Carbon flow in belowground food webs assessed by isotope tracers

Soil food webs are complex networks that consist of several trophic levels and taxonomic groups including soil microorganisms, protists, nematodes, annelids and soil arthropods. Interactions between and within trophic levels and taxonomic groups regulate important ecosystem functions such as the cycling of carbon (C) and nutrients, with soil microorganisms channeling resources from the base of the food web to higher trophic levels of meso- and macrofauna decomposers and predators. Root exudates and decomposing plant residues are the major basal resources of C, and recent research highlighted the dominant role of root C for forest soil food webs. However, despite the large importance of agroecosystems for the global energy budget, channeling of C and nutrients in arable systems still is little understood. The present thesis focused on the flux of shoot residue- and root-derived C within arable soil food webs. In three field experiments I investigated soil animal community responses and the incorporation of shoot residue- and root-derived C into soil meso- and macrofauna at the species level. In the experiment presented in Chapter 2 I investigated the effects of aboveground resources on abundances and community composition of the soil animal food web of two arable fields planted with wheat and maize, respectively, by adding hackled maize shoot residues to the fields. Addition of shoot residue-derived resources did not affect the soil animal food web, suggesting that aboveground resources are of minor importance for soil animal communities. However, independent of shoot residue addition, the abundance and diversity were much higher and more fluctuating in wheat as compared to maize fields, due to more favourable habitat conditions and more pronounced pulses of root-derived resources in form of root exudates and decomposing root residues in wheat. Taking advantage of the differences in natural 13C/12C signatures of wheat and maize I tracked the incorporation of shoot residue- and root-derived resources into the body tissue of soil animals (Chapter 3). In general, one year after the start of the experiment incorporation of root-derived resources exceeded that of shoot residue-derived resources by a factor of two, highlighting the importance of root-derived resources for arable soil food webs. Furthermore, at higher taxonomic resolution only few soil animal taxa predominantly relied on shoot residue-derived resources, while approximately 30% preferred root-derived resources, and half of the taxa were generalist feeders incorporating both shoot residue- and root-derived resources. In a pulse labelling experiment (Chapter 4) I investigated the short-term incorporation of root-derived C and fertilizer N into the soil animal food web using 13CO2 and K15NO3. Ratios of 13C/12C and 15N/14N were measured in bulk soil, maize shoots, roots and meso- and macrofauna, plus 13C/12C in nematodes and microbial phospholipid fatty acids over a period of 25 days. Both 13C and 15N were incorporated into all compartments of the soil food web, with saprotrophic fungi incorporating by far the highest amounts of 13C, while higher trophic levels, i.e. nematodes and meso- and macrofauna, were less enriched. This suggests a prominent role of saprotrophic fungi in C and nutrient cycling in arable fields, but also that the majority of root-derived C remains locked up at the base of the food web. Further, higher amounts of 13C in predators than decomposers of meso- and macrofauna indicate a prominent role of nematodes for transferring resources to higher trophic levels. Overall, the present thesis highlights the importance of root-derived as compared to shoot residue-derived resources for arable soil food webs, thereby contributing to a better understanding of C and nutrient fluxes in agroecosystems.