Post-tillage evolution of structural pore space and saturated and near-saturated hydraulic conductivity in a clay loam soil

Abstract

Soil structure varies considerably with time in agricultural soils, as a result of complex interactions between soil management (i.e. tillage and traffic) and site-specific environmental factors. The resulting temporal variations in soil hydraulic properties significantly affect the soil water balance (e.g. partitioning between infiltration and runoff), but are still poorly understood. Thus, post-tillage decreases in saturated and near-saturated hydraulic conductivities have been frequently observed, although the underlying changes in the properties of the structural pore-space have not been studied. We used X-ray tomography to quantify the temporal changes in the volume, size distribution and connectivity of structural pores in the harrowed layer of a conventionally-tilled agricultural field over one growing season. We also determined the extent to which these properties could explain temporal variations in saturated hydraulic conductivity measured on the same samples and near-saturated hydraulic conductivities measured in the field with tension disc infiltrometers. Early-season rainfall events caused significant decreases in the imaged porosity in the uppermost 5mm of soil within one month of harrowing and also at 2.5–5cm depth but only for pores >0.5mm in diameter. Measurements of critical pore diameter showed that the sample-scale connectivity of these pores was very limited at all times. Temporal changes in the pore network characteristics were reflected in the field measurements of near-saturated hydraulic conductivity, but not in saturated hydraulic conductivity measured in the laboratory. Saturated hydraulic conductivity was, however, correlated (Spearman ρ=0.50) with the connectivity probability which is a measure of global pore space connectivity. Large spatial variation combined with a small sample size limited our ability to quantify temporal variations in the soil structural pore-space. Increased automation of image processing would enable greater replication and therefore enhance the utility of X-ray tomography as a technique for investigating the properties of the structural pore space.