Three-dimensional quantification of intra-aggregate pore-space features using synchrotron-radiation-based microtomography

Pore network geometries of intra-aggregate pore spaces are of great importance for water and ion flux rates controlling C sequestration and bioremediation. Advances in non-invasive three-dimensional imaging techniques such as synchrotron-radiation-based x-ray microtomography (SR-[micro]CT), offer excellent opportunities to study the interrelationships between pore network geometry and physical processes at spatial resolutions of a few micrometers. In this paper we present quantitative three-dimensional pore-space geometry analyses of small scale (~5 mm across) soil aggregates from different soil management systems (conventionally tilled vs. grassland). Reconstructed three-dimensional microtomography images at approximate isotropic voxel resolutions between 3.2 and 5.4 [micro]m were analyzed for pore-space morphologies using a suite of image processing algorithms associated with the software published by Lindquist et al. Among the features quantified were pore-size distributions (PSDs), throat-area distributions, effective throat/pore radii ratios as well as frequency distributions of pore channel lengths, widths, and flow path tortuosities. We observed differences in storage and transport relevant pore-space morphological features between the two aggregates. Nodal pore volumes and throat surface areas were significantly smaller for the conventionally tilled (Canv. T.) aggregate (mode [approximately equal to] 7.9 x [10.sup.-7] [mm.sup.3]/[approximately equal to] 63 [micro][m.sup.2]) than for the grassland aggregate (mode [approximately equal to] 5.0 x [10.sup.-6] [mm.sup.3]/[approximately equal to] 400 [micro][m.sup.2]), respectively. Path lengths were shorter for the Canv. T. aggregate (maximum lengths < 200 [micro]m) compared with the grassland aggregate (maximum lengths > 600 [micro]m). In summary, the soil aggregate from the Canv. Tsite showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system. Abbreviations: Conv. T., conventional tillage; DU, down to upward flow; IK, indicator kriging; LR, left to right flow; PSD, pore-size distribution; SOC, soil organic carbon; SR-[micro]CT, synchrotron radiation microtomography; TSD, throat-size distribution.