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 remain 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 studied 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 modulated soil pH depending on soil type, increasing pH in Cambisol while decreasing pH in Luvisol. Species-specific effects on cast properties revealed a physicochemical gradient: L. castaneus and L. terrestris produced casts with the highest nitrate, dissolved organic carbon, total organic carbon, and moisture levels, whereas endogeic species had 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 earthworm’s 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 to future studies of the role of earthworms in soil biogeochemical cycling and a framework for advancing our understanding of their impact on soil organic matter dynamics.