Trophic cascades in simplified soil food webs and consequences for carbon cycling

The soil microbiome is widely recognized as an important driver of soil carbon (C) cycling but the role of soil fauna is largely overlooked. It is proven that microbivores, e.g., bacterivorous nematodes, can alter the microbiome composition and activity, but the microbivores themselves can be controlled by their predators. How these higher trophic interactions impact the soil microbiome, through trophic cascades, remains to be investigated, as well as the consequences for soil C cycling. We tested the existence and direction of trophic cascades in soil food webs by manipulating the presence of bacterivorous-dominated nematode communities and their predators (nematode-feeding mite Gaeolaelaps aculeifer) in a full factorial design. After microbial re-inoculation of sterilized grassland soil and soil food web reconstruction, we monitored the decomposition of added grass litter and associated C mineralization during five weeks. We also characterized the soil microbiome composition by phospholipid fatty acid analysis and 16S sequencing. While the presence of nematodes mostly did not affect C cycling, the addition of predators decreased C mineralization by 10 %. However, litter decomposition rates were unaffected by soil food web composition. Taken together, these results suggest that the presence of predators may result in enhanced soil C stabilization, at least in the short term. The presence of predators also resulted in a shift in microbiome composition, notably with higher Gram(+):Gram(-) ratios, but no change in microbial biomass, suggesting that nematodes may shift their diet because of predation. Our results confirm that the effects of nematodes and their predators on the soil microbiome are not additive and that predators can alter soil C cycling trough trophic cascades. As predators are often sensitive to land use change and intensification, these findings suggest that loss of belowground predators may result in increased C losses following litter decomposition.