Recent studies demonstrate that microorganisms are sensitive to environmental change, and that their community composition influences ecosystem functioning. However, it is unknown whether microbial composition interacts with the environment to affect the response of ecosystem processes to changing abiotic conditions. To investigate the potential for such interactive effects on leaf litter decomposition, we manipulated microbial composition and three environmental factors predicted to change in the future (moisture, nitrogen availability, and temperature). We isolated fungal and bacterial taxa from leaf litter and used them to construct unique communities. Communities were inoculated into microcosms containing sterile leaf litter and exposed to four environmental treatments (control conditions, increased temperature, decreased moisture, and elevated nitrogen availability). Respiration was tracked over 60 days, and communities were pyrosequenced to assess compositional changes. As hypothesized, composition and environmental treatment interacted to influence respiration rates. In particular, microbial composition interacted more strongly with changing nitrogen availability and less so with changing moisture or temperature. Further, the magnitude of a community's response to a particular environmental change was partly explained by changes in composition over the course of the experiment; microcosms that showed a large change in respiration rate included more taxa whose relative abundance changed as well. Together, these results suggest that information about microbial composition may be more useful for predicting functional responses to some types of environmental changes than others.