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

Correctly estimating the trophic fractionation factors (Delta N-15 and Delta C-13) 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 N-15/N-14 and small C-13/C-12 trophic fractionation, but in field studies litter-dwelling microarthropods and other invertebrates are consistently enriched in C-13 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, Delta N-15 and Delta C-13 averaged 1.4 +/- A 0.1 and 1.0 +/- A 0.2 aEuro degrees, respectively. In microcosms with fungi-inoculated litter, the difference in delta N-15 between collembolans and plant litter averaged 1.5 +/- A 0.2 aEuro degrees, confirming the relatively small N-15/N-14 trophic fractionation at the basal level of detrital foodwebs reported in numerous field studies. In full agreement with field observations, the difference in delta C-13 between bulk litter and collembolans in laboratory microcosms averaged 3.6 +/- A 0.1 aEuro degrees and only little depended on collembolan species identities or the presence of water-soluble compounds in the litter. We conclude that increased delta C-13 values typical of litter-dwelling decomposers are largely determined by an increased C-13 content in saprotrophic microorganisms.