Soil macroinvertebrate activity and pore size distribution: characterization by X ray CT and 3D image analysis

Soil organisms play a role in soil structure dynamics mainly by their activities like burrowing and mixing of materials, which in turn influence water and matter fluxes. Since the beginning of the 90s non-invasive technologies such as X-ray CT have been applied to directly observe and quantify macroporosity and bioturbation induced by soil fauna. However, in comparison with earthworms, termites and ants, the direct quantification of the effects of the activities of other soil macroinvertebrates on soil pore system development remains largely unexplored. In this work we carried out a short-term microcosm experiment combined with tridimensional image analysis to identify and quantify separately the contribution of five different taxa of macroinvertebrates to the formation of the soil pore system. Biopore size distribution produced by earthworms, millipedes, centipedes, campodeiform and elateriform lavae has been calculated from X-ray CT images of repacked soil samples inoculated each with one individual and then incubated in the field where soil and invertebrates were collected. Pore system has then been analysed appling the "successive opening" algorithm. All taxa exhibited specific signs in term of shape of the pore size distribution: except for elateriform larvae and centipedes, all studied macroinvertebrates induced evident multimodality in the pore size spectrum, with negative skewness for the worms and strongly positive one for the Campodeiform larvae due, this latter, to the excavation of caves. Earthworms showed an individual burrowing ratio significantly lower than that of the other soil invertebrates, being such parameter inversely correlated with the size of the individuals for all the taxa. Overall, results confirm the need to properly consider the specific role of the different soil macroinvertebrates as ecosystem engineers, while the applied experimental approach has provided data suitable for modeling the contribution of macroinvertebrates in soil structure formation.