Population- and ecosystem-level effects of predation on microbial-feeding nematodes

We studied the role of nematode predation in the functioning of detrital fo od webs assembled in microcosms. The microcosms contained defaunated humus and litter materials, a diverse microbial community with bacteria, fungi an d protozoa, and a birch (Betula pendula) seedling infected with mycorrhizal fungi. Different levels of top-down control upon microbivorous nematodes w ere set up by assembling food webs either without predators, or in combinat ions with a specialist and a non-specialist predatory mite (Mesostigmata). The nematode community was composed of either (1) three species of bacteriv orous, or (2) three species of fungivorous nematodes or (3) both groups tog ether. After two growing periods for the birch (38 weeks), the microcosms w ere destructively sampled for animal and microbial biomasses, concentration of mineral N in the soil, plant biomass and plant N concentration. The spe cialist predator reduced biomasses of both bacterial- and fungal-feeding ne matodes by more than 50%, whereas the non-specialist predator weakly increa sed the biomass of fungivorous nematodes. Thus, under high predation pressu re, the biomass of microbivores changed as predicted by trophic dynamic mod els assuming strong top-down control and uniformly behaving trophic levels. Despite this, microbial biomass was unaffected by the predators. However, microbial respiration increased slightly in the presence of predators. Assu ming that microbial respiration correlates with microbial productivity, the increase in microbial respiration indicates a cascading productivity regul ation. The composition of the microbivore community had only a minor effect on the outcome of the top-down control on microbes. The > 50% reduction in nematode biomass and respiration coincided with < 16% increase in microbia l respiration and did not affect microbial biomass. Presence of the special ist predator slightly reduced soil NH4+ concentration in communities with f ungivore nematodes but plant growth and N uptake remained unchanged. Thus, the structure of the community only weakly controlled nutrient mineralisati on.