Characterization of soil aggregate structure by virtual erosion of X-ray CT imagery

Abstract

The characterization of soil aggregates and structure has been tedious and intrusive in the past. A typical analysis of soil structure is done through bulk density, aggregate density or manual aggregate erosion. X-ray computed tomography (CT), is a non-invasive technique that can be used to understand the spatial configuration and nature of soil components, and how they are related to soil processes and behaviour. The objectives of this research were to evaluate the utility of virtual erosion/distance mapping, combined with analysis of the 3D semivariance of CT imagery, for quantifying the internal structure of soil aggregates and, specifically, to determine its applicability for comparing the impact of management practices on soil structure. Samples were collected from the long-term rotation experimental plots at the University of Guelph's Elora Research Station, in Elora, Ontario Canada. This research focused on Continuous Corn Mouldboard Plough (CCMP); Complex Rotation Mouldboard Plough (RotMP); Continuous Corn No-Till (CCNT); and Complex Rotation No-Till (RotNT). A total of 16 plots (four from each treatment) were utilized, for a total of 32 core samplings and 96 aggregates. An average histogram, mean radiodensity, distance mapping and semivariance analysis were run on the greyscale imagery. Aggregates from CC, regardless of tillage, had higher aggregate density and radiodensity overall, as well as at the centroid (the central layer determined from the distance mapping technique) and surface layers, coherent with reduced void space. Aggregates from both RotMP and RotNT had the lowest aggregate radiodensity, suggesting that crop rotation is a key factor in lowering bulk density/radiodensity. Distance mapping and aggregate peeling techniques were useful to distinguish differences in aggregate structure when considering soil management, as it enabled the quantification of aggregate layering and structural formation. Eroded aggregates were found to be more isotropic and homogeneous at the surface when compared to uneroded aggregates, showing the erosion technique allows for an analysis of the aggregates in a more in depth analytical way. The Balaguar parameters were also able to determine significant differences between the eroded and uneroded aggregates as well as between practices.