribution of the simulated water velocity and the results indicated that the transverse velocity components had a non-Gaussian symmetric distribution, while the longitudinal velocity component had a skewed-forward distribution. Both distributions had a marked peak for velocity close to zero, indicating a significant portion of stagnant water.

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm-3; penetration resistance 0.06 MPa) and compact soil (1.4 g cm-3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.

Soil surrounding a growing root must be displaced to accommodate the increased root volume. To ease soil penetration, root caps produce border cells and mucilage that lubricate the root surface, decreasing friction at the root‐soil interface. Rhizosphere deformations caused by roots with or without a functional root cap were compared to determine the effects of the root cap on sand displacement and penetration. Intact (KYS wild type) and decapped ( agt1 dec mutant) primary maize roots were grown in observation chambers filled with sand. Non‐destructive time‐lapse micro‐imaging combined with particle image velocimetry was used to visualize and quantify sand displacements as small as 0.5 µm caused by growing roots. Decapped ( agt1 dec ) roots displayed typical responses of mechanically impeded roots at sand densities that did not affect intact KYS roots. Sand displacement decreased exponentially with distance from the root and extended four to eight root radii into the sand. The calculated mean sand density increase and the compressed sand area were doubled by decapping. Maximum density often occurred in front of the apex of decapped roots whereas it occurred along the sides of intact roots. Periodic variation in sand deformation was observed, probably associated with root circumnutation, which may also facilitate soil penetration. Sand particles moved alongside KYS roots more easily than they did alongside agt1 dec roots. A functional exuding cap was therefore essential for efficient rhizosphere deformation and penetration by roots. Manipulating root tip, and specifically root cap, properties is a possible target for improving root penetration in hard soil.

This paper reports a series of geotechnical centrifuge model tests conducted to investigate the mechanical reinforcement of slopes by vegetation. Some of the model slopes contained young willow trees, which were grown in controlled conditions to provide different root distributions and mechanical properties. Slopes were brought to failure in the centrifuge by increasing water pressures. The failure mechanisms were investigated photographically and using post-test excavation. By measuring the soil properties and pore pressures in each test when failure occurred, slope stability calculations could be performed for each slope failure. These back-calculations of stability suggest that only a small amount of reinforcement was provided by the root system even when it was grown for 290 days before testing. In contrast, the use of the measured root properties and a commonly used root reinforcement model suggests that significant reinforcement should have been provided by the roots. This disparity is probably due to either inappropriate assumptions made in the root reinforcement model or soil alteration produced by root growth. Such disparities may exist in the application of root reinforcement models to full-scale slopes and therefore require additional study. The modelling technique outlined in this paper is suitable for further investigation of root mechanical interactions with slopes.

Biological and physical interactions in unsaturated soil, the vadose zone, have received a surge of research interest over the past several years. This article reviews recent research, focusing on the limitations imposed by the complexity of soil, the use of model systems to understand processes, new technologies, and the understanding of how biology changes soil structure. Research using model systems to mimic natural structure, such as rough planar surfaces or packed columns, has made it possible to demonstrate and quantify microbial interactions at very small spatial scales, including the coexistence of competing microbes and the invasion of soil pores by organisms that should be too large to fit. It is now possible to see inside soil at micrometer resolution in three dimensions, either by the use of noninvasive imaging techniques on intact soils or a model transparent soil with the same refractive index as water. Soil biology also changes soil structure. Techniques from engineering such as fracture mechanics and rheology have measured enhanced particle bonding, dispersion, and aggregation caused by root and microbial derived exudates. Models of soil structure dynamics are beginning to use these data. Concurrent research on naturally structured soil is essential, but using model systems that allow for the application of material science approaches or the detection and modeling of specific processes will enable the building of complexity by piecing together simpler systems. A major challenge for future research is gaining a quantitative understanding of how soil biology changes structure and incorporating this knowledge with studies of soil biodiversity, microbial functions, and root–soil interactions. Upscaling from microbial processes at micrometer resolution to the whole plant, field or catchment presents an even greater challenge.

A controlled environment study investigated the interactions between soil compaction and N availability on the growth and root tissue composition of young barley plants. Plants were grown for 14 days in a mixture of sand and calcined clay (fired clay granules) at two levels of compaction (low and high; dry bulk densities of 0.94 and 1.08 g cm−3 respectively) and two levels of N supply (high, resulting in N sufficient plants and low giving plants deficient in N). High compaction reduced total root length by 23%, leaf area by 21% and altered biomass partitioning (reduced leaf area ratio and increased root weight ratio), but had no effect on total biomass production over the time-course of the experiment. By contrast low N supply, reduced root biomass by 42% and shoot biomass by 47%, but had less effect on shoot morphology than compaction. There was no significant interaction between compaction and N supply on growth and biomass partitioning, although towards the end of the experiment, the rate of N uptake per unit root dry weight was reduced by about 50% by high compaction when N supplies were low, but not when they were high. Compaction altered the concentration of some root tissue components independently of N supply. For example, high compaction reduced the concentration of cellulose plus hemi-cellulose by 30% and increased the mineral content by 38%, whilst N supply had no effect. The concentration of several other components was altered by compaction and N supply in the same direction. Both high compaction and low N supply increased the lignin concentration whilst reducing the concentration of organic N compounds and nitrate, thereby increasing the C:N and lignin:N ratios. Compaction and low N supply increased C:N by a factor of 1.3 and 1.8 respectively, whilst the lignin:N ratio was increased by 1.7 and 2.1 respectively. Thus, both compaction and low N availability altered root tissue composition in a way that might reduce the rate of root degradation by soil microbes. The implications of these findings for modelling nutrient cycling are briefly discussed.

Abstract How rainfall infiltration rate and soil hydrological characteristics develop over time under forests of different ages in temperate regions is poorly understood. In this study, infiltration rate and soil hydrological characteristics were investigated under forests of different ages and under grassland. Soil hydraulic characteristics were measured at different scales under a 250‐year‐old grazed grassland (GL), 6‐year‐old (6yr) and 48‐year‐old (48yr) Scots pine ( Pinus sylvestris ) plantations, remnant 300‐year‐old individual Scots pine (OT) and a 4000‐year‐old Caledonian Forest (AF). In situ field‐saturated hydraulic conductivity ( K fs ) was measured, and visible root:soil area was estimated from soil pits. Macroporosity, pore structure and macropore connectivity were estimated from X‐ray tomography of soil cores, and from water‐release characteristics. At all scales, the median values for K fs , root fraction, macroporosity and connectivity values tended to AF &gt; OT &gt; 48yr &gt; GL &gt; 6yr, indicating that infiltration rates and water storage increased with forest age. The remnant Caledonian Forest had a huge range of K fs (12 to &gt;4922 mm h −1 ), with maximum K fs values 7 to 15 times larger than those of 48‐year‐old Scots pine plantation, suggesting that undisturbed old forests, with high rainfall and minimal evapotranspiration in winter, may act as important areas for water storage and sinks for storm rainfall to infiltrate and transport to deeper soil layers via preferential flow. The importance of the development of soil hydrological characteristics under different aged forests is discussed. Copyright 2015 British Geological Survey. Ecohydrology © 2015 John Wiley &amp; Sons, Ltd.

A series of centrifuge model tests were conducted to investigate the contribution of root reinforcement to slope stability. A compacted sandy clay slope, inclined at 45°, was reinforced with model roots. The model roots were varied in material, architecture, and numbers. They had stiffness values corresponding to upper and lower values found for plant roots. The architecture included taproots and branched roots. Slope collapse was triggered by raising the water table while soil displacements, pore-water pressures, and root strains were measured. The mode of failure was changed by the presence of roots from a progressive block failure to translational failure. The tests revealed how axial strains and bending strains were mobilized in the roots and how the roots influenced the slope failure mechanism. Different limit equilibrium slope stability calculations were performed at slope failure conditions to quantify the amount of reinforcement provided by different root types. These measured root reinforcement contributions were compared with those predicted according to common root reinforcement models. A reinforcement calculation method allowing for root pull-out was found to give the best agreement.

The biomechanics of root systems influence plant lodging resistance and soil structural stabilisation. Tissue age has the potential to influence root biomechanical properties through changes in cell wall chemistry, root anatomy and morphology. Within a root system the internal structures of roots are known to vary markedly within different root types. Nodal, seminal and lateral roots of Barley (Hordeum vulgare) have differing biomechanical behaviour in tension. This study examines the effects of root age on biomechanical properties of barley root types (Hordeum vulgare) under abiotic stress. Root age was determined as a function of the distance from root tip with abiotic stresses consisting of waterlogging and restriction to root elongation rate through increased soil bulk density. Linear regression analyses were performed on log-transformed tensile strength and Young’s modulus data with best fits determined for single and multiple parameter models to root morphological properties. Regression co-efficients and Akaike values showed that distance from root tip (taken as a proxy of root age) was the best single variable for prediction of both root tensile strength and Young’s modulus. Incorporation of both distance from root tip and root diameter and root type increased the reliability of predictions for root biomechanical properties from 47 to 57 % for tensile strength and 35 to 62 % for Young’s modulus. The age effect may partly explain some scatter in both Young’s modulus and tensile strength to diameter relationship, commonly cited in the literature.

The use of standard dynamic root architecture models to simulate root growth in soil containing macropores failed to reproduce experimentally observed root growth patterns. We thus developed a new, more mechanistic model approach for the simulation of root growth in structured soil. In our alternative modelling approach, we distinguish between, firstly, the driving force for root growth, which is determined by the orientation of the previous root segment and the influence of gravitropism and, secondly, soil mechanical resistance to root growth. The latter is expressed by its inverse, soil mechanical conductance, and treated similarly to hydraulic conductivity in Darcy’s law. At the presence of macropores, soil mechanical conductance is anisotropic, which leads to a difference between the direction of the driving force and the direction of the root tip movement. The model was tested using data from the literature, at pot scale, at macropore scale, and in a series of simulations where sensitivity to gravity and macropore orientation was evaluated. Qualitative and quantitative comparisons between simulated and experimentally observed root systems showed good agreement, suggesting that the drawn analogy between soil water flow and root growth is a useful one.

Mechanical root reinforcement is an important parameter to evaluate for stability analysis of rooted slopes. The contribution of roots is however difficult to quantify in situ without time-consuming methods or heavy equipment. Here we report field testing using the newly developed “corkscrew” method at two different sites with plantings of conifers and blackcurrant. In both sites we found positive correlations between root quantity and root reinforcement in surface layers where many roots were found. Below 125 mm depth, no correlations could be found, probably due to variability in soil stress and gravel content. Roots were shown not only to increase the soil peak strength, but also to add ductility to the soil, i.e., adding strength over much larger displacement ranges. Measured reinforcement, although similar to other experimental studies, was smaller than predicted using existing models. This may be attributed to the distinct difference in shear displacement required to mobilize the strength of rooted soil as compared with fallow soil. At displacements sufficient to mobilize root strength, the soil strength component has reduced from peak to a much smaller residual strength. The corkscrew method proved a promising tool to quantify root reinforcement in field conditions due to its ease of use and short test duration.

Soil bioengineering using vegetation is an environmentally friendly technique for slope stabilisation. Plants stabilise slopes by way of mechanical reinforcement (through root anchorage) and hydrological reinforcement (through transpiration-induced matric suction). However, little is known about the effects of a plant's functional group on slope hydrology and stabilisation. This makes it difficult for engineers to select appropriate species for soil bioengineering. In this study, full-scale field monitoring of a 20 m long vegetated embankment was conducted, with the aim to quantify and compare the hydro-mechanical reinforcement provided by three contrasting woody species (deciduous Corylus avellana, evergreen Ilex aquifolium and evergreen Ulex europaeus) native to a European temperate climate. The rainfall interception, matric suction and in situ soil strength were measured over two growing seasons. Evergreen species differed greatly in their water uptake, and sometimes exceeded the deciduous species. The evergreen U. europaeus induced the greatest suction (&gt; 70 kPa) and soil shear strength (e.g. up to 136 kPa) because of its better developed above-ground shoot architecture for rainfall interception (up to 50% of incoming rainfall) and greater root length density for water uptake. This suggests that careful choice of species could greatly enhance slope stabilisation by increasing the soil shear strength.

We investigated the influence of root border cells on the colonisation of seedling Zea mays roots by Pseudomonas fluorescens SBW25 in sandy loam soil packed at two dry bulk densities. Numbers of colony forming units (CFU) were counted on sequential sections of root for intact and decapped inoculated roots grown in loose (1.0 mg m(-3)) and compacted (1.3 mg m(-3)) soil. After two days of root growth, the numbers of P. fluorescens (CFU cm(-1)) were highest on the section of root just below the seed with progressively fewer bacteria near the tip, irrespective of density. The decapped roots had significantly more colonies of P. fluorescens at the tip compared with the intact roots: approximately 100-fold more in the loose and 30-fold more in the compact soil. In addition, confocal images of the root tips grown in agar showed that P. fluorescens could only be detected on the tips of the decapped roots. These results indicated that border cells, and their associated mucilage, prevented complete colonization of the root tip by the biocontrol agent P. fluorescens, possibly by acting as a disposable surface or sheath around the cap.

Plant root systems frequently permeate both natural and engineered soil slopes, influencing slope stability via mechanical reinforcement and soil drying. These root systems are often loaded by external forces during slope movements and when plant stems are subject to animal foraging or wind gusts. A series of physical model tests were conducted to examine how root geometries, root properties, and soil effective stress states affect the pullout capacity of simple unbranched model roots. Lengths of wood, rubber, and real roots were pulled from dry and partially saturated sand. The tests revealed the importance of the root to soil stiffness ratio during progressive failure, the mechanical properties of soil (and interfaces) at low effective stresses, the root diameter, and the tortuosity of the root material. Scaling issues due to shear banding are more important, and effective stresses under wet conditions are smaller than in conventional geotechnical practice because roots have a relatively small diameter (typically 10 −4 –10 −1 m) and are at shallow depth (typically 0–2 m). After careful consideration of these effects, predictions were made for the pullout capacity of small root sections based on material data for willow roots (Salix viminalis) in sand.

To penetrate soil, a root requires pressure to expand the cavity it is to occupy and to overcome root‐soil friction. We quantified these two pressures and showed that the root‐soil friction can be a substantial component of root penetration resistance. This provides evidence suggesting that modifying root‐tip traits has potential to improve plant performance.

Cereal farming is moving rapidly towards reduced tillage, with over 100 million ha of land currently tilled using minimum tillage implements. This study reports five years of data from a field experiment investigating the response of different barley cultivars and mixtures to soil tillage practice and nitrogen fertiliser levels. Five tillage treatments were established that imposed different amounts of soil disturbance: (T1) zero tillage, (T2) minimum tillage to 7 cm depth and ploughed treatments followed by power harrowing consisting of (T3) conventional plough to 20 cm depth, (T4) plough to 20 cm followed by compaction by wheeling the entire plot with a tractor fitted with 8.8 Mg total load and (T5) deep plough to 40 cm depth, all on the same site each year. Four winter barley cultivars (Sumo, Fanfare, Pastoral and Pipkin) were selected based on contrasting rooting characteristics, disease resistance and yield sensitivity. They were planted in plots as monocultures and as all 2-, 3- and 4-component mixtures thereof. Significant differences in soil physical properties and carbon content were observed over the five years of the study. Grain yield varied by 13% between tillage treatments, with conventional and deep plough conditions generally the highest yielding and zero tillage the lowest. Sumo gave the highest yield overall under deep plough conditions, whereas Pipkin was the best cultivar under conventional and zero tillage conditions. Rhynchosporium was the most common disease and the mixture gave decreased infection in all years and tillage conditions. Complex mixtures gave around 32% less disease than the simple mixtures. There was an overall differential cultivar response to soil tillage conditions that was buffered by cultivar mixtures. Mixtures offered benefits in both yield response and disease control under all soil tillage conditions.

Major research efforts are targeting the improved performance of root systems for more efficient use of water and nutrients by crops. However, characterizing root system architecture (RSA) is challenging, because roots are difficult objects to observe and analyse. A model-based analysis of RSA traits from phenotyping image data is presented. The model can successfully back-calculate growth parameters without the need to measure individual roots. The mathematical model uses partial differential equations to describe root system development. Methods based on kernel estimators were used to quantify root density distributions from experimental image data, and different optimization approaches to parameterize the model were tested. The model was tested on root images of a set of 89 Brassica rapa L. individuals of the same genotype grown for 14 d after sowing on blue filter paper. Optimized root growth parameters enabled the final (modelled) length of the main root axes to be matched within 1% of their mean values observed in experiments. Parameterized values for elongation rates were within ±4% of the values measured directly on images. Future work should investigate the time dependency of growth parameters using time-lapse image data. The approach is a potentially powerful quantitative technique for identifying crop genotypes with more efficient root systems, using (even incomplete) data from high-throughput phenotyping systems.

Strong regions and physical barriers in soils may slow root elongation, leading to reduced water and nutrient uptake and decreased yield. In this study, the biomechanical responses of roots to axial mechanical forces were assessed by combining 3D live imaging, kinematics and a novel mechanical sensor. This system quantified Young's elastic modulus of intact poplar roots (32MPa), a rapid <0.2 mN touch-elongation sensitivity, and the critical elongation force applied by growing roots that resulted in bending. Kinematic analysis revealed a multiphase bio-mechanical response of elongation rate and curvature in 3D. Measured critical elongation force was accurately predicted from an Euler buckling model, indicating that no biologically mediated accommodation to mechanical forces influenced bending during this short period of time. Force applied by growing roots increased more than 15-fold when buckling was prevented by lateral bracing of the root. The junction between the growing and the mature zones was identified as a zone of mechanical weakness that seemed critical to the bending process. This work identified key limiting factors for root growth and buckling under mechanical constraints. The findings are relevant to crop and soil sciences, and advance our understanding of root growth in heterogeneous structured soils.

Mechanical root reinforcement is one of the mechanisms by which vegetation enhances slope stability. Common approaches to quantify this effect include either in situ shear box testing or destructive root sampling combined with a theoretical model to estimate reinforcement parameters. Both approaches, however, are time consuming. Here four new in situ techniques are evaluated to quantify mechanical root reinforcement and then compared under laboratory conditions. All four methods yield distinct results in soils reinforced with woody root analogues (acrylonitrile butadiene styrene rods), fine root analogues (polypropylene fibres) or stones. Two methods (adaptations of penetrometer testing, dubbed ‘blade penetrometer’ and ‘pull-up’) are suitable for spatially locating rooted zones and individual roots, while the other two (‘pin vane’ and ‘corkscrew’ extraction) demonstrate potential for directly quantifying the rooted soil stress–strain behaviour. These simple methods are suitable for use on difficult-to-access terrain where many measurements are needed to quantify spatial and temporal variability of root-zone properties for geotechnical calculations. The techniques are quicker to use than conventional methods and so should improve the reliability of slope stability predictions.

This paper presents a comparative study of three different classes of model for estimating the reinforcing effect of plant roots in soil, namely (i) fibre pull-out model, (ii) fibre break models (including Wu and Waldron’s Model (WWM) and the Fibre Bundle Model (FBM)) and (iii) beam bending or p-y models (specifically Beam on a Non-linear Winkler-Foundation (BNWF) models). Firstly, the prediction model of root reinforcement based on pull-out being the dominant mechanism for different potential slip plane depths was proposed. The resulting root reinforcement calculated were then compared with those derived from the other two types of models. The estimated rooted soil strength distributions were then incorporated within a fully dynamic, plane-strain continuum finite element model to assess the consequences of the selection of rooted soil strength model on the global seismic stability of a vegetated slope (assessed via accumulated slip during earthquake shaking). For the particular case considered in this paper (no roots were observed to have broken after shearing), root cohesion predicted by the pull-out model is much closer to that the BNWF model, but is largely over-predicted by the family of fibre break models. In terms of the effects on the stability of vegetated slopes, there exists a threshold value beyond which the position of the critical slip plane would bypass the rooted zones, rather than passing through them. Further increase of root cohesion beyond this value has minimal effect on the global slope behaviour. This implies that significantly over-predicted root cohesion from fibre break models when used to model roots with non-negligible bending stiffness may still provide a reasonable prediction of overall behaviour, so long as the critical failure mechanism is already bypassing the root-reinforced zones.

The aim of the present work was to determine the factors limiting growth in mechanically impeded roots. Pea roots were grown in compressed and uncompressed sand cores, and then removed and transferred to hydroponics. Root elongation was slowed in impeded sand cores and did not recover to the unimpeded rates until 60 h after transfer to the hydroponics system. Root diameter was greater in impeded roots, and only after 36 h in hydroponics was new root tissue produced of the same diameter as the unimpeded controls. The turgor pressure of the growing cells was measured with a turgor probe and was the same in both treatments. The slower elongation rate of the previously impeded roots was, therefore, the result of axial tightening of the cell walls. Cell length profiles suggested that axial cell wall tightening persisted in the unrestricted hydroponics system. Production of new cells in unrestricted conditions was required before root elongation returned to the unimpeded state. Osmotic potential was decreased by approximately 0.2 MPa in previously impeded roots compared with the unimpeded ones. This corresponds to a decrease in water potential of 0.2 MPa. These data are discussed in relation to regulation of cell extension, solute unloading and the penetration of compacted soils by roots.

Summary • To assess the influence of mechanical impedance on cell fluxes in the root cap, maize (Zea mays) seedlings were grown in either loose or compacted sand with penetration resistances of 0.2 MPa and 3.8 MPa, respectively. Numbers of cap cells were estimated using image analysis, and cell doubling times using the colchicine technique. • There were 5930 cells in the caps in the compact and 6900 cells in the loose control after 24 h growth in sand. Cell production rates were 2010 cells d−1 in compact and 1570 cells d−1 in loose sand. • These numbers represent accumulations of 4960 and 3540 detached cells d−1 around the cap periphery following the two types of treatment. The total number of detached cells was estimated as sufficient to completely cover the whole root cap in the compact sand, but only 11% of the root cap in the loose sand. • In conclusion, mechanical impedance slightly enhanced meristematic activities in the lateral region of the root cap. The release of extra border cells would aid root penetration into the compact sand.

Root systems show considerable plasticity in their morphology and physiology in response to variability within their environment. Root elongation below a water-table was expected to slow due to hypoxia, whilst roots above the waterlogged zone were expected to compensate by increasing elongation rates. Tomato plants (Solanum lycopersicum L.) were grown in peat in root chambers (300 × 215 × 6 mm) with a transparent front. Root chambers were maintained in flatbed scanners tilted at 30° to vertical and scanned every 3 h before, during and after waterlogging the lower layer for 24 h or 5 days. Root elongation rates were calculated from the displacement of randomly selected root tips between successive scans. Oxygen content was determined in the waterlogged layer and plant and root parameters were determined at cessation of the experiment. Root elongation rates decreased rapidly when waterlogged. Growth rates of the waterlogged roots decreased, while growth rates of roots above the waterlogged zone increased. In 24 h waterlogged roots new lateral root growth occurred in the lower layer of the root chamber when water was drained while after 5 day waterlogging new root growth had to be initiated from roots above the waterlogged zone. Plants increased growth rates in roots above the waterlogged zone probably as compensation for the suboptimal conditions in the waterlogged zone which eventually led to roots dying.

Vegetation can improve slope stability by transpiration-induced suction (hydrologic reinforcement). However, hydrologic reinforcement varies with seasons, especially under temperate climates. This study aims to quantify and compare the hydrologic reinforcement provided by contrasting species during winter and summer. One deciduous (Corylus avellana) and two evergreens (Ilex aquifolium and Ulex europaeus) were planted in 1-m soil columns. Soil columns were irrigated, left for evapotranspiration and then subjected to extreme wetting events during both summer and winter. Soil water content, matric suction and strength were measured down the soil profile. Plant water status and growth (above- and below-ground) were also recorded. The tested species showed differing abilities to remove water, induce suction and hence influence soil strength. During summer, only Ulex europaeus provided a soil strength gain (up to six-fold the value at saturation) along the entire depth-profile inducing high suction (e.g. 70 kPa), largely maintained after wetting events in deeper soil (0.7 m). During winter, the evergreen species could remove water but at slower rates compared to summer. Evergreens could slowly induce suction and hence potentially stabilise slopes during winter. However, there were large differences between the two evergreens because of different growth rate and resource use.

A combined root growth and water extraction model is described that simulates the affects of mechanical impedance on root elongation in soil. The model simulates the vertical redistribution of water in the soil profile, water uptake by plant roots, and the effects of decreasing water content on increasing soil strength and decreasing the root elongation rate. The modelling approach is quite general and can be applied to any soil for which a relation can be defined between root elongation and penetrometer resistance. By definition this excludes soils that contain a large proportion of continuous channels through which roots can grow unimpeded. Root elongation rate is calculated as a function of the penetrometer resistance which is determined by the soil water content. Use of the model is illustrated using input data for a sandy loam soil. The results confirm reports in the literature that the depth of water extraction can exceed the rooting depth. The increase in mechanical impedance to root growth due to this water extraction restricted the maximum rooting depth attained, and this limited the depth of soil from which a crop could extract water and nutrients. This study highlighted the lack of published data sets for single crop/soil combinations containing both the strength/root growth information and the hydraulic conductivity characteristics necessary for this type of model.

A new method was developed to measure simultaneously, continuously, and non-destructively the elongation rate and the force exerted by the roots of seedlings grown in moist air. A pea (Pisum sativum L. cv. Helka) seedling was suspended inside a modified sample tube on one side of a pulley, with the tip of the radicle pushing on to a force transducer through a hole in the tube. The force on the root tip was monitored by the force transducer and could be adjusted by adding or removing mass from the counterweight on the other side of the pulley. As the root grew, the sample tube was raised and the elongation of the root was monitored using a linear variable differential transformer (LVDT) attached to the thread connecting the sample tube and counterweight. The changes in elongation rate were recorded which occurred in response to increases and decreases in the applied force. Forces of up to 125 mN were exerted on the root, corresponding to forces per unit final cross-sectional area (i.e. root growth pressures) of up to 0.1 MPa. As soon as the force on the root was changed there was a rapid reversible compression or extension of the root. Superimposed on this elastic/viscoelastic deformation, the root elongation rate slowed by more than 50% within 30 min of increasing the force applied to the root by 100 mN. A similarly fast but smaller increase in growth rate occurred when the force was removed. Both of these ‘fast’ responses were followed by a longer period of more gradual change in the root elongation rate over a period of 20 h or longer. Both ‘fast’ and ‘slow’ responses may be explained in terms of a modified Lockhart model of growth. The initial ‘fast’ response of the root is probably due to the immediate change in the effective pressure (i.e. the turgor pressure minus the yield stress and external resisting pressure) available to drive cell elongation. The reason for the second slower adjustment of the elongation rate is not known, but is probably due to some combination of a decrease in the rate of cell production and/or a stiffening of the cell walls in the longitudinal direction with increasing mechanical resistance. The increase in root diameter in response to mechanical impedance decreased the root growth pressure that the root exerted, but was associated with a slower root elongation rate.

Soil compaction is a serious issue that affects the growth of crop roots. Penetrometer resistance is often used as an indicator of the resistance of soil to root elongation. We developed a simple function to estimate penetrometer resistance from the soil compression characteristic. Five soils with contrasting textures and organic matter contents were used in this work. Air-dry soil samples were saturated on a tension table and then drained to different matric potentials not less than −30 kPa. The equilibrated soils were compacted in a uniaxial compression device to give precompression stresses in the range of 30 to 1000 kPa. The penetrometer resistance of the soils was then measured with a 2-mm-diameter 60° cone penetrometer. Soil compression characteristics varied with soil texture, organic matter content, and initial soil water content. Penetrometer resistance values increased with decreasing void ratio and could be explained by the precompression stress and the slope of the compression characteristic, using a simple equation with parameters independent of soil type. Our equation explained 84% of the variance in penetrometer resistance.

This paper investigates the seismic performance of rooted granular slopes using dynamic finite-element analysis, validated against recently published centrifuge test data. The importance of selecting suitable strength parameters to represent soil response within a strain-hardening constitutive model was demonstrated and the simulations suggested that any boundary effects introduced through the use of the equivalent shear beam container in the centrifuge are negligible and can be represented by a semi-infinite lateral boundary condition. Using the validated model, a parametric study investigated the effects of different rooted soil properties on the performance of slopes of different heights. Vegetation was effective in reducing deformations at the crest of slopes of modest height, although the benefit reduced as slope height or soil apparent cohesion increased. The effectiveness was significantly affected by the extent of the root system, but only moderately sensitive to root cohesion, and insensitive to stiffness or damping of the rooted soil. Plant species possessing deep and extensive root systems are therefore recommended for seismic stabilisation rather than those with the strongest roots. For modelling purposes, it is sufficient to be able to quantify only the strength of the rooted soil and its area of influence. The magnitude of improvement from vegetation in terms of decreasing seismic permanent slip was also found to be insensitive to the construction method used (i.e. compacted/uncompacted embankment or cutting) for drained granular slopes.

We studied the effect of mechanical impedance on cell flux and meristematic activity in pea roots. Pea seedlings ( Pisum sativum L. cv. Helka) were grown in cores of sand packed to dry bulk densities of either; 1.4 Mg m −3 with an additional 2.4 kg uniaxial load applied to the surface to increase the mechanical resistance to growth (penetration resistance of 1.5 MPa); or 1.0 Mg m −3 (penetration resistance of 0.05 MPa). A water content of 0.06 g g −1 was chosen for optimum root growth. After 3 days, the seedlings were transferred to hydroponics, colchicine was added and the rate of cell doubling, mitotic index and length of the cell cycle was assessed. Cell flux in the third cortical layer was calculated for roots immediately removed from sand.Mechanical impedance slowed root extension to about 20% of the unimpeded rate, and final cell length was reduced to 50% of the unimpeded length. The rate of cell doubling was 3.4 times slower for roots recovering from mechanical impedance mostly as a result of a longer period spent in interphase. Cell flux in impeded roots was approximately half that of unimpeded roots (5 cells h −1 ), and contributed to a shorter cell file and elongation zone, and a slower rate of root elongation.

Roots of plants growing in dry soil often experience large mechanical impedance because the decreased soil water content is associated with increases in soil strength. The combined effect of mechanical impedance and water stress hinders the establishment of seedlings in many soils, but little is known about the interaction between these two stresses. A method has been designed that, for the first time, measured the maximum axial force exerted by a root growing under controlled water stress. Using this technique the axial force exerted by a pea radicle was measured using a shear beam, while the seedling was suspended in an aerated solution of polyethylene glycol 20 000 at osmotic potentials between 0 and −0.45 MPa. The maximum growth force was then divided by the crosssectional area of the root to give the maximum axial growth pressure. The value of maximum axial growth pressure decreased linearly from 0.66 to 0.35 MPa as the osmotic potentials of the solution of PEG decreased from 0 to −0.45 MPa. In dry soil, therefore, the maximum strength of soil that a root can penetrate is decreased because of the decrease in maximum growth pressure. The elongation rates of unimpeded roots were similar whether the roots were subject to either a matric potential in soil or to an osmotic potential in a solution of PEG.

This paper presents an experimental methodology using a geotechnical centrifuge and an environmental chamber to explore long-term embankment performance in light of the more severe conditions expected due to global climate change. The environmental chamber applies water and air input to control inundation and evaporation conditions at the soil surface. Example results show that a well-compacted intermediate-plasticity model embankment performs well when subjected to 19 years of alternating wet and dry periods. Finally, the paper reports an experimental methodology that will allow further model tests to be conducted to examine how different climate scenarios, soil types and compaction levels may affect long-term embankment response.

The parameters in Richards' equation are usually calculated from experimentally measured values of the soil–water characteristic curve and saturated hydraulic conductivity. The complex pore structures that often occur in porous media complicate such parametrization due to hysteresis between wetting and drying and the effects of tortuosity. Rather than estimate the parameters in Richards' equation from these indirect measurements, image-based modelling is used to investigate the relationship between the pore structure and the parameters. A three-dimensional, X-ray computed tomography image stack of a soil sample with voxel resolution of 6 μm has been used to create a computational mesh. The Cahn–Hilliard–Stokes equations for two-fluid flow, in this case water and air, were applied to this mesh and solved using the finite-element method in COMSOL Multiphysics. The upscaled parameters in Richards' equation are then obtained via homogenization. The effect on the soil–water retention curve due to three different contact angles, 0°, 20° and 60°, was also investigated. The results show that the pore structure affects the properties of the flow on the large scale, and different contact angles can change the parameters for Richards' equation.

Soil compaction represents a major impediment to plant growth, yet wild plants are often observed thriving in soil of high bulk density in non-agricultural settings. We analysed the root growth of three non-cultivated species often found growing in compacted soils in the natural environment. Plants of ribwort plantain (Plantago lanceolata), dandelion (Taraxacum officinale), and spear thistle (Cirsium vulgare) were grown for 28 d in a sandy loam soil compacted to 1.8 g cm-3 with a penetration resistance of 1.55 MPa. X-Ray computed tomography was used to observe root architecture in situ and to visualise changes in rhizosphere porosity (at a resolution of 35 μm) at 14 d and 28 d after sowing. Porosity of the soil was analysed within four incremental zones up to 420 μm from the root surface. In all species, the porosity of the rhizosphere was greatest closest to the root and decreased with distance from the root surface. There were significant differences in rhizosphere porosity between the three species, with Cirsium plants exhibiting the greatest structural genesis across all rhizosphere zones. This creation of pore space indicates that plants can self-remediate compacted soil via localised structural reorganisation in the rhizosphere, which has potential functional implications for both plant and soil.

Abstract Two similar, sandy loam soils from the same geographical region but with distinct nematode communities were used to determine the extent to which water, soil and inoculum factors affected nematode community structure. Treatments were established in pots containing a middle layer of frozen defaunated soil, sandwiched between an inoculum that was either fresh soil from the same site ('self') or a mixture of soils to give a more diverse inoculum ('mixed'). During year 2, half the pots were watered at regular intervals while the other half received only rainfall. For individual nematode taxa, soil layer and watering regime were the main factors discriminating between treatments, while initial inoculum had a larger influence than soil type. Acrobeloides was most affected by the watering regime, being more abundant under variable water conditions, whereas Hoplolaimidae, Longidorus and Pratylenchus were more abundant in deeper soil layers in contrast to other taxa. For the community as a whole, when analysed by principal component analysis, soil factors clearly influenced composition and also indicated that the biological properties of the soils were important.

The main difficulty in the use of 3D root architecture models is correct parameterization. We evaluated distributions of the root traits inter-branch distance, branching angle and axial root trajectories from contrasting experimental systems to improve model parameterization. We analyzed 2D root images of different wheat varieties (Triticum aestivum) from three different sources using automatic root tracking. Model input parameters and common parameter patterns were identified from extracted root system coordinates. Simulation studies were used to (1) link observed axial root trajectories with model input parameters (2) evaluate errors due to the 2D (versus 3D) nature of image sources and (3) investigate the effect of model parameter distributions on root foraging performance. Distributions of inter-branch distances were approximated with lognormal functions. Branching angles showed mean values <90°. Gravitropism and tortuosity parameters were quantified in relation to downwards reorientation and segment angles of root axes. Root system projection in 2D increased the variance of branching angles. Root foraging performance was very sensitive to parameter distribution and variance. 2D image analysis can systematically and efficiently analyze root system architectures and parameterize 3D root architecture models. Effects of root system projection (2D from 3D) and deflection (at rhizotron face) on size and distribution of particular parameters are potentially significant.

Summary The introduction of impressive technologies in the search for life's diversity and activity in soil has led to remarkable new techniques and knowledge concerning the soil microbial community. These have led to finding some important links to function. However, we attest that the general lack of causality found between the many metrics of microbial diversity and populations of soil microbes and function is due, at least in part, to the lack of understanding of the links between microbial populations and dynamics to their physical habitat and attendant moisture conditions. In this opinion paper we explore the importance of this interplay between organism and habitat. Further, as an example of this interplay, we introduce the potential importance of nematode movement and gene transfer in bacterial populations. Highlights The importance of the physical habitat is highlighted in soil microbiology studies. The interplay between the soil–root–habitat is emphasized. Seeking a functional understanding of biodiversity rather than a ‘biology of numbers and differences’ approach is proposed. The movement of nematodes with respect to horizontal gene transfer is discussed.

Vegetation on railway or highway slopes can improve slope stability through the generation of soil pore water suctions by plant transpiration and mechanical soil reinforcement by the roots. To incorporate the enhanced shearing resistance and stiffness of root-reinforced soils in stability calculations, it is necessary to understand and quantify its effectiveness. This requires integrated and sophisticated experimental and multi-scale modelling approaches to develop an understanding of the processes at different length scales, from individual root–soil interaction through to full soil-profile or slope scale. One of the challenges with multi-scale models is ensuring that they sufficiently closely represent real behaviour. This requires calibration against detailed high-quality and data-rich experiments. This study presents a novel experimental methodology, which combines in situ direct shear loading of a willow root-reinforced soil with X-ray computed tomography to capture the three-dimensional chronology of soil and root deformation within the shear zone. Digital volume correlation (DVC) analysis was applied to the computed tomography dataset to obtain full-field three-dimensional displacement and strain information. This paper demonstrates the feasibility and discusses the challenges associated with DVC experiments on root-reinforced soils.

Abstract Four similar, agricultural soils with distinct nematode communities were used to determine the extent to which soil and inoculum factors affected nematode community structure. The soils all had a sandy loam texture from the same geographical area and had been in pasture or arable rotation for the last 10 years. Treatments were established in pots containing a middle layer of frozen defaunated soil, sandwiched between an inoculum that was either fresh soil from the same site ('self') or a mixture of soils to give a more diverse inoculum ('mixed'). Principal component analysis indicated that a single soil type given different inocula developed different community structures (i.e., the community under 'self' differed from that under 'mixed') suggesting an inoculum effect. It was also true that different soil types under a single inoculum soil also developed different community structures (i.e., community under 'mixed' differed with soil type), suggesting a soil effect. It is likely that the nematode community structure is influenced by a combination of antecedent land use, soil factors, species introductions and inter-species competition, which should be considered in any interpretation of nematode communities as a biotic indicator.