Nematode grazing increases the allocation of plant-derived carbon to soil bacteria and saprophytic fungi, and activates bacterial species of the rhizosphere

Microbially driven soil processes are crucial for ecosystem functions such as soil carbon storage and nutrient cycling. The functioning of the microbial community, in turn, largely depends on plant-derived carbon inputs. However, besides the energy supply by plants, the microbial community also underlies a top-down control by higher trophic levels, which has often not been considered when investigating soil microbial processes. Here we investigated the impact of bacterivore and fungivore-grazing nematodes on the net assimilation of carbon into the bacterial and fungal biomass in a microcosm experiment. We manipulated bacterivorous or fungivorous nematodes to obtain different grazing densities. In combination with a 13CO2 pulse labelling we traced the carbon flow from plants into the soil microbial community using PLFA biomarkers. Furthermore, we investigated if nematode grazing, independent of the carbon flow, shifted the active bacterial community using H218O DNA stable isotope probing. We found that the microbial biomass was unaffected by nematode grazers. However, bacterivore nematodes stimulated the incorporation of the 13C label directly into biomarkers of bacteria groups and indirectly into biomarkers of saprophytic fungi. The nematode grazing furthermore enhanced the activity of bacterial taxa typically for the rhizosphere microbiome that utilize mainly root deposits, while bacterial groups that degrade already stored soil organic matter were inhibited. We conclude that grazing of higher trophic levels impacts soil processes and likely the entire soil biota via two mechanisms: firstly, through the constant grazing on soil microorganisms and their constant regrowth, the allocation of plant-derived carbon that enters the soil food web increases; and secondly, it preferentially activates bacteria of the rhizosphere microbiome that putatively contribute to soil organic matter storage.