Let's get functional: Relationship between earthworm traits and physicochemical cast properties

Although earthworms play a crucial role in soil biogeochemical processes, the importance of their traits in shaping the physicochemical properties of their casts remains poorly understood. This study aimed (1) to evaluate the influence of earthworm species and soil types on cast properties and (2) investigate the relationship between earthworm morphological, anatomical, physiological and behavioral traits and the physicochemical properties of their casts. Nine temperate earthworm species were introduced under controlled conditions in two contrasting soil types (Luvisol and Cambisol), and 21 traits were determined for each species. Casts were analyzed for their physicochemical properties, total and available organic carbon and nitrogen, and compared to control soil incubated under similar conditions without earthworms. Results showed that earthworm activity changed soil pH depending on soil type, with pH increasing in Cambisol, and decreasing in Luvisol. Species-specific effects on cast properties revealed a physicochemical gradient: the epigeic L. castaneus and epi-anecic L. terrestris produced casts with the highest nitrate, dissolved organic carbon, total organic carbon, and moisture levels, whereas cast from endogeic species showed the lowest values. Moreover, earthworm species exerted a stronger overall influence on cast properties than soil type (59 % vs. 24 %), underscoring the dominant role of species-specific traits in shaping cast characteristics. We identified nine key traits related to the earthworms' morphology, anatomy, physiology and behavior, that influenced cast properties directly or indirectly. Direct effect traits included mouth area, gizzard size and location, typhlosole complexity, intestinal mucus and cast production. Indirect effect traits, such as pigmentation, litter ingestion and litter-to-cast ratio, reflected the ecological behavior of earthworm species. This trait-based approach provides a promising avenue for future studies on the role of earthworms in soil biogeochemical cycling and a framework for improving our understanding of their impacts on soil organic matter dynamics. • Earthworm species influenced cast physicochemical properties more than soil type. • Earthworm activity modulated soil pH depending on soil type. • Nine functional traits were identified as key determinants of cast properties. • Traits may affect soil properties directly or indirectly, reflecting species-specific feeding behaviour.