The effects of earthworm species on organic matter transformations and soil microbial communities are only partially related to their bioturbation activity

Earthworms are pivotal in shaping soil ecological processes through their bioturbation activity and organic matter consumption. Earthworm species are known to have different impacts on soil structure, but only a small number of species have been studied so far, and few studies have examined how earthworms simultaneously affect soil functions. Here, we measured the impact of different earthworm species on soil structure (bioturbation function), carbon (C) and nitrogen (N) dynamics and the microbial community (organic matter transformation function), while exploring the links between these functions, across distinct soil compartments (surface casts, below-ground drilosphere, and bulk soil). Six earthworm species (Lumbricus terrestris, Allolobophora chlorotica, Octolasion cyaneum, Octodrilus complanatus, Aporrectodea caliginosa meridionalis and Microscolex dubius) of different ecological categories and functional groups were incubated in soil cores with soil and alfalfa litter for 6 weeks. Our results on the bioturbation function illustrated a great diversity of behaviors and confirmed the relevance of a functional classification based on bioturbation metrics. The main microbial hotspots were surface casts, whose characteristics allowed to distinguish two groups of species. Octod. complanatus, L. terrestris and M. dubius induced high humidity (respectively, +57, +48, +74%), high C (respectively, 19.9, 24.8, 33.2 g kg⁻¹ dry soil) and N (respectively, 2, 2.3, 3.2 g kg⁻¹ dry soil) content and microbial community selection, promoting C and N mineralization. The three other species had a lower impact. The below-ground drilosphere only showed specific characteristics in the case of L. terrestris. The effects of the studied species on the organic matter transformation function did not align with their bioturbation activities nor with their ecological category. These findings show that the contribution of earthworms to C and N turnover is only partially dependent on their bioturbation effects and suggest the usefulness of developing distinct functional groups based on the specific soil functions under consideration.