Evaluation of X-ray computed tomography for quantifying macroporosity of loamy pasture soils

Abstract

Macropores are important pathways for rapid infiltration of water into soil as large soil pores allow roots, air, and water to penetrate into the soil. Soil compaction due to dairy cattle trampling may reduce soil macroporosity to the extent that growth or even survival of roots is limited by oxygen availability and soil strength. This study aims to evaluate the feasibility of X-ray computed tomography (CT) for determining macroporosity in dairy pasture soils and to assess the effects of sample volume (both within and between cores of varying sizes) and locations (i.e. site and soil depth) on measures of porosity. Undisturbed soil cores (50 and 65mm diameter) were collected from two depths (0 to 10cm and 20 to 30cm) and from three sampling sites (representing moderately to highly trafficked zones) within a commercial dairy paddock. The intact soil cores were scanned at three resolutions (30, 109 and 138μm) using X-ray CT, and porosity and mean pore diameter measured in various volumes of these scans. Porosity and mean pore diameter decreased with depth but did not differ across the three sampling sites; this was consistent with measures of porosity based on a traditional bulk density based method and soil water retention curves. There was no significant (P>0.05) effect of analysing porosity at increasing distances from the core edge. Likewise, increasing the volume of soil that was analysed within each core was not found to have a significant (P>0.05) effect on macroporosity. However, mean pore diameter was found to significantly (P<0.05) increase with increasing volume of soil measured (both within a soil core and with increasing core size) and significantly (P<0.05) decrease with increasing resolution of the scans. The results suggest that while absolute measures of macroporosity might not change with core size or the volume of soil analysed, the pore-space characteristics that are captured differ significantly. Macroporosity values for various pore size classes (0.2 to 298μm pore diameters) assessed using soil–water retention curves compared with those determined using the X-ray CT were found to be comparable. Consequently, X-ray CT is a valuable tool for characterising pore-space from the macro- to the micro-scale, however, sampling and analysis strategies must be appropriate for the specific research aims. The practical implications of the results are discussed.