Productivity and trophic-level biomasses in a microbial-based soil food web

Basic trophic-dynamic models using prey-dependent prey-predator interactions typically predict (1) that the limiting factors, resources and predation, should alternate at adjacent trophic levels, and (2) that only biomasses at resource limited levels should increase when the productivity of a system is increased. However, experimental studies on aquatic systems have shown that biomasses tend to respond to increased productivity at all trophic levels. To test the predictions in a terrestrial environment, we performed an experiment with a soil food web. We established three food webs with one, two, or three trophic levels in microcosms containing an initially sterilized mixture of leaf litter and raw humus, and increased the productivity with additional glucose in half of the replicates of each food web. The first trophic level contained 22 species of bacteria and fungi, the second level a bacterivorous nematode (Caenorhabditis elegans) and a fungivorous nematode (Aphelenchoides sp.), and the third level a predatory nematode (Prionchulus punctatus). We sampled the microcosms destructively four times during the 22-week experiment to estimate the trophic-level biomasses and soil NH4+-N concentration. Evolution of CO2 was measured to estimate microbial productivity. Microbial productivity was greater and the amount of NH4+-N lower in the communities provided with additional energy. The presence of microbivores also resulted in greater microbial productivity than in the pure microbial community. The biomass of microbes and microbivores increased when provided with supplementary energy independently of the number of trophic levels in the food web, while the biomass of the predatory nematode did not significantly respond to additional energy. The predatory and the bacterial feeding nematodes limited the biomass of their resources, whereas the fungal biomass was unaffected by the fungivore. The results infer that the biomasses at the first and second trophic level were simultaneously limited by resources and predation, which contradicts basic prey-dependent models. Prey refuges provided by soil structure may explain the inability of predators to control their preys as effectively as predicted by these models. Moreover. the results suggest that the nature of trophic interactions may differ at the bottom and top of soil food webs, and between the fungal and bacterial channels.