Interactions between litter-feeders and the burrowing earthworm A. caliginosa do not affect mineral-associated C addition in recent spoil material

Soil macrofauna affects litter decomposition by modifying organic matter availability for microorganisms. Litter-feeders accumulate partially decomposed organic matter at the soil surface, while burrowing earthworms mix the organic matter with mineral particles. Synergistic effects of the combined presence of litter-feeders and burrowing earthworms through facilitation are expected but seldom tested. We performed a three-months microcosm experiment in which we tested the effect of the presence of litter-feeders (an epigeic earthworm and an isopod) on the biomass of the burrowing earthworm Aporrectodea caliginosa and on C incorporation into individual soil organic matter fractions. We used a recent post mining soil, limited in food resources to promote interactions, and alder litter which is suitable food for soil invertebrates. We hypothesized that A. caliginosa would be positively affected by the presence of litter feeders, and that this would impact C addition to the more stable fractions. Contrary to our hypotheses, we found no variation in the biomass of A. caliginosa among treatments, as this species could feed directly on alder leaves. The accumulation of poorly stabilized particulate organic C was negatively affected by the presence of both earthworm species (i.e. less than cumulative), suggesting either that A. caliginosa fed on the particulate organic matter produced by the epigeic earthworm, or that competition for the limiting food resources took place. However, A. caliginosa promoted a similar increase in stable mineral-associated C in all treatments. This confirms the plasticity of A. caliginosa regarding food resources observed in other experiments, and shows that the effect of the burrowing earthworm A. caliginosa on stable C accumulation is not necessarily mediated by litter-feeders in the presence of litter of good quality.