Behavioral and functional responses of Hormogaster samnitica to different organic amendments

The decline of soil organic matter in Mediterranean agroecosystems, coupled with the underutilization of crop residues, calls for sustainable strategies that enhance soil health while valorizing local organic resources. This study investigates the behavioral and functional responses of Hormogaster samnitica, a large-bodied, endogeic earthworm, to four organic amendments derived from agricultural residues: vine pruning compost (PRUN), vine pruning compost with a commercial accelerator (COMM), vine pruning compost inoculated with Indigenous Microorganisms (INDG), and vermicompost from spent coffee grounds (VERM). The experiment was conducted in laboratory mesocosms using repacked soil columns. Earthworms were introduced at three body size classes (“big”, “medium” and “small”), and their impact on soil porosity and infiltration was assessed via X-ray tomography and infiltration tests. Results showed that body size significantly affected earthworm activity, with larger individuals creating wider and deeper burrows and markedly increasing water infiltration. Among treatments, COMM significantly enhanced macroporosity in the amended soil layer and induced a marked shift in the vertical distribution of macropores. However, despite increased porosity, compost-based amendments generally reduced infiltration rates compared to the CTRL. VERM showed limited effects on both bioturbation and infiltration. A two-choice preference test revealed that H. samnitica exhibited weak substrate preferences, while the more generalist Aporrectodea caliginosa clearly favored compost-based treatments over VERM and CTRL. Overall, these findings highlight the importance of considering species-specific traits and amendment properties when evaluating the ecological impact of organic inputs, and support the use of compost as a biostimulant for soil fauna and structure enhancement in Mediterranean systems.