PRODUCTIVITY AND TROPHIC-LEVEL BIOMASSES IN A MICROBIAL-BASED SOIL FOOD-WEB

Basic trophic-dynamic models using prey-dependent prey-predator intera ctions typically predict (1) that the limiting factors, resources and predation, should alternate at adjacent trophic levels, and (2) that o nly biomasses at resource limited levels should increase when the prod uctivity of a system is increased. However, experimental studies on aq uatic systems have shown that biomasses tend to respond to increased p roductivity at all trophic levels. To test the predictions in a terres trial 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 co ntained 22 species of bacteria and fungi, the second level a bacterivo rous nematode (Caenorhabditis elegans) and a fungivorous nematode (Aph elenchoides sp.), and the third level a predatory nematode (Prionchulu s punctatus). We sampled the microcosms destructively four times durin g the 22-week experiment to estimate the trophic-level biomasses and s oil NH4+-N concentration. Evolution of CO2 was measured to estimate mi crobial productivity. Microbial productivity was greater and the amoun t of NH4+-N lower in the communities provided with additional energy. The presence of microbivores also resulted in greater microbial produc tivity than in the pure microbial community. The biomass of microbes a nd microbivores increased when provided with supplementary energy inde pendently of the number of trophic levels in the food web, while the b iomass of the predatory nematode did not significantly respond to addi tional energy. The predatory and the bacterial feeding nematodes limit ed the biomass of their resources, whereas the fungal biomass was unaf fected by the fungivore. The results infer that the biomasses at the f irst and second trophic level were simultaneously limited by resources and predation, which contradicts basic prey-dependent models. Prey re fuges provided by soil structure may explain the inability of predator s 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 fu ngal and bacterial channels.