There are few experimental data on the consequence of varying the composition and diversity of soil animals communities, or soil food-webs, on ecosystem properties. Here, we tested the hypothesis that varying the diversity and composition of soil animals within a trophic group, the microbial-feeders, affects litter decomposition and nutrient flux in grassland. Microcosms containing grassland plant litter were inoculated with individual species of Collembola Folsomia candida, Pseudosinella alba, and Protaphorura armata,and all possible two and three species combinations of these species. Our data show that towards the end of the experiment individual species of Collembola, and especially F. candida, had markedly different, but positive, effects on measures of litter mass loss, microbial activity (CO2 respiration) and the leaching of dissolved organic carbon (DOC) and nitrate-N. Two and three species combinations of Collembola revealed that effects of fauna on ecosystem processes were due to differences in the composition of the collembolan community, rather than the number of species present. In comparison to a treatment that had no fauna, significantly higher rates of litter mass loss, microbial activity, and DOC and nitrate release were detected only in microcosms that contained F. candida. There was no evidence of effects of F. candida in combination with other species, relative to effects of F. candida alone, on the above properties. These findings support the notion that changes in the diversity of microbivorous fauna may not have a predictable effect on decomposition processes rates and that the functioning of the microbial-feeding trophic group is influenced mainly by the physiological attributes of the dominant animal species present, in this case F. candida.