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-mu 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 (similar to 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 mu 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 (Conv. T.) aggregate (mode approximate to 7.9 x 10(-7) mm(3)/approximate to 63 mu m(2)) than for the grassland aggregate (mode approximate to 5.0 x 10(-6) mm(3)/approximate to 400 mu m(2)), respectively. Path lengths were shorter for the Conv. T. aggregate (maximum lengths < 200 mu m) compared with the grassland aggregate (maximum lengths > 600 mu m). In summary, the soil aggregate from the Conv. T site showed more gas and water transport limiting micromorphological features compared with the aggregate from the grassland management system.