Spatial and temporal variation of resource allocation in an arable soil drives community structure and biomass of nematodes and their role in the micro-food web

• Across depth nematodes metacommunities were shaped by resource availability and quality.
• Biomass allocation points to stronger predation pressure in the rooted zone.
• Bottom-up control was apparent in the root and of top-down forces in the fungal energy pathway.
• Assemblages at root-free depth were mainly excluded from the processes above.
• Mass transfer of resources by seepage water at fallow periods linked metacommunities.

The assemblages of organisms residing at different soil horizons are important drivers for carbon and nutrient cycle in arable systems, yet knowledge on food web dynamics below the plough zone is sparse. The present study investigated the effects of management practice on the nematode community structure and biomass in the top soil, rooted zone below plough layer, and root free deeper soil in two successive vegetation periods. Resource quality was manipulated by crop plant (maize and wheat) and organic amendment (with/without maize litter), to differently fuel the major soil carbon pathways based on roots, bacteria and fungi. Nematode assemblages were used as model to determine changes in the structure, function and carbon allocation of the soil micro-food web.The biomass of nematode families as well as the fungal to bacterial feeder ratio F/(F + B) assigned a predominance of the bacterial carbon channel in the arable soil. The differences in spatial and temporal availability of major plant resources, i.e. litter (recalcitrant substrate) and rhizodeposits (labile carbon), separated the nematode fauna into meta-communities along the depth profile. The seasonal pattern in population density, and the positive effects of litter amendment predominantly in the top soil, indicated low migration between these patches during crop growth. However, mass-flow as resource subsidy for communities at depth takes place under fallow during winter. Crop type was most important for carbon allocation, with wheat generally resulting in a greater biomass build-up of nematodes. The biomass distribution of trophic groups differed with depth, pointing to a strong bottom-up effect in the root channel, whereas predation likely was the primary regulating force in the fungal channel. Overall, combining nematode community composition with biomass revealed spatially separated micro-food web assemblages, differing in structure and carbon allocation. Thereby transport processes cross community boundaries, linking top soil and subsoil, notably in periods without a crop.