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Highlights • Compression index, from drained triaxial compression, is independent of SOC. • Reported effects of SOC on compression may be due to soil hydraulic effect. • Void ratio at any effective stress is strongly correlated with SOC. • The plastic limit test is a useful test to compare of soil physical behaviour., We investigated the effect of soil organic carbon (SOC) on the consolidation behaviour of soil from two long term field experiments at Rothamsted; the Broadbalk Wheat Experiment and Hoosfield Spring Barley. These experiments are located on soil with similar particle size distributions, and include treatments with SOC contents ranging from approximately 1–3.5 g/100 g. Soils taken from plots with contrasting SOC contents were compressed and deformed in a triaxial cell and the normal consolidation and critical state lines were determined. We found that the compression index was independent of SOC, but the void ratio at any given effective stress was highly correlated with organic carbon content. By comparison with uniaxial compression data, the apparent influence of SOC on the compression index is likely to be due to its effect on soil hydraulic properties rather than any intrinsic effects of strength. The plastic limit test appears to be a useful and simple test to allow direct comparison of soil physical behaviour and expected soil density.

Efficient use of nitrogen (N) by plants and particularly crops, is of global importance. In agriculture, high crop yields and protein content depend upon extensive N-inputs, however fertilizer N is costly to the farmer, and overuse can be damaging to the environment. A critical component of optimised usage is efficient capture by crop root systems. This chapter focusses on principal mechanisms of uptake and factors influencing efficiency. Genetic variation in root architecture and in an array of transporters known to be involved in nitrogen capture is detailed. The impacts of abiotic stress factors such as soil structure are described. Finally prospects and opportunities for crop improvement are discussed.

Summary We explore the effect of microbial activity stimulated by root exudates on the penetrometer resistance of soil and its elastic modulus. This is important because it is a measure of the mechanical strength of soil and it correlates closely with the rate of elongation of roots. A sandy soil was incubated with a synthetic root exudate at different temperatures, for different lengths of time and with selective suppression of either fungi or bacteria. The shape of the temperature response of penetrometer resistance in soil incubated with synthetic exudate was typical of a poikilothermic temperature response. Both penetrometer resistance and small strain shear modulus had maximum values between 25 and 30°C. At temperatures of 20°C and less, there was little effect of incubation with synthetic root exudate on the small strain shear modulus, although penetrometer resistance did increase with temperature over this range (4–20°C). This suggests that in this temperature range the increase in penetrometer resistance was related to a greater resistance to plastic deformation. At higher temperatures (> 25°C) penetrometer resistance decreased. Analysis of the DNA sequence data showed that at 25°C the number of Streptomyces (Gram‐positive bacteria) increased, but selective suppression of either fungi or bacteria suggested that fungi have the greater role with respect to penetrometer resistance. Highlights Effect of microbial activity stimulated by synthetic root exudates on the mechanical properties.We compared penetrometer measurements and estimates of elastic modulus with microbial community.Penetrometer resistance of soil showed a poikilothermic temperature response.Penetrometer resistance might be affected more by fungi than bacteria.

In this study, we explored the effects of microbial activity on the evaporation of water from cores of a sandy soil under laboratory conditions. We applied treatments to stimulate microbial activity by adding different amounts of synthetic analogue root exudates. For comparison, we used soil samples without synthetic root exudates as control and samples treated with mercuric chloride to suppress microbial activity. Our results suggest that increasing microbial activity reduces the rate of evaporation from soil. Estimated diffusivities in soil with the largest amounts of added root exudates were one third of those estimated in samples where microbial activity was suppressed by adding mercuric chloride. We discuss the effect of our results with respect to water uptake by roots.

The velocities of shear waves Vs in two soils, a loamy sand and a sandy clay loam, were measured at various matric potentials and confining pressures. We used a combination of Haines apparatus, pressure plate apparatus and a Bishop and Wesley tri-axial cell to obtain a range of saturation and consolidation states. We proposed a single effective stress variable based on a modification to Bishop’s equation which could be used in a published empirical model (Santamarina et al., 2001) to relate shear wave velocity to soil physical conditions. Net stress required a nonlinear transformation. Matric potential was converted into suction stress with the function proposed by Khallili and Khabbaz (1998), thus requiring an estimate of the air entry potential. We found it was possible to fit Vs to void ratio, net stress and matric potential with a set of four parameters which were common to all soils at various states of saturation and consolidation. In addition to the data collected for this study we also used previously published data (Whalley et al., 2011). The utility of shear wave measurements to deduce soil physical properties is discussed.

RATIONALEThis study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously. METHODSSoil incubations were performed under a He/O-2 atmosphere and the denitrification product ratio [N2O/(N-2+N2O)] was determined by direct measurement of N-2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine O-18, N-15 and N-15 site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects () of both N2O production and reduction, taking into account emissions from two distinct soil pools. RESULTSA clear relationship was observed between N-15 and O-18 isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller. CONCLUSIONSThe fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the O-18/N-15 ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions. Copyright (c) 2014 John Wiley & Sons, Ltd.

In horticulture, the soil structure, moisture content and strength have profound effects on plant growth. When soils are regarded as porous media, sub-surface wave propagation can be indicative of the soil status. Such propagation can be initiated by sources of airborne sound through acoustic-to-seismic coupling. Measurements of the ratio of near-surface sound pressure to acoustically induced solid particle motion can be exploited to estimate the acoustic and elastic properties of soils. Traditional methods of monitoring seismic signals by use of buried geophones are invasive and may affect the soil samples of interest. This paper describes a non-invasive acoustic-seismic technique. A loudspeaker was used to generate airborne sound, and the reflected sound and the subsequent vibration of the soil surface were recorded by microphones and a laser Doppler vibrometer respectively. These data were used to estimate acoustic and elastic soil parameters through an optimisation process minimising the differences between the data and model predictions based on incorporation of a modified Biot theory in a Fast Field Program. Example results of laboratory and field measurements are reported.

X-ray sedimentation is potentially attractive for estimating the particle-size distribution of soil, but has been thought to overestimate clay fractions because of the iron they contain. We have examined X-ray sedimentation for estimating the particle-size distribution of fully dispersed and of aggregated clay. We obtained good agreement between the pipette and X-ray sedimentation methods using two different kaolinite samples with a small iron concentration. We also studied montmorillonite saturated with Li, Na, K, Mg or Ca to investigate possible effects of hindered settling. For the Ca montmorillonite, we obtained good agreement between X-ray sedimentation and the pipette method at concentrations up to 40 g l(-1). We also found that X-ray sedimentation could be used at concentrations less than those recommended by the manufacturer. Hindered settling was observed at concentrations > 20 g l(-1) for Li- and Na-saturated montmorillonite. Our data suggest that conducting experiments at different mass concentrations is a sensitive test to detect such problems. Finally we used a fractal model to investigate the possible errors that might result from the size dependence of aggregate density and show that the error caused by assuming a single value for particle density of solids is small.