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There has been recent interest in the use of surface‐deployed geophysical methods to estimate soil moisture profiles. In this study, we applied multicoil, frequency domain, electromagnetic induction (EMI) geophysical surveys to determine electrical conductivity (σ) profiles of the root zone of four winter wheat (Triticum aestivumL.) genotypes grown in a randomized block experiment with four replicates. Field measurements of apparent electrical conductivity (σa) were obtained at sites with two different soil textures. We used the cumulative sensitivity model to predict EMI conductivity data from the conductivity profile measured with electrical resistivity tomography (ERT) on a subset of the plots we investigated. During the inversion of the EMI data, conductivities were adjusted on all plots so that they were consistent with the ERT data. Changes in electrical conductivity of field soil, with depth computed from inversion of the EMI data, during the growth period were compared with measured changes in soil water content. Laboratory measurements confirmed a positive correlation between electrical conductivity and soil water content. Between crop emergence and maturity, water extraction by the different wheat genotypes reduced the water content by up to 30% Comparing changes in electrical conductivity between reference profiles determined shortly after crop emergence and electrical conductivity profiles at later dates as the crop matured, we were able to use EMI to remotely monitor moisture extraction by the roots of different wheat genotypes with depth and time.

Biogeochemical reactions occurring in soil pore space underpin gaseous emissions measured at macroscopic scales but are difficult to quantify due to their complexity and heterogeneity. We develop a volumetric-average method to calculate aerobic respiration rates analytically from soil with microscopic soil structure represented explicitly. Soil water content in the model is the result of the volumetric-average of the microscopic processes, and it is nonlinearly coupled with temperature and other factors. Since many biogeochemical reactions are driven by oxygen (O2) which must overcome various resistances before reaching reactive microsites from the atmosphere, the volumetric-average results in negative feedback between temperature and soil respiration, with the magnitude of the feedback increasing with soil water content and substrate quality. Comparisons with various experiments show the model reproduces the variation of carbon dioxide emission from soils under different water content and temperature gradients, indicating that it captures the key microscopic processes underpinning soil respiration. We show that alongside thermal microbial adaptation, substrate heterogeneity and microbial turnover and carbon use efficiency, O2dissolution and diffusion in water associated with soil pore space is another key explanation for the attenuated temperature response of soil respiration and should be considered in developing soil organic carbon models.

Carbon sequestration, infrastructure rehabilitation, brownfields clean-up, hazardous waste disposal, water resources protection and global warming—these twenty-first century challenges can neither be solved by the high-energy consumptive practices that hallmark industry today, nor by minor tweaking or optimization of these processes. A more radical, holistic approach is required to develop the sustainable solutions society needs. Most of the above challenges occur within, are supported on, are enabled by or grown from soil. Soil, contrary to conventional civil engineering thought, is a living system host to multiple simultaneous processes. It is proposed herein that ‘soil engineering in vivo’, wherein the natural capacity of soil as a living ecosystem is used to provide multiple solutions simultaneously, may provide new, innovative, sustainable solutions to some of these great challenges of the twenty-first century. This requires a multi-disciplinary perspective that embraces the science of biology, chemistry and physics and applies this knowledge to provide multi-functional civil and environmental engineering designs for the soil environment. For example, can native soil bacterial species moderate the carbonate cycle in soils to simultaneously solidify liquefiable soil, immobilize reactive heavy metals and sequester carbon—effectively providing civil engineering functionality while clarifying the ground water and removing carbon from the atmosphere? Exploration of these ideas has begun in earnest in recent years. This paper explores the potential, challenges and opportunities of this new field, and highlights one biogeochemical function of soil that has shown promise and is developing rapidly as a new technology. The example is used to propose a generalized approach in which the potential of this new field can be fully realized.

The soil water release characteristic is controlled by the soil pores and so alteration of the pore system will have an effect. We sought to examine the effects of various pore deformations in a range of arable and grassland soils from the UK. Sieved topsoil materials were compressed to 50 or 200 kPa, or remolded to simulate shear deformation at the plastic limit. They were then subjected to matric potentials from 0 to ‐1500 kPa using conventional tension and pressure plate apparatus. Volume changes were monitored to assess shrinkage. Further samples in the compressed state were subjected to x‐ray computed tomography scanning to nondestructively characterize the soil pore system. Increasing the compression from 50 to 200 kPa mainly affected the >30‐μm pores, changing a dual porous system of inter‐ and intraaggregate pores to one mainly dominated by intraaggregate pores, as confirmed by gravimetric water release characteristic data and the scans. Shear‐deformed soils retained more water than the compressed soil and shrank more, such that they remained tension saturated at low (negative) matric potentials. We developed a function to predict the soil saturation state as a function of matric potential and porosity. This explained 28 to 63% of the variance, irrespective of the initial structural state, but up to 94% of the variation when one of the fitted parameters was allowed to vary for the different initial states. This was sufficiently systematic to suggest that a general model of the effects of soil deformation and shrinkage on the water release characteristic may be possible.

To understand the hydraulic properties of soils, accurate prediction of the unsaturated hydraulic conductivity is needed, which is commonly derived from the soil water release characteristic. In fine‐textured soils, the modified Mualem–van Genuchten (MMVG) function has been used extensively to predict the change in relative conductivity with changes in matric potential, but this assumes that the soil is rigid with a constant porosity (CP), which is unrealistic for shrinking soils. We used a triaxial cell apparatus to accurately monitor soil volume changes during water release in a clay soil. From this we derived relative hydraulic conductivity functions based on either MMVG (CP), or based on geometrically similar shrinkage (GSS) of all pore sizes by a constant factor as calculated from the measured shrinking porosity (SP). The MMVG (CP) function predicted a decline in relative conductivity of up to three orders of magnitude from 0 to −400 kPa matric potential. This was similar to published data on the response of conductivity in the same soil to consolidation of larger pores. Based on SP, however, the soil remained effectively tension saturated (>0.97) down to a matric potential of −85 kPa due to normal shrinkage. The corresponding GSS (SP) function predicted only a modest decrease in relative conductivity from 1 to 0.85 across this range. In this case the larger pores, although reduced in size, are assumed to be still water filled and conducting. The GSS function was probably the most realistic portrayal of hydraulic functioning in a tension‐saturated shrinking clay soil.

The aggregation of clays after the addition of organic materials is described. Clays were incubated with or without added organic matter in the form of grass, straw, or charcoal and needed to be dried to a water potential of −1.5 MPa or less to aggregate. It was the fine fraction of the clay (<0.5 μm) that aggregated after organic additions, and this was apparent even in clays or clay mixtures that had a broad particle size distribution. Grass was more effective than straw at aggregating the clay. When chloroform was added to the samples, there was little aggregation, suggesting that micro-aggregation after the addition of organic material to clay is mainly microbially mediated. The aggregation of clay in the presence of added substrate was temperature dependent, with an optimum between 20 and 30 °C, but in the absence of substrate aggregation increased with temperature without a maximum. This supports the hypothesis that biological mechanisms are playing an important role in aggregation.

Abstract As a soil dries, the earthworms in that soil dehydrate and become less active. Moisture stress may weaken an earthworm, lowering the radial pressure that the animal can produce. This possibility was investigated for the earthworm Aporrectodea caliginosa (Savigny). Pressures were compared for saturated earthworms (worms taken from saturated soil) and stressed earthworms (worms that had been partially dehydrated by leaving them in dry soil). A load cell was used to record the forces that earthworms produced as they moved through artificial burrows (holes that had been drilled through blocks of aluminium or Perspex). The radial pressure was calculated using the forces exerted and the dimensions of the artificial burrows. There was a negative correlation between burrow diameter and radial pressure, although radial pressure was independent of the length of the block through which the earthworms had burrowed. The highest radial pressures were produced by the anterior segments of the animal. Partial dehydration caused the earthworms to become quiescent, but did not decrease the radial pressure that the earthworms produced. It is suggested that coelomic fluid is retained in the anterior segments while the rest of the animal dehydrates. Dehydrated earthworms became lethargic, and we suggest that lethargy is due to the loss of coelomic fluid from the posterior segments. Coelomic fluid is known to be lost through dorsal pores. In burrowing species of earthworm such as Aporrectodea caliginosa, these pores are only present on the posterior segments.