Earthworms ingest large amounts of soil and have the potential to radically alter the biomass, activity, and structure of the soil microbial community. In this study, the diversity of eight bacterial groups from fresh soil, gut, and casts of the earthworms Lumbricus terrestris and Aporrectodea caliginosa were studied by single-strand conformation polymorphism (SSCP) analysis using both newly designed 16S rRNA gene-specific primer sets targeting Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Bacteroidetes, Verrucomicrobia, Planctomycetes, and Firmicutes and a conventional universal primer set for SSCP, with RNA and DNA as templates. In parallel, the study of the relative abundance of these taxonomic groups in the same samples was performed using fluorescence in situ hybridization. Bacteroidetes, Alphaproteobacteria, and Betaproteobacteria were predominant in communities from the soil and worm cast samples. Representatives of classes Flavobacteria and Sphingobacteria (Bacteroidetes) and Pseudomonas spp. (low-abundant Gammaproteobacteria) were detected in soil and worm cast samples with conventional and taxon-targeting SSCP and through the sequence analysis of 16S rRNA clone libraries. Physiologically active unclassified Sphingomonadaceae (Alphaproteobacteria) and Alcaligenes spp. (Betaproteobacteria) also maintained their diversities during transit through the earthworm intestine and were found on taxon-targeting SSCP profiles from the soil and worm cast samples. In conclusion, our results suggest that some specific bacterial taxonomic groups maintain their diversity and even increase their relative numbers during transit through the gastrointestinal tract of earthworms.

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Correctly estimating the trophic fractionation factors (Δ15N and Δ13C) in controlled laboratory conditions is essential for the application of stable isotope analysis in studies on the trophic structure of soil communities. Laboratory experiments usually suggest large 15N/14N and small 13C/12C trophic fractionation, but in field studies litter‐dwelling microarthropods and other invertebrates are consistently enriched in 13C relative to plant litter. In the present study, we report data from two laboratory experiments investigating both fungi–collembolans and litter–fungi–collembolans systems. In the fungi–collembolans system, Δ15N and Δ13C averaged 1.4 ± 0.1 and 1.0 ± 0.2 ‰, respectively. In microcosms with fungi‐inoculated litter, the difference in δ15N between collembolans and plant litter averaged 1.5 ± 0.2 ‰, confirming the relatively small 15N/14N trophic fractionation at the basal level of detrital foodwebs reported in numerous field studies. In full agreement with field observations, the difference in δ13C between bulk litter and collembolans in laboratory microcosms averaged 3.6 ± 0.1 ‰ and only little depended on collembolan species identities or the presence of water‐soluble compounds in the litter. We conclude that increased δ13C values typical of litter‐dwelling decomposers are largely determined by an increased 13C content in saprotrophic microorganisms.

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