Soil animal food webs in temperate forests: effects of forest management on trophic structure as indicated by molecular gut content, stable isotope and fatty acid analyses

The soil system comprises a high diversity of coexisting species interacting in close association. Generally, soil animals are assumed to be trophic generalists feeding on resources of even different trophic levels. The complex structure of soil habitats hampers locating specific resources and this results in feeding on a broad range of resources. It has been assumed that most decomposers rely on labile resources of high nutritional value, such as high quality litter and microorganisms. However, there is increasing evidence that recalcitrant carbon sources being physically and chemically stabilised in soil aggregates are mobilised by certain decomposer groups. However, the soil system with its multitude of trophic interactions and ways of resource utilisation by consumers is still poorly understood. The aim of this thesis was to examine utilisation of food resources and energy channels of important key invertebrate taxa as representatives of different trophic levels in the soil food web. We aimed at identifying trophic niches of closely related species and the driving factors for its variations. Classical techniques, such as microscopic identification of gut contents and feeding experiments, examining trophic interactions and energy flows in soil food webs are increasingly replaced by novel molecular techniques. We used stable isotope, fatty acid and molecular gut content analyses providing reliable time-integrated and indirect methods to unravel trophic linkages between cryptic organisms in opaque soil systems.&#13;\nIn Chapter 2 We investigated feeding strategies of two centipede species to identify their trophic niches and variations therein with forest type. In combination with body size measurements, we conducted stable isotope and fatty acid analyses on centipedes allowing insight into trophic levels and the utilisation of basal resources. The results indicated centipedes to occupy distinct trophic niches with the smaller species relying more on bacterial resources and root-derived carbon as compared to the larger species. Differences in trophic niches were more pronounced in coniferous than in beech forests. The results suggest that habitat structure and body size may act as determinants for variations in trophic niches of soil arthropod predators.&#13;\nComplementing the study in Chapter 2, in Chapter 3 we investigated centipede gut contents of the same individuals for three prey taxa (collembolans, dipterans and earthworms) applying specific PCR assays. In this approach insight into mechanisms was deepened by including a second region, the Schorfheide-Chorin, and factors, such as prey abundance, litter mass and soil pH in the analyses. The results indicated that forest type did not affect prey choice, but factors representing habitat characteristics, such as litter mass and soil pH, were responsible for differences. Similar to Chapter 2 the results suggest body size and habitat structure to act as important factors in trophic niche differentiation.&#13;\nIn Chapter 4 we investigated the use of carbon resources of ecologically different earthworm species being abundant decomposers in soil and preferred prey of centipedes (Chapters 2 and 3). Using compound-specific 13C stable isotope analysis of fatty acids, we related earthworm 13C signatures to resources of different stability. Endogeic earthworm species, in contrast to epigeic species, were found to predominantly assimilate recalcitrant carbon sources. Results also pointed to the utilisation of stable resources associated with clay attached to soil aggregates. This study revealed specific stable soil carbon pools to essentially contribute to nutrition of endogeic earthworm species.&#13;\nIn Chapter 5 we combined fatty acid and stable isotope analyses to investigate trophic niches of ecologically different collembolan species and their association with litter and root resources. Furthermore, we investigated feeding interactions by relating collembolan fatty acids to that of litter-dwelling microorganisms, which are regarded as major food resource. Results indicate that collembolans occupy several trophic niches acting as decomposers of litter- and root-derived resources as well as as predators. Moreover, we found collembolan fatty acid patterns to not correspond with that of microorganisms suggesting that they feed on specific microsites in soil keeping their trophic niche constant.&#13;\nThis thesis allowed insight into soil animals as generalistic species with distinct trophic niches. The three soil animal taxa occupying different trophic levels were documented to differ in strategies of resource utilisation and variation therein. Furthermore, the studies challenge the view of fixed energy channels with definite associated animal groups in soil food webs. Overall, this thesis considerably contributes to the understanding of soil food web structure and functioning. Using novel techniques it presented insights into trophic interactions on different trophic levels in an elegant way.

Topic: Isotope Analysis in Ecology
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences