Characterization of Porosity Gradient within the Soil Aggregates Using X-Ray Microtomography.

While changes in soil properties due to different long-term land management types have been intensively studied on a scale of large soil samples and cores, land use effects at the soil aggregate level still call for better understanding. We hypothesize that knowledge of these effects will contribute to a better understanding of the key mechanisms governing carbon sequestration by soils. In particular, porosity structure within soil aggregates is likely to be associated with spatial heterogeneity in distributions of moisture, organic matter, and bacteria within the aggregate. Recent advances in X-ray microtomography allow us to examine internal structure of soil aggregates with micrometer-level precision. The objectives of this study are (i) to develop methodology for detecting porosity gradient within the soil aggregate, (ii) to examine porosity gradients computed from X-ray images of soil aggregates and to compare them to measurements obtained by physical peeling of the aggregates of the same size and soil type; and (iii) to compare intra-aggregate porosity gradients in the aggregates from soil subjected to contrasting long-term land uses.
The aggregates from two long-term experiments located in Michigan, USA and Shenyang, China are used in the study. The land use practices that are compared are native succession vegetation, soil long-term maintained without vegetation, and conventional row-crop agriculture.
Exterior regions of the aggregates were found to be more porous mainly due to large amount of pores directly connected to the outside of the aggregate. However, this effect depends on the definition of the aggregate boundary, and is less pronounced in sandier aggregates.