Characterisation of soil pore structure anisotropy caused by the growth of bio-subsoilers

Abstract

Deep-rooting cover crops (tap-rooted plants and grasses) are suggested to enhance soil properties when used as bio-subsoilers in compacted soils. However, there is limited knowledge the impact of bio-subsoilers on anisotropy of the soil pore system of compacted zones. The objective of this study is to evaluate the impact of different bio-subsoiler species on the anisotropy of the soil pore characteristics of a compacted sandy loam subsoil. The experimental treatments consisted of spring barley as control and chicory, lucerne and tall fescue as bio-subsoilers. The soil samples were taken in both vertical and horizontal directions at 0.3–0.4 m depth. Air permeability (ka) and air-filled porosity (εa) were quantified at −10, −30, −50, −100 and −300 hPa matric potential, and X-ray computed tomography (CT) scans were obtained for samples drained at −100 hPa. Results showed no significant treatment effect (p < 0.05) on ka, the ratio of the non-Darcian to the Darcian air permeability (R), or the PO1 index (ka/εa) in either vertical or horizontal directions. However, the factor of anisotropy of ka, PO1 and R-ratio was significantly affected by chicory and lucerne (p < 0.05), indicating the creation of more vertically than horizontally oriented pores. The results from measured parameters were supported by CT image analysis as macropores connecting from end-to-end of the soil cores were mainly captured for vertically oriented samples by the CT scanner. Lucerne had significantly larger proportion of CT-macroporosity (∼50%) and CT-total branch length (∼40%) for vertically connected pore network as compared to spring barley (∼20% for both parameters). This study suggests that the impact of growing bio-subsoilers on the anisotropy of the soil pore system of the studied compacted subsoil is species-dependent.