Interplay of omnivory, energy channels and C availability in a microbial-based soil food web

To study the effects of omnivory on the structure and function of soil food webs and on the control of trophic-level biomasses in soil, two food webs were established in microcosms. The first one contained fungi, bacteria, a fungivorous nematode (Aphelanchoides saprophilus) and a bacterivorous nemat ode (Caenorhabditis elegans), and the second one fungi, bacteria, the fungi vore and an omnivorous nematode (Mesodiplogaster sp.) feeding on both bacte ria and the fungivore. Half of the replicates of each food web received add itional glucose. The microcosms were sampled destructively at 5, 9, 13 and 19 weeks to estimate the biomass of microbes and nematodes and the soil NH4 +-N concentration. The evolution of CO2 was measured to assess microbial re spiration. Microbial respiration was increased and soil NH4+-N concentratio n decreased by the addition of glucose, whereas neither was affected by the food-web structure. Supplementary energy increased the biomass of fungi an d the fungivore, but decreased the biomass of bacteria, the bacterivore and the omnivore. The omnivore achieved greater biomass than the bacterivore a nd reduced the bacterial biomass less than the bacterivore. The biomass of the fungivore was smaller in the presence of the omnivore than in the prese nce of the bacterivore at three sampling occasions. Fungal biomass was not affected by food-web structure. The results show that the effects of the om nivore were restricted to its resources, whereas more remote organisms and soil processes were not substantially influenced. The results also indicate that the presence of an omnivore does not necessarily alter the control of populations as compared with a food web containing distinct trophic levels , and that the fungal and bacterial channels may respond differently to cha nges in energy supply.