William Rickard

Bio: William Rickard is an academic researcher from University of Nottingham. The author has contributed to research in topics: Soil structure & Macropore. The author has an hindex of 1, co-authored 1 publications receiving 5 citations. Previous affiliations of William Rickard include Rothamsted Research.

Evolution of the transport properties of soil aggregates and their relationship with soil organic carbon following land use changes

Abstract: Aggregates are functional units to describe the impact of soil structural changes on physical and biogeochemical processes in soil. Both incubation and field experiments have shown that changing agricultural practices could reshape the intra-aggregate structure in a matter of days, but most such data were obtained from a single time-point and it is hence impossible to interpret that such a change was just a temporal transition or the new equilibria towards which the aggregates had evolved following the management changes. Understanding this is indispensable as intra-aggregate structure and its ability to transport substrates modulate all biogeochemical processes involved in soil carbon and nutrient cycle. This paper investigates this using soil samples archived from a reversion experiment initiated in 2008 at Rothamsted Research (UK), where parts of a plot that had been fallow since the 1950 s were converted to wheat or grass in 2008. We used X-ray Computed Tomography images, acquired at voxel size 1.5 µm, of aggregates in the archived soils to investigate the evolution of transport property of the aggregates over time, as well as its relationship with soil organic carbon (SOC). We also evaluated the development of pore connectedness following the conversion. The results show that the transport ability of the aggregates explains the SOC change much better than the porosity, and that noticeable changes in porosity of the connected pores and their ability to transport substrates did not emerge until the sixth year after the conversion. Ten years after the conversion, there was still no sign of the porosity of the connected pores and the bulk diffusion coefficient to plateau. In addition, we found the conversion to grass changed the intra-aggregate pore geometry significantly in that the bulk diffusion coefficients of their aggregates trends with their porosities in a way differing significantly from those for the bare fallow and arable treatments. All these suggest that the intra-aggregate reconfiguration following the conversion is a slow process, and that the ability of pore space to transport substrates is more important than the habitat they provide in SOC stabilisation.

Evolution of the transport properties of soil aggregates and their relationship with soil organic carbon following land use changes

Abstract: Aggregates are functional units to describe the impact of soil structural changes on physical and biogeochemical processes in soil. Both incubation and field experiments have shown that changing agricultural practices could reshape the intra-aggregate structure in a matter of days, but most such data were obtained from a single time-point and it is hence impossible to interpret that such a change was just a temporal transition or the new equilibria towards which the aggregates had evolved following the management changes. Understanding this is indispensable as intra-aggregate structure and its ability to transport substrates modulate all biogeochemical processes involved in soil carbon and nutrient cycle. This paper investigates this using soil samples archived from a reversion experiment initiated in 2008 at Rothamsted Research (UK), where parts of a plot that had been fallow since the 1950 s were converted to wheat or grass in 2008. We used X-ray Computed Tomography images, acquired at voxel size 1.5 µm, of aggregates in the archived soils to investigate the evolution of transport property of the aggregates over time, as well as its relationship with soil organic carbon (SOC). We also evaluated the development of pore connectedness following the conversion. The results show that the transport ability of the aggregates explains the SOC change much better than the porosity, and that noticeable changes in porosity of the connected pores and their ability to transport substrates did not emerge until the sixth year after the conversion. Ten years after the conversion, there was still no sign of the porosity of the connected pores and the bulk diffusion coefficient to plateau. In addition, we found the conversion to grass changed the intra-aggregate pore geometry significantly in that the bulk diffusion coefficients of their aggregates trends with their porosities in a way differing significantly from those for the bare fallow and arable treatments. All these suggest that the intra-aggregate reconfiguration following the conversion is a slow process, and that the ability of pore space to transport substrates is more important than the habitat they provide in SOC stabilisation.

An analysis of three XCT-based methods to determine the intrinsic permeability of soil aggregates

Abstract: Nowadays there are many papers dealing with the analysis of the soil porous system through X-ray computed tomography (XCT). However, a reduced number of studies focus on modeling the intrinsic permeability (k) of complex porous media, such as soil, based on three-dimensional (3D) high-resolution images. The determination of k is fundamental to understanding several processes taking place in soil such as water and air transmission. Thus, this paper compares three XCT-based methods for estimating the k of the pore system of soil aggregates: i) Finite volume-based simulation (APES - Absolute Permeability Experiment Simulation), ii) Image-based parametrization of the Kozeny-Carman (IBP-KC) equation, and iii) Pore network model (PNM). The 3D image considered in the comparison of methods was acquired at the X-ray microtomography beamline at the Brazilian Synchrotron Light Facility, with a voxel size of 1.64 μm. APES was considered as a reference method for being less dependent on the configuration of sensitivity parameters. APES and PNM demonstrated to be more time consuming than the IBP-KC, the first taking longer among all for the k computation. PNM presented the drawback of higher sensitivity to thin pore ramifications, resulting in unrealistic values of k and requiring disconnecting one thinner pore to yield permeabilities comparable to the ones obtained by APES. This study demonstrated that a fine alternative to compute k for large image datasets is to calibrate faster methods (e.g., IBP-KC and PNM) against a slower but more reliable reference method (e.g., APES), using at least one 3D image, before applying the faster methods on the remaining images of the dataset.

Response of subsoil organic matter contents and physical properties to long‐term, high‐rate farmyard manure application

Abstract: Application of farmyard manure (FYM) is common practice to improve physical and chemical properties of arable soil and crop yields. However, studies on effects of FYM application mainly focussed on topsoils, whereas subsoils have rarely been addressed so far. We, therefore, investigated the effects of 36‐year FYM application with different rates of annual organic carbon (OC) addition (0, 469, 938 and 1875 g C m−2 a−1) on OC contents of a Chernozem in 0–30 cm (topsoil) and 35–45 cm (subsoil) depth. We also investigated its effects on soil structure and hydraulic properties in subsoil. X‐ray computed tomography was used to analyse the response of the subsoil macropore system (≥19 μm) and the distribution of particulate organic matter (POM) to different FYM applications, which were related to contents in total OC (TOC) and water‐extractable OC (WEOC). We show that FYM‐C application of 469 g C m−2 a−1 caused increases in TOC and WEOC contents only in the topsoil, whereas rates of ≥938 g C m−2 a−1 were necessary for TOC enrichment also in the subsoil. At this depth, the subdivision of TOC into different OC sources shows that most of the increase was due to fresh POM, likely by the stimulation of root growth and bioturbation. The increase in subsoil TOC went along with increases in macroporosity and macropore connectivity. We neither observed increases in plant‐available water capacity nor in unsaturated hydraulic conductivity. In conclusion, only very high application of FYM over long periods can increase OC content of subsoil at our study site, but this increase is largely based on fresh, easily degradable POM and likely accompanied by high C losses when considering the discrepancy between OC addition rate by FYM and TOC response in soil.