Stable isotope natural abundances (δ13C and δ15N) of the earthworm Eisenia fetida and other soil fauna living in two different vermicomposting environments

Manure heaps and vermicomposting systems are hotspots of heterotrophic activity supporting a high-detritivore biomass where epigeic earthworms interact intensively with bacteria, fungi and other soil fauna. We carried out a prospective study of the vermicomposting food webs using the natural abundance of stable C and N isotopes in 66 samples of soil fauna and the substrates in which the animals live in two systems: (i) a high-feeding-rate vermireactor, fed with pig slurry, and (ii) a farm manure vermicomposting heap fed with cattle manure. The aims of the study were specifically (i) to test the extent to which the isotopic signals in the earthworms resemble those of the substrates in which they live, (ii) to further our knowledge of the ontogenic changes in resource utilization of the earthworm Eisenia fetida, and (iii) to obtain information about the relative trophic position of the soil fauna in the food web of vermicomposting systems. Tissues of earthworms were significantly 15N-enriched (by 4–8‰) relative to fresh and mature manures in both vermicomposting systems. The δ13C values of adult earthworms were not different from those of the fresh animal wastes in both vermicomposting systems, suggesting that adult worms preferred fresh manure than worked materials as carbon source. The little but significant enrichment in 15N observed in hatchlings living in the pig slurry vermicomposting bins relative to adult tissues likely reflect different feeding strategies, not observed in the cattle manure heap. Besides, hatchlings in the cattle manure heap appeared markedly depleted in 13C (by ∼5‰) relative to the adult earthworms, suggesting the use of a different source of carbon in the early stage. Diptera larvae presented very low values of δ13C, likely suggesting a relevant role of methanotrophic bacteria in their diet. Based on the shifts in δ15N, a taxon in the pig slurry vermicomposting bins may be assigned at least to three relative trophic positions separated by a 15N shift of 2‰, with Enchytraeida clearly in the lower position, adults and hatchlings of E. fetida and nematodes in an intermediate level, and Collembola at the higher position showing an enrichment of 9‰ relative to the substrate. In the cattle manure heap three trophic levels may be also identified, with larvae of Diptera and Coleoptera as the less 15N-enriched level, a general detritivore group in intermediate position, and finally a predatory taxa with a +9‰ shift comprised by Staphylinidae.