Dual amino acid stable isotope analyses (13C and 15N) as a novel method to unravel energy fluxes and trophic relationships in soil animal food webs

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Soil animal food webs are complex and heterogeneous systems. Despite their fundamental role for ecosystem functioning, basic processes therein are not well understood. Differentiation between fungal, bacterial and plant derived resources, quantification of energy fluxes as well as estimation of trophic levels remain major challenges. Building on findings gained from bulk stable isotope and compound-specific fatty acid analyses, the proposed project aims at adapting the analysis of 13C and 15N signatures of individual amino acids (AAs) in animals and resources for analyzing soil animal food webs. Compound-specific analysis of AAs provides significant advantages over the analysis of bulk isotopic signatures, circumventing problems such as isotopic variation of basal resources and varying trophic fractionation. By utilizing 15N signatures of so-called trophic and source AAs within one organism, trophic levels can be unambiguously identified independently of differing baselines. 13C signatures of essential AAs can be used to distinguish between energy fluxes originating from different basal resources using stable isotope fingerprinting. Plants, fungi and bacteria differ in metabolic pathways of AA synthesis and therefore produce unique 13C patterns that are transferred to consumers. Relative contributions of basal resources can be quantified using mixing models. The combination of 13C and 15N analyses of AAs is a novelty for food web analyses; as yet very few studies adopted this approach. To adapt the dual analysis of AAs for soil animal food webs, I will conduct controlled laboratory experiments to establish trophic enrichment factors (TEFs) for individual AAs, and to investigate physiological parameters such as the half-life of AAs in soil animals, and the influence of diet quality on TEFs. Feeding experiments with mixed diets will allow developing mixing models to quantify relative energy fluxes from basal resources to consumers. In a mesocosm labeling experiment featuring major groups of decomposer organisms, I will utilize the dual isotopic analysis of AAs to separate contributions of aboveground vs. belowground resources, and at the same time quantify fluxes via fungal/bacterial/plant channels. The dual isotopic analysis of AAs represents a novel and very promising approach to clarify the trophic structure of soil animal food webs in unprecedented detail.