Data from: Compound-specific isotope analysis of amino acids as new tool to uncover trophic chains in soil food webs

Food webs in soil differ fundamentally from those above ground; they are based on inputs from both living plants via root exudates, and from detritus, which is a complex mixture of fungi, bacteria and dead plant remains. Trophic relationships are difficult to disentangle due to cryptic lifestyle of soil animals and inevitable microbial contributions to their diet. Compound-specific isotope analysis of amino acids (AAs) is increasingly used to explore complex food webs. The combined use of AA δ13C and δ15N values is a promising new approach to disentangle trophic relationships since it provides independent but complementary information on basal resource and trophic position of consumers. We conducted a controlled feeding study by reconstructing trophic chains from main basal resources (bacteria, fungi, plants) to primary consumers (springtails, oribatid mites) and predators (gamasid mites, spiders). We analyzed dual compound-specific isotope values in AAs of resources and consumers, simultaneously applying an approach termed “stable isotope (13C) fingerprinting” to identify basal resource, and calculating trophic positions using 15N values of trophic and source AAs in consumers. In the 13C fingerprinting analysis, consumers in general grouped close to their basal resources. However, higher than usual offsets in AA δ13C between diet and consumers suggest either gut microbial supplementation, or the utilization of specific resource fractions. Identification of trophic position crucially depends on correct estimates of the trophic discrimination factor (TDFGlu-Phe), which was close to the commonly applied value of 7.6‰ in primary consumers feeding on microbial resources, but considerably lower in arachnid predators (~2.4‰), presumably due to higher diet quality, excretion of guanine, and fluid feeding. Primary consumers feeding on lime leaf litter were substantially depleted in 15NPhe, possibly due to selective utilization of more readily available nitrogen pools within leaves. Whilst our feeding study demonstrates that dual compound-specific AA analyses holds great promise in delineating trophic linkages among soil dwelling consumers and their resources, it also highlights that a “one-size-fits-all” approach to TDFGlu-Phe does not apply to soil food webs