Isotopic niche (13C and 15N values) of soil macrofauna in temperate forests

RATIONALE Stable isotope analysis (SIA) is used widely for reconstructing trophic links of vertebrate and invertebrate animals. Soil macrofauna form a substantial food source for a range of predators including amphibians, reptiles, birds and mammals. SIA-based estimations of their trophic niches require knowledge on the full range of isotopic signatures of potential preys. Considering the extremely high diversity of soil animals, this information is not easy to obtain. METHODS We estimated a typical range of the isotopic signatures of soil macrofauna by compiling published and original data on soil macroinvertebrates in 11 temperate forests. We examined whether the baseline correction (i.e. subtracting 13C or 15N values of local litter) would decrease the between-site variability in the 13C and 15N values of soil animals. The dataset was subsequently used to estimate the frequency distribution of 13C and 15N values in saprophagous and predatory soil animals. RESULTS The baseline correction reduced the between-site variability in 15N, but not in 13C values of soil animals. Over 95% of the taxa or individuals examined fell into an isotopic space with uncorrected 13C values ranging from -27.9 to -22.5 parts per thousand, and litter-normalized 15N values from 0.8 to 9.6 parts per thousand. Saprophagous and predatory soil animals were on average enriched in 13C relative to plant litter by 3.5 and 2.7 parts per thousand, respectively, which is likely to reflect the importance of saprotrophic microorganisms as the main energy source in soil food webs. The difference in 15N values between saprophages and predators averaged 2.8 parts per thousand, which fits the anticipated trophic enrichment per trophic level. CONCLUSIONS Our results indicate that the range of possible 15N values of soil macrofauna in temperate forest ecosystems can roughly be predicted based on the 15N values of plant litter. On the other hand, no site-specific normalization is usually required when predicting the range of 13C values of soil macroinvertebrates. Copyright (c) 2014 John Wiley & Sons, Ltd.