Do interactions between litter-feeders and endogeic earthworms impact soil organic matter content and its forms? Results of a microcosm experiment

Soil saprophagous fauna strongly impacts litter decomposition through its modification of microbial communities and activity. However, soil fauna is very diverse, with different feeding strategies. For example, arthropods such as woodlice and millipedes and epigeic earthworms (litter-feeders) are known to feed on the litter layer, while endogeic earthworms feed on organic particles mixed with the soil mineral layer. Distinct fauna may have a very different impact on the final forms of the soil organic matter. They could act synergistically as endogeic earthworms may require the fragmentation and incorporation of litter into the soil by litter-feeders to access their food. We performed a three-months microcosm experiment in which we tested the effect of litter fragmentation and presence of litter-feeders (isopods or epigeic earthworms of Dendrodrilus rubidus species) on the survival and change in biomass of endogeic earthworms (of Aporrectodea caliginosa species). We also tested whether different combinations of fauna and litter fragmentation affect microbial biomass and respiration, and the forms of the soil organic matter. We used a recent post mining soil, to create a stressful environment with poor food resources for the endogeic species. The hypotheses were that endogeic earthworms would be positively affected by the fragmentation and mixing of litter with the soil, whether it would be done manually or by epigeic earthworms or isopods, and that this would impact soil properties. First results showed no decrease in the biomass of A. caliginosa in any of the treatments. A. caliginosa was able to consume the alder litter added at the soil surface at a similar rate than the epigeic D. rubidus, even without previous fragmentation and in the absence of litter-feeders. A loss of carbon through respiration and an increase of dissolved organic carbon content in the soil were associated with the presence of epigeic earthworms, while an increase in soil microbial biomass carbon was induced only by endogeic earthworms when litter was added at the soil surface. Nitrates content was increased when both types of earthworms were present. Interestingly, endogeic earthworms had an opposite impact on microbial biomass and dissolved organic carbon content when litter was mixed in small pieces with the soil or added at the soil surface. First conclusions are that though the endogeic earthworm A. caliginosa do not seem to require a first fragmentation of the litter to access it, the final effect on decomposition differ according to the other fauna present and to the fragmentation of the litter. Analyses of the forms of the organic matter in the different treatments will allow to determine how this impacted the incorporation of organic matter into the soil.