Reliability of isotopic fractionation (Δ15N, Δ13C) for the delimitation of trophic levels of oribatid mites: Diet strongly affects Δ13C but not Δ15N

Stable isotope analysis has become an analytical tool of central importance in trophic ecology. The actual degree of isotopic fractionation, however, remains a black-box in most studies. Hence, mean values for trophic enrichment from other taxa are commonly used to delimitate trophic levels. On the other hand, resource composition as well as consumer physiology both influence fractionation patterns. Especially in soil food webs, high variability of isotopic fractionation has been found for both 15N and 13C. Here, we investigate effects of diet on trophic enrichment in a no-choice feeding experiment with the oribatid mite model species Archegozetes longisetosus and a set of fungal, animal, algal and plant resources (seeds and vegetative tissues). We found consistent trophic enrichment for nitrogen irrespective of diet (Δ15N = 3.9‰), but no reliable fractionation for carbon being negative for fungi (Δ13C = −1.8‰), neutral for algae, and positive for the remaining plant and animal resources (Δ13C = 2.3‰). The results suggest that δ15N is a reliable marker for delimiting the trophic level of oribatid mites and presumably other soil detritivores, while understanding δ13C signals needs a priori knowledge about isotopic fractionation with respect to consumed resources.