Fluxes of root-derived carbon into the nematode micro-food web of an arable soil

Organic carbon (C) released from living roots forms a major resource for microorganisms controlling energy, C pathways and, hence, food web structure and dynamics. However, knowledge on quantitative C fluxes into food web compartments is scarce. Nematodes, with functional groups at each trophic level, served as a model community for assessing root C fluxes into the micro-food web. Maize, grown on soil cores from an arable field, was pulse-labeled with 14CO2 followed by sampling 2, 5, 10 and 16days after labeling. Nematode population density, community structure, trophic groups and their 14C activities were analyzed. Overall, 55 genera of 22 families were detected. Plant-feeders, which had the highest density, showed the fastest and highest incorporation of root C. Bacterial-feeders incorporated more root-derived 14C than fungal-feeders. This was consistent with a bacterial- to fungal-feeder-ratio of 0.63 and a moderate to low Channel Index (average 38), a nematode faunal index that assigns the magnitude of carbon flow via the bacterial or fungal channel, both indicating a major energy flux in the bacterial decomposition pathway. Predators and omnivores showed low incorporation of root-derived C, pointing to a basal food web structure with short food chains and low energy transfer to higher trophic levels. Combining 14C tracing with taxonomic identification of nematodes allowed quantification of root C fluxes into food web compartments. The incorporation of root C into nematodes was small (~0.1% of that in microbial biomass), yet forms an important part of belowground C channeling as it links microbial and faunal food web.