Free-living nematodes as prey for higher trophic levels of forest soil food webs

Nematodes are the most abundant invertebrates in soils and are key prey in soil food webs. Uncovering their contribution to predator nutrition is essential for understanding the structure of soil food webs and the way energy channels through soil systems. Molecular gut content analysis of consumers of nematodes, such as soil microarthropods, using specifi c DNA markers is a novel approach for studying predator – prey interactions in soil. We designed new specifi c primer pairs (partial 18S rDNA) for individual soil-living bacterial-feeding nematode taxa ( Acrobeloides buetschlii, Panagrellus redivivus, Plectus velox and Plectus minimus ). Primer specifi city was tested against more than 100 non-target soil organisms. Further, we determined how long nematode DNA can be traced in the gut of predators. Potential predators were identifi ed in laboratory experiments including nine soil mite (Oribatida, Gamasina and Uropodina) and ten springtail species (Collembola). Finally, the approach was tested under fi eld conditions by analyzing fi ve mite and three collembola species for feeding on the three target nematode species. Th e results proved the three primer sets to specifi cally amplify DNA of the respective nematode taxa. Detection time of nematode DNA in predators varied with time of prey exposure. Further, consumption of nematodes in the laboratory varied with microarthropod species. Our fi eld study is the fi rst defi nitive proof that free-living nematodes are important prey for a wide range of soil microarthropods including those commonly regarded as detritivores. Overall, the results highlight the eminent role of nematodes as prey in soil food webs and for channelling bacterial carbon to higher trophic levels.