Influence of decomposer food web structure and nitrogen availability on plant growth

We studied the sensitivity of soil microbial communities and ecosystem proc esses to variation in the vertical and horizontal structure of decomposer f ood web under nitrogen poor and N-enriched conditions. Microcosms with humu s and litter layer of boreal forest floor, birch seedlings infected with my corrhizal fungi, and decomposer food webs with differing trophic group and species composition of soil fauna were constructed. During the second growi ng period for the birch, we irrigated half of the microcosms with urea solu tion, and the other half with de-ionised water to create two levels of N co ncentration in the substrate. During the experiment night time respirations of the microcosms were measured, and the water leached through the microco sms was analysed for concentration of mineral N, and nematode numbers. The microcosms were destructively sampled after 37 weeks for plant biomass and N uptake, structure of soil animal and microbial community (indicated by PL FA profiles), and physical and chemical properties of the humus and litter materials. Predatory mites and nematodes had a negative influence on the bi omass of their microbivorous and microbi-detritivorous prey, and microbi-de tritivores affected the biomass and community structure of microbes (indica ted by PLFA-analysis). Moreover, predatory mites and nematodes increased mi crobial biomass and changed the microbial community structure. The decompos er food web structure affected also N uptake and growth of plants. Microbi- detritivorous fauna had a positive effect, whereas predators of microbial a nd detritus feeding fauna exerted a negative influence on plant N uptake an d biomass production. The impact of a trophic group on the microbes and pla nt was also strongly dependent on species composition within the group. Nit rogen addition magnified the influence of food web structure on microbial b iomass and plant N uptake. We suggest that addition of urea-N to the soil m odified the animal-microbe interaction by increasing microbial growth and a ltering community structure of microbes. The presence of microbi-detritivor es and predators reduced loss of carbon from the microcosms, and the food w eb structure influenced also water holding capacity of the materials. The c hanges in plant growth, nutrient cycling, size of N and C pools, and in the physical properties of the soil emphasize the importance and diversity of indirect consequences of decomposer food web structure.