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.