Quantification of Soil Macropore Characteristics using X-ray Computed Tomography

Abstract

Soil macropores, such as root channels and earthworm burrows, constitute only a small fraction of the total soil volume but contribute largely to the transport of water and solutes in subsurface flows. One of the challenges in soil hydrology is quantification of soil macropore characteristics. The X-ray computed tomography (CT) is a powerful technique that allows quantification of the 3D soil macropore structure without altering the sample. The goal of this study was to quantify and characterize two-dimensional (2D) and three-dimensional (3D) soil macropore characteristics including, macroporosity, macropore size distribution, macropore length density, and interconnectivity (node density) using CT. The first part of this study provided quantitative information of different soil macropore characteristics at different topographical locations (upslope, midslope, and downslope) and depths in a 0.40 ha pasture field located in Alabama, USA. The downslope location had significantly less macropore number and macroporosity values in the 0-100 mm soil layer. In addition, macropores at the surface (0-100 mm) of upslope and midslope soils were highly connected as compared to the soils of the downslope location. The results of this study suggest that macropores at the surface (0-100 mm) of the downslope locations are largely affected by trampling induced compaction due to presence of higher soil moisture content at the downslope location, via runoff and seepage losses from the upper slopes. Besides, it was found that macroporosity and ECD measurements could be largely biased because of higher values of coefficient of variation (CV) for a smaller diameter sub-volume cores compared to the larger diameter cores.

In the second part of the study, quantitative evaluation was made on both the spatial and temporal variability in 3D soil macropore structure in a pasture field. The results of this study showed that 3D macropore characteristics may change significantly with time (~0.33 years) and topographical position in the topsoil surface (0-100 mm) layer. Also, pores smaller than 2 mm in diameter were found to be highly sensitive to topographical differences suggesting need of further studies using higher resolution X-ray CT to quantify smaller pores (<0.70 mm). In the third part of this study, soil macropore characteristics were quantified as a function of land use type and tillage practices in soil cores collected from Wisconsin, USA. The macropores in soil cores collected from fields under conventional tillage were smaller and mostly concentrated near the surface soil layer as compared to the no-till soils, which can be attributed to the disaggregation of the soil structure due to tillage operations. On the other hand, soil cores collected from no-till field and alfalfa had relatively larger and vertical macropores. Overall results of this study will contribute to the enhanced evaluation of soil macropore features with significant implications for flow and contaminant transport modeling in soils.