The purpose of this study was to determine whether changes in microbial community composition alter soil enzyme activity and ultimately soil carbon transformation. Tropical forest and plantation soils were incubated at 3 temperatures (5, 20, and 35 °C) for 103 days in the laboratory. A natural isotopic signature allowed for the determination of microbial utilization of young and old C from the delta ¹³C signal in respired CO₂. The composition of the microbial community was characterized by two methods: phospholipid fatty acid analysis (PLFA) and intergenic transcribed spacer region analysis (ITS) of whole soil DNA. The activities of soil enzymes involved in carbon degradation were assayed using standardized fluorometric and spectroscopic-based methods. Carbon dioxide production was assayed over the incubation period in order to model the kinetics of microbial respiration using a variable-substrate-pool approach. Analysis of the kinetics of respiration indicated that the size of the available substrate pool increased at higher temperatures, with little effect on the decomposition rate constant. Isotope data revealed that microbial communities accessed older C at higher temperatures. Both PLFA and ITS patterns revealed differences in microbial community composition in soils that had been incubated at different temperatures. Assays of enzyme activity performed at the end of the incubation period revealed that, in general, the highest standardized enzyme activities occurred in soil from the lowest temperature incubations (5 °C). It appears that, as soil temperature changes, the composition of the microbial community changes, the physiological capabilities of the community change, and the soil carbon substrates utilized by these communities change. Thus, soil carbon cycling under changing climatic regimes may be effected by compositional and physiological adaptation of the soil microbial community to new conditions.