Stable isotopes of amino acids indicate that soil decomposer microarthropods predominantly feed on saprotrophic fungi

Abstract

Soil microarthropods are essential for nutrient cycling in forest ecosystems as they are integral components of decomposer food webs. They channel carbon and nutrients from leaf litter and roots to higher trophic levels; however, knowledge on the relative importance of different channels and on their variation with forest type is lacking. Although the importance of root‐derived inputs for sustaining soil food webs is increasingly recognized, the pathways by which they are channeled to higher trophic levels are little understood. For the channeling, ectomycorrhizal fungi may play a significant role, but until now methods allowing to separate the contribution of ectomycorrhizal and saprotrophic fungi to the nutrition of soil animal communities are lacking. Using dual analysis of 15N and 13C in amino acids (AAs), we investigated trophic positions and basal resources of two major groups of soil microarthropods, Collembola and Oribatida, in beech and spruce forests in Germany. By applying a 13C fingerprinting approach and Bayesian mixing models, we separated in a first step the relative contribution of bacteria, fungi, and plants to the nutrition of soil microarthropods. As fungi were identified as the major food source, in a second step we attempted to separate the contribution of ectomycorrhizal vs. saprotrophic fungi. For the first time, we provide direct evidence that soil microarthropods mainly rely on saprotrophic fungi, whereas ectomycorrhizal fungi are consumed by only few species. While trophic niches of Collembola and Oribatida species generally varied little between beech and spruce forests, plant detritus as basal resource of soil microarthropods was somewhat more important in beech forests, whereas in spruce forests microbial resources dominated. Overall, the dual analysis of carbon and nitrogen in AAs provided insight into food web structure of soil microarthropods in unprecedented detail, and for the first time allowed to estimate the relative importance of mycorrhizal and saprotrophic fungi for soil food web nutrition, a long‐standing riddle in soil food web ecology. The technique provides the perspective for a comprehensive understanding of the trophic structure and energy channeling in soil food webs.

High consistency of trophic niches in soil microarthropod species (Oribatida, Acari) across soil depth and forest type

“…Our results suggest that Oribatida may have access to their required resources irrespective of the soil depth they colonize. Further, stable isotope ratios in litter well explained the variation in stable isotope values of Oribatida species, suggesting that Oribatida species predominantly rely on fresh organic carbon in litter or on rootderived resources irrespective of the actual depth they inhabit (Pollierer et al, 2007;Okuzaki et al, 2009;Melguizo-Ruiz et al, 2017;Potapov et al, 2019;Pollierer and Scheu, 2021).…”

“…The importance of plant rhizosphere as microhabitats has been supported by a recent labeling experiment for Collembola (Li et al, 2021). Further, the high consistency of the trophic positions of Oribatida species supports that soil food webs are resistant to changes in forest types (Pollierer et al, 2021), agreeing with recent studies on variations in trophic niches in other mesofauna groups, such as Mesostigmata mites and Collembola between land-use systems (Klarner et al, 2017;Susanti et al, 2021). In tropical ecosystems ranging from rainforest to monoculture plantations, trophic plasticity of Oribatida has been suggested to depend on species (Krause et al, 2019), which we could not confirm for Oribatida species in temperate forest ecosystems.…”

“…Trophic niche consistency in Oribatida species also indicates that intraspecific competition may be stronger than interspecific competition, thereby facilitating niche differentiation of Oribatida species in soil (Hart et al, 2016). Although the contribution of bacteria and plants as basal resources to Oribatida species varies depending on species, saprotrophic fungi are the major food resource of primary and secondary decomposer Oribatida species as indicated by compound-specific stable isotope analysis of amino acids (Pollierer and Scheu, 2021). In contrast to different life forms in Collembola , the vertical distribution of Oribatida species has not yet been found to be related to morphological and life-history traits (Pande and Berthet, 1975;.…”

Out of the dark: Using energy flux to connect above‐ and belowground communities and ecosystem functioning

“…These include the analysis of stable isotopes (e.g., Potapov et al 2018) and fatty acids (Ferlian et al, 2012;Ruess et al, 2005). First advances have been made to follow the signature of basal resources across multiple trophic levels Pollierer & Scheu, 2021). However, although these methods provide a rather general picture of energy channels, their ability to provide species-specific trophic interaction data is limited.…”

“…Being able to look at multiple individual ecosystem processes has value in itself, particularly as they do not need to be measured individually but are simply inferred based on the presence of the respective players and their energy demands. However, using the energy-flux approach, we can not only look at these multiple functions in isolation but we can use the single functions to calculate trophic multifunctionality (Barnes et al, 2018;Potapov, Klarner, Sandmann, Widyastuti, & Scheu, 2019).…”