Detritivorous earthworms directly modify the structure, thus altering the functioning of a microdecomposer food web

Epigeic earthworms are key organisms in fresh organic matter mounds and other hotspots of heterotrophic activity. They turn and ingest the substrate intensively interacting with microorganisms and other soil fauna. By ingesting, digesting and assimilating the surrounding substrate, earthworms could directly modify the microdecomposer community, yet little is known of such direct effects. Here we investigate the direct effects of detritivore epigeic earthworms on the structure and function of the decomposer community. We characterized changes in the microfauna, microflora and the biochemical properties of the organic substrate over a short time (72 h) exposure to 4 different densities of the earthworm Eisenia fetida using replicate mesocosms (500 ml). We observed a strong and linear density-dependent response of the C and N mineralization to the detritivore earthworm density. Earthworm density also linearly increased CO2 efflux and pools of labile C and inorganic N. This effect on the function was likely a direct consequence of earthworm activity. Furthermore, earthworms affected the microbial metabolic activity, but this response was not linearly related to the earthworm density, possibly because of indirect effects through the microbial community. Earthworms also had strong effects on the structure of the two trophic levels examined; they enhanced the fungal populations and reduced the numbers of bacterivore nematodes. The effect on the fungi was clearly dependent on the earthworm density, and the reduction of bacterivorous nematodes was also related to the earthworm density, but only marginally. In contrast, earthworms did not have significant effects on microbial biomass carbon, flagellate protozoa or ciliate protozoa. A meaningful part of the short term changes in microflora and microfaunal communities after some hours might be attributable to the earthworm gut associated processes. Hence, detritivore earthworms can directly and quickly modulate the decomposer community altering the decomposition rates of organic matter.