Isotope analyses of amino acids in fungi and fungal feeding Diptera larvae allow differentiating ectomycorrhizal and saprotrophic fungi-based food chains

1- Both ectomycorrhizal (ECM) and saprotrophic fungi are fundamental to
carbon and nutrient dynamics in forest ecosystems; however, the relative
importance of these different fungal functional groups for higher trophic
levels of the soil food web is virtually unknown. 2- To explore
differences between fungal functional groups and their importance for
higher trophic levels, we analysed isotopic composition of nitrogen and
carbon in amino acids (AAs) and bulk tissue of leaf litter, fungi, and
fungal-feeding Diptera larvae. 3- By accounting for isotopic variability
of utilized substrates, compound-specific isotope analyses of nitrogen in
AAs yielded more realistic results for the trophic position of fungi than
bulk isotope analyses, with converging trophic positions of saprotrophic
and ECM fungi. 4- Saprotrophic and ECM fungi possessed different AA δ13C
signatures separating fungal functional groups and their consumers in
fingerprinting approaches, thereby allowing to trace energy fluxes from
these basal resources to higher trophic levels. 5- A pronounced isotopic
fractionation even in essential/source AAs of fungal-feeding Diptera
larvae necessitates further studies on tissue-/compound-specific isotopic
differences in fungi and on potential supplementation by gut
microorganisms. 6- The results highlight the potential of
compound-specific isotope analysis of amino acids to identify and
integrate contributions of different fungal functional groups to higher
trophic levels in soil food webs.