Analysis of Within-Aggregate Soil Porosity Using Microtomographic Images.

Description of carbon sequestration in soils is of major importance in quantifying soil’s role in global carbon cycle. However, complete understanding of this process at soil aggregate level is still lacking. One of the key factors to be studied in order to understand transport and transformations of organic matter at microscale is the structure of soil aggregates and the structural changes that take place within the aggregates subjected to impact of natural forces, such as wetting-drying cycles. With the advent of X-ray computed microtomography detailed information about soil aggregate structure can be obtained in non-destructive and non-invasive way, providing micrometer-order precision of soil aggregate inner matter representation. However, methods of processing X-ray soil images in order to obtain reliable representation of within aggregate pore structures are still not well established. The first objective of this study is to examine existing methods for segmenting a grayscale image into a binary (pore/solid) image and to discuss limitations of their applicability to soil images. The second objective is to examine changes in porosity with depth within the soil aggregates as determined based on X-ray images and to compare them with measurements obtained from SAE (Soil Aggregate Erosion) peeling of the aggregates of the same soil type. Preliminary studies showed coherence between computational and physical porosity measurements. Therefore, as the third objective, soil aggregate image analysis will be applied to compare total porosity and porosity gradients in naturally air-dry soil aggregates and aggregates subjected to several cycles of wetting and drying.