Morphological description of soil structure patterns produced by earthworm bioturbation at the profile scale

In cultivated soils, Soil structure mainly results from climatic, anthropogenic and biological processes. Nevertheless, few field methods evaluating the quality of soil structure consider the contribution of biological processes. In order to include earthworm biostructures in the field description of soil-structure, an original method is proposed in this paper. Soil profiles under different agricultural practices were examined to distinguish soil-structure patterns, notably those resulting from earthworm bioturbation. The relevance of naked eye observation was tested by a micromorphological approach, using image analysis on thin sections. Then, the application of this method was illustrated by mapping soil profiles.Our study led to the creation of a typology (i.e. classification system) of eleven soil-structure patterns, taking into account anthropogenic processes (e.g. compaction, soil tillage), root activity and earthworm activity. Seven patterns were attributed to earthworm activity in the form of burrows or casts. Three burrow features were distinguished, differentiating between filled burrows, or empty burrows with a brown cutan or without visible cutan. Four patterns of cast packing were distinguished, differentiating between cast aggregates that were fresh, welded, compacted, or combined with burrow features. This typology appears relevant for developing a field tool to describe and spatially quantify soil structure.

► Earthworm bioturbation strongly impacts on morphological features in soil profiles.
► We identified and characterized 7 soil-structure patterns (casts and burrows).
► Cast patterns differ by their morphological appearances (granular, welded, compact).
► Each cast pattern presents a specific macroporosity (qualitatively, quantitatively).