Quantification of 3-D soil macropore networks in different soil types and land uses using computed tomography

SummaryThe importance of soil macropores as preferential pathways for water, air, and chemical movement in different soils has long been recognized. However, quantification of complex macropore structures and their relationships to soil types and land uses remains elusive. The objectives of this study were to (1) quantify 3-D macropore networks in intact soil columns using an improved approach and (2) investigate the effects of soil type and land use on soil macropore characteristics. Two soils with contrasting textures and structures (Hagerstown silt loam and Morrison sand) from two land uses (row crop and pasture) were investigated. Intact soil columns, 102 mm in diameter and about 350 mm in length, were taken for each soil type-land use combination. The soil columns were scanned using X-ray computed tomography at a voxel resolution of 0.234 mm × 0.234 mm × 2.000 mm. After reconstruction, characteristics of macropore networks were quantified, including continuous macroporosity change along depth, macropore size distribution, network density, surface area, length density, length distribution, mean hydraulic radius, tortuosity, inclination (angle), and connectivity (path number and node density). The approach we developed provided an improved quantification of complex 3-D macropore networks. The analysis of variance indicated that soil type, land use, and their interaction significantly influenced macroporosity, network density, surface area, length density, node density, and mean angle. The interaction of soil type and land use also influenced mean tortuosity and hydraulic radius. Within the same soil type, the soils under pasture land use had greater macroporosity, length density, and node density than that under row crop, especially in the subsoil. This was due to greater organic matter content and more biota activities in the pasture. Within the same land use, the Morrison sand displayed lower overall macroporosity than the Hagerstown silt loam because of weaker structure and higher amount of rock fragments in the Morrison soil and thus less suited for biota activities. The results from this study provide improved quantitative evaluation of a suite of soil macropore features that have significant implications for non-equilibrium flow prediction and chemical transport modeling in field soils.