Large 13C/12C and small 15N/14N isotope fractionation in an experimental detrital foodweb (litter–fungi–collembolans)

Abstract

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.