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1. Impacts of environmental changes on carbon cycling in terrestrial ecosystems are typically correlated with shifts in the composition, biomass and activity of soil faunal communities. Given the role of soil fauna in decomposition, shifts in soil faunal communities may further modify the carbon dynamics of a perturbed system. 2. To test this possibility, we manipulated soil community composition in model grassland ecosystems and used a 13CO2-tracer to follow the assimilation, retention and pathways of recent, photosynthetically fixed carbon. The community treatments formed a nested hierarchy of functional complexity: (1) microbiota only; (2) microbiota and mesofauna; (3) microbiota, mesofauna and macrofauna. 3. These treatments significantly affected the rate of decline in 13C-label respiration rate and the amount of 13C-label retained by the communities: while the rate of decline in 13C-label respiration rate was greatest in the microbiota treatment, the treatment with both micro- and mesofauna retained less 13C-label than either the more functionally complex or simple treatments. The presence of macrofauna altered the utilization of 13C-label by Collembola and Enchytraeidae: they decreased the mass of 13C-label utilized by Collembola and increased that utilized by Enchytraeidae. 4. Our results suggest that soil community composition may play a key role in regulating the dynamics of recent, photosynthetically fixed carbon. [ABSTRACT FROM AUTHOR]