Changes in trophic structure of decomposer communities with land use in Central European temperate forests

Soil animals affect major ecosystem processes, such as nutrient cycling and soil formation, and thereby play a central role in aboveground - belowground interactions. Soil animal food webs are complex trophic networks comprising a diversity of species and resources. Due to the fact that soil animals are exceptionally diverse and feeding interactions in the soil habitat are obscured from direct observation, the structure and functioning of soil animal food webs are little understood. The present thesis focuses on the composition and trophic ecology of the soil animal food web of Central European forests and the mechanisms responsible for changes with forest management. In the first study (Chapter 2) we analyzed effects of forest type and management on species composition and functional structure of soil animal communities in a replicated design including three different study regions. The results show that the turnover of species between forest sites is primarily driven by regional abiotic factors, such as soil pH, but additionally relates to forest type and management. Soil animal communities of coniferous forests differ most from those of unmanaged beech forests. The analysis of habitat conditions suggests that this is due to increasing disturbance of the microbial decomposer community and decreasing rates of litter decomposition with forest management intensity. However, little changes in functional structure and diversity of soil animal communities with forest type indicate that the architecture of soil animal food webs is buffered against disturbances by forest land use. The second study (Chapter 3) investigated dietary changes of soil animals with forest type and management by analyzing natural variations in stable isotope signatures. The results show that trophic niches of soil animal species are little affected by management of beech forests. However, the signatures suggest that soil animals shift towards utilizing more litter derived nutrients in coniferous forests, with the pattern being consistent across different species and trophic levels. Furthermore, the results indicate that the importance of litter resources for soil animals increases with the thickness of leaf litter layers but is little related to litter C-to-N ratios and concentrations of litter microorganisms. Presumably, thick leaf litter layers hamper the access of soil animals to root derived resources and thereby foster the utilization of leaf litter in coniferous forests. Overall, the results suggest that structural characteristics of organic layers outweigh litter quality as driving factor of soil animal food webs. The third study (Chapter 4) focuses on the trophic ecology of mesostigmatid mites (Mesostigmata) in old-growth beech forests. We investigated natural stable isotope signatures of 40 species common in Central European forests. The results show that the prey spectrum of Mesostigmata comprises mostly secondary decomposers with primary decomposers and intra-guild prey being of little importance. Distinct prey spectra of species suggest that trophic niche differentiation facilitates species richness of this diverse predator group. The signatures furthermore indicate that trophic niches are consistent across different sexes and ontogenetic stages. Presumably, Mesostigmata species heavily rely on bacterial and root feeding nematodes as prey and thereby occupy trophic niches inaccessible to larger sized predators. In contrast to larger predators, the trophic position of Mesostigmata is independent from body size, most likely due to the variable body size of their prey. By uncovering mechanisms responsible for changes in soil animal communities with forest type and details on the trophic ecology of an important predator group, results of this thesis represent major advances in understanding soil animal food webs.