Use of X-ray microcomputed tomography for characterizing earthworm-derived belowground soil aggregates

Soil structure is closely linked to biological activities. However, identifying, describing and quantifying soil aggregates remain challenging. X-ray microcomputed tomography (X-ray μCT) provides a detailed view of the physicalstructure at a spatial resolution of a few microns. It could be a useful tool todiscriminate soil aggregates, their origin and their formation processes for a better comprehension of soil structure properties and genesis. Our study aims to (a) determine different X-ray μCT-based aggregate parameters for differentiating earthworm casts belowground (earthworm aggregates) from aggregates that are not formed by earthworms (non-earthworm aggregates), and (b) to evaluate if these parameters can also serve as specific “tomographic signatures” for the studied earthworm species. For this purpose, we set up a microcosm experiment under controlled conditions during 8 weeks, including three species of earthworms tested separately: the epigeic Lumbricus rubellus, the anecic Lumbricus terrestris and the endogeic Allolobophora chlorotica. Our results show that X-ray μCT analysis helps distinguish earthworm aggregates from non-earthworm ones using (a) the relative volume of the components within aggregates and (b) the volumetric mass of aggregates and their global volume. In particular, the volume ratio of mineral grains within the aggregates is significantly different according to earthworm species. So, X-ray μCT is a powerful and promising tool for studying the composition of earthworm casts and their formation. However, future research is needed to take into account the shapes and spatial distribution of the aggregates' components, in particular the different states of organic matter decomposition.