Soil food webs
Understanding the trophic organisation of decomposer systems is the prerequisite to uncover the driving forces for species composition and the causal mechanisms responsible for the spatial and temporal variation of soil animal populations. A wide spectrum of animal groups is investigated, with a focus on earthworms, oribatid mites, springtails and protozoa. Methods include the quantitative measure of density and biomass of soil animal populations and microorganisms, the analysis of natural variations in stable isotope ratios (13C, 15N) and the use of lipids and gene fragments as markers in food chains.
Stable isotope analysis
The elements carbon and nitrogen each occur in two stable isotopes (13C, 12C, 15N and 14N). In organisms preferentially lighter isotopes are metabolised and excreted resulting in the enrichment of consumer tissue in heavier isotopes. In particular the 15N-to-14N ratio increases with higher trophic levels in food webs, whereas the 13C-to-12C ratio remains more stable. This allows evaluating the trophic level of species (N isotopes) and main food resources (C isotopes) of animals in soil. Further, stable isotopes can be used to label nutrients and carbon, and track their channelling through food webs (pulse labelling).
Fatty acid analysis
Fatty acids form part of membranes of living cells (phospholipids) and are used for energy storage in the fat body of animal consumers (neutral lipids). In particular prokaryotes but also major eukaryote taxa differ in their membrane lipids. These lipids may be deposited without change in the fat body of consumers leaving signatures of their diet (‘dietary routing’). Such marker fatty acids may originate exclusively from the diet of consumers (absolute markers) or may be enriched by consuming certain diets (relative markers). Thereby, fatty acids can be used for unravelling trophic relationships and identifying basal resources of soil food webs. Further, phospholipids in litter and soil can be used to characterize the structure of soil microbial communities.
Molecular gut content analysis
Detecting prey remains in a predator's gut allows identifying and quantifying trophic links within food webs. As many invertebrate predators do not ingest hard prey remains, microscopic analysis of gut contents is of limited use. Species or taxon specific molecular markers allow to detect and identify small remnants of prey DNA even several days post feeding. The method is a powerful tool to uncover trophic relationships and prey preferences of generalist predators particularly in opaque habitats such as the soil.