Sensitivity of primary production to changes in the architecture of belowground food webs

The proposed mechanisms for the species diversity-function relationship in plant communities stress the recognition of functional properties of specie s, and interactions between plants and soil processes. As resource availabi lity to plants is influenced by the architecture of decomposer food webs, i t has been hypothesised that the diversity of decomposers can also control ecosystem processes, including primary production. We manipulated the complexity of soil animal communities in a miniecosystem experiment in which a boreal forest floor with birch seedlings infected wi th mycorrhizal fungi was created. The soil animal diversity ranged from zer o to typical species richness of soil fauna (approximately 50 taxa) in coni ferous forests. Between these extremes was a nested factorial design with m anually assembled communities consisting of two sets of one-species, and tw o sets of five-species animal communities within Fungivorous and microbi-de tritivorous trophic groups. To investigate the role of predators in system functioning, the miniecosystems with fungivorous and microbidetritivorous f auna were established either with or without mesostigmatid mites as top pre dators. The miniecosystems were incubated in a climate chamber with varying illumination and temperature regimes for 40 weeks. Our experiment provides evidence that primary productivity is generally ins ensitive to variation at the species level or even at the level of trophic groups. Although top predators generally reduced prey population size, no e ffect was found on system functioning. However; the removal of microbe- or detritus-feeding fauna, especially the microbi-detritivore Cognettia sphagn etorum reduced plant N uptake and accumulation of plant biomass. The functi onal importance of soil fauna was inversely related to the trophic position of the group. Our results suggest that ecosystem functioning is robust aga inst species extinctions in belowground food webs, and that primary product ion is predominantly controlled by organisms at low trophic positions in th e decomposer food web.