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Studies across multiple soils find increasing pH decreases water repellency. In this study, water repellency and a range of other soil physical properties of bulk soils, aggregates and intact specimens were measured on a long-term pH field experiment on a single sandy loam soil under a ley-arable crop rotation, with soil pH adjustments occurring annually by adding FeSO4 or CaCO3, to lower or raise the pH, respectively. Crop impacts were investigated by comparing 3rd year grass-white clover to spring oats, at the beginning (May) and end (September) of the growing season to allow soil structure comparisons. As in previous research, increased CO2 microbial respiration (p<0.05) was found with increasing pH along the gradient, but in this study, we found only the aggregate and soil bulk density affected by soil pH. Soil-water contact angles differed between crops (p<0.05), as well as the repellency index of soil aggregates, however, there was no soil pH effect. Overall, differences in data were found to be a result of the various crops in the rotation rather than by soil pH, indicating only minor impacts on soil physical characteristics after > 55 years of chemical additions to amend soil pH. [ABSTRACT FROM AUTHOR]

Alternate wetting and drying (AWD) irrigation can save large amounts of water in rice cultivation. By repeatedly wetting and drying the soil under AWD, accentuated pore structure of the rhizosphere compared to flooded rice may occur. This could affect root growth and resource capture, but to date the physical structure and behaviour of the rhizosphere of rice under AWD has not been explored. In a controlled glasshouse experiment, two different textured soils were used in split rhizotrunks to separate a root‐zone from bulk soil using mesh. To mimic a paddy field, the top of the rhizotrunk was filled with puddled soil and below the puddled layer there was a sieved soil layer. Root‐zone physical properties were measured using a combination of high resolution X‐ray CT imaging (pore structure), a miniaturised infiltrometer (hydrological) and a small indenter (mechanical). Soil under AWD irrigation had 46% greater macroporosity and 20% more pore connectivity compared to continuous flooding (CF). Compared to the bulk soil, root‐zone soil under AWD or CF had greater macroporosity, water sorptivity and mechanical hardness. In the root‐zone, AWD compared to CF increased the rate of water absorption by around 36%, but did not affect mechanical hardness. Our results suggest AWD interacting with rice roots could promote more effective water transmission through a more stable, larger and better‐connected pore system. The results of this study also suggest that soil physical changes by AWD could improve the utilization of resources in a rice production system. [ABSTRACT FROM AUTHOR]

Great potential exists to harness plant traits at the root–soil interface, mainly rhizodeposition and root hairs, to 'build' soils with better structure that can trap more carbon and resources, resist climate stresses, and promote a healthy microbiome. These traits appear to have been preserved in modern crop varieties, but scope exists to improve them further because they vary considerably between genotypes and respond to environmental conditions. From emerging evidence, rhizodeposition can act as a disperser, aggregator, and/or hydrogel in soil, and root hairs expand rhizosheath size. Future research should explore impacts of selecting these traits on plants and soils concurrently, expanding from model plants to commercial genotypes, and observing whether impacts currently limited to glasshouse studies occur in the field. Root hairs and rhizodeposits are root traits that vary between plant species and crop genotypes and have a large impact on both plants and soils. Targeting these traits may benefit both plants and soil, improving food and environmental security at the same time. Soils may store more carbon (greenhouse gas mitigation), trap more water (drought tolerance) and nutrients, and resist erosion. From limited research, rhizosheath size has been maintained or improved in modern crop varieties, but potential exists to increase it further. Whether this will lead to improved yield or soil properties, however, requires greater field testing to verify. Laboratory and glasshouse research using root trait ideotypes has found marked impacts on soil biophysical properties. Rhizodeposits vary in behaviour between species from hydrogels to surfactants, and as soil dispersers (miners) or aggregators (builders). [ABSTRACT FROM AUTHOR]

Soil structure has a huge impact on plant root growth, but it is difficult to isolate from other soil properties in field studies, and generally overlooked in laboratory studies that use sieved and homogenised repacked soil. This study aimed to compare root and shoot growth under controlled soil conditions where only soil structure varied. Soil treatments used soil sieved to < 2 mm, packed in uniform layers to create a homogenous structure. A heterogeneous structure was packed from artificially formed aggregates created by breaking apart the homogeneous soil after intense compaction. Barley, peas and Arabidopsis, selected for contrasting root sizes, were grown under three levels of compaction (1.25 g cm−3, 1.40 g cm−3, 1.55 g cm−3) in both homogeneous and heterogeneous structured soils for 10 days. Penetration resistance increased from about 0.4 MPa at 1.25 g cm−3 to 1.3 MPa at 1.55 g cm−3 for either soil structure. Soil structure was quantified from water retention characteristics and X-ray Computed Tomography (CT) as complementary methods to assess the soil's pore size distribution and properties. Heterogenous soil had 50% more macropores at 1.55 g cm−3 when compared to homogenous soils. Pore structure complexity in the heterogeneous structure was found to be beneficial for root growth of peas and barley but not Arabidopsis. Shoot biomass of peas grown in heterogeneous soil at 1.55 g cm−3 increased by 65% when compared to homogenous soil, whereas barley and Arabidopsis shoot biomass did not differ significantly between any treatments. Chlorophyll, flavonoid, and nitrogen content could only be measured on barley or peas due to shoot size, but only minor differences were observed between soil structures. Soil structural heterogeneity influenced many root properties and above-ground biomass, with impacts found to be species-dependent and likely caused by the interaction between root size and preferential growth in macropores. [Display omitted] • Controlled approach to create soil structural heterogeneity. • The method uses artificial soil aggregates to create a complex pore structure. • At same bulk density soil structure influences root growth and above-ground biomass. • Impacts of soil structure heterogeneity found to be species-dependent. • Suitable to study the effects of soil physical properties on root growth. [ABSTRACT FROM AUTHOR]

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 (<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. [ABSTRACT FROM AUTHOR]

New techniques and technologies are unraveling processes that underpin biological and physical interactions in soils. From a review of recent literature, we argue for concurrent studies of model and natural soil systems, and show the benefits of taking approaches from other disciplines. 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. [ABSTRACT FROM AUTHOR]

A stable isotope ( 13 C-residue, 15 N-NO 3 – fertiliser) approach combined with measurements of soil pore space gas concentrations was used to investigate spatial and temporal mechanisms of residue carbon (C) affecting denitrification. Whilst relationships between residue addition and N 2 O fluxes have previously been well characterised, the influence of residues on production and reduction of N 2 O at depth is less well understood. Here we investigated the relationship between residue- 13 C addition (0, 1 and 2 mg C g −1 soil) and denitrification ( 15 N-N 2 O and 15 N-N 2 production) at 2, 5 and 8 cm soil depths and also fluxes from the soil surface. Hydrophobic probes that equilibrate with the soil gas phase were used to extract gases at soil depth, followed by analysis for 15 N-N 2 O, 15 N-N 2 , 13 C-CO 2 and O 2 concentrations. 15 N-N 2 O and CO 2 surface fluxes peaked one day after 14 NH 4 15 NO 3 application (1 mg N g −1 soil), with residue application resulting in a more than 20-fold greater 15 N-N 2 O emission rate compared to the non-amended control. Eight days after N application 15 N-N 2 O pore space concentrations had significantly increased 20-fold at 8 cm depth below the residue layer compared to no residue application. However, simultaneous increases in 15 N-N 2 surface fluxes and profile concentrations showed efficient reduction of the N 2 O at shallow depth (3–10 cm depth) resulting in surface emission of N 2 rather than N 2 O. Our results have implications for management to lower emissions as denitrifier activity at greater depth, and the greater reduction of N 2 O to N 2 , appeared to be indirectly driven by residue addition via the depletion of O 2 during aerobic heterotrophic respiration in the surface layer. In contrast, net surface fluxes of N 2 O were more directly related to the residue addition through substrate provision for denitrification. [ABSTRACT FROM AUTHOR]

The hillslopes of red soils (Ultisols) in southern China are intensively cultivated for cash crops and fruit trees. During the rainy monsoon, soil erosion is prevalent, whereas a summer/autumn dry season induces drought stress. Crops respond differently to these stresses, and have different effects on soil water regime. This study used a combination of field observation and HYDRUS-2D modeling to assess the soil water dynamics and plant available water for peanut ( Arachis hypogaea ) and citrus ( Citrus sinensis ) at Sunjia Red Soil Critical Zone Observatory (CZO). Between April 1, 2012 and March 31, 2014, surface runoff and moisture content at 5, 20, 40, and 80 cm depths in the soil under both land uses were monitored at up, middle and foot slope positions along a hillslope. Results indicate that the citrus plot had higher soil water content at 5 cm depth during the dry season, and lower at 20, 40 and 80 cm depths throughout the year than the peanut plot. As expected, the soil water content was higher at foot slope, compared to up slope, and in deeper soil than near surface. We observed limited soil water availability to peanut during mid-July to August, and to citrus from mid-July to mid-November. Compared to the peanut plot, the citrus plot generally showed 12–28% greater evapotranspiration, 3–4 times less runoff, and 2–57% greater deep drainage. These differences were greater at the up slope position. Our data and HYDRUS-2D simulation suggest that the deep-rooted citrus reduced runoff during the rainy season by increasing macropore flow and canopy interception, and minimized the soil water stress during the dry season by utilizing water from deeper soil. Thus, we recommend trench planting of citrus along with peanut intercropping on hilly red soils as sustainable agricultural practices. [ABSTRACT FROM AUTHOR]

The dynamics of soil structure is caused by biotic and abiotic processes, with the onset and magnitude of deformation controlled by soil rheological and mechanical properties. Quantification of such properties is challenging because soil behaviour changes with soil moisture, but common rheological tests are not applicable over all consistency ranges. Here, we combine oscillation shear rheometry with spherical indentation mechanical measurements of soil to obtain greater characterization over a broader range of water contents. The elastic modulus could be measured with either approach, with good agreement found for measured silt and clay textured remoulded agricultural soils. For shear rheometry, plastic viscosity, complex modulus and shear yield stress were also obtained. The spherical indentation provided measurements of hardness and yield stress. Although yield stress was correlated between approaches, the values were orders of magnitude greater for the indenter (0.54 ± 0.33 kPa vs. 34.4 ± 31.2 kPa), presumably because of different loading and failure conditions. At drier water contents, yield stress varied more between the two tests on the clay soil, which corresponded with brittle fracture creating artefacts in shear rheometry measurements. Spherical indentation has not been widely applied to the testing of soils, but the good agreement over a wide water content range between elastic modulus obtained from spherical indentation measurements (0.66 ± 0.27 MPa in wetter zone to 4.45 ± 2.53 MPa in drier zone) and shear rheometry (0.47 ± 0.11 MPa in wetter zone to 2.02 ± 0.98 MPa in drier zone) is promising. Moreover, spherical indentation can be applied to materials varying from brittle to viscous and allows testing on structurally intact soil aggregates. The geometry of a spherical indenter may more closely mimic contacting soil aggregates, so scope exists to extend the approach to explore the slumping of aggregated seedbeds produced by tillage. • Spherical indentation and shear rheometry tests on soil pastes were combined • Testing range extended to broader water contents, from viscous to brittle behaviour. • Elastic modulus measurements similar with both approaches. • Yield stress differed by orders of magnitude between tests. • Future potential to apply spherical indentation to structured soils. [ABSTRACT FROM AUTHOR]

Abstract: Soil wettability affects physical properties such as aggregate stability, infiltration rate, or erodibility. To describe the wetting properties of soil, the soil–air–water contact angle (CA) is often used. At present, a direct measurement of the CA determined on intact soil aggregates and a direct comparison with corresponding homogenized aggregates is still lacking, mainly because standard methods have not been defined to measure the wettability of soil aggregates. In this study, the Capillary Rise Method (CRM) was used to assess contact angles of single intact 1–2 mm soil aggregates, packings of intact aggregates and packings of crushed (homogenized) aggregates of 9 topsoils and 3 humus subsoils from 5 sites in Germany. In general, CAs of the homogenized aggregates were quite similar for all soils (65°±10°), while CAs for aggregate packings and single aggregates generally were larger for grassland and forest soils (60° to 80°) than for arable soils (0° to 20°). It was concluded that all soils contain potentially hydrophobic pore surfaces, but the effectiveness of these surfaces to create water repellency depends on the small-scale architecture of the pore space and the distribution/position of hydrophobic components inside the matrix. These findings suggest that it is misleading, particularly for agricultural soils, to quantify wetting properties by only analyzing homogenized soil. We propose that CRM-CA measurements should be extended to investigations of intact aggregates and the wetting coefficient k =cosθ should also be displayed to distinguish more clearly between completely wettable soils and soils with low subcritical repellency. Finally, it should be noted that the methods proposed are only applicable to non-hydrophobic soil exhibiting subcritical water repellency (CA >0°,…, CA ≤90°). [Copyright &y& Elsevier]