Background/Question/Methods Grasslands dominated by C3 species typically differ in soil C and N cycling rates compared to C4-dominated grasslands. However, few studies have examined whether these differences in soil processes are related to differences in microbial community function, abundance, or composition. We performed a laboratory incubation study to determine whether soil microbial communities associated with C3-dominated and C4-dominated grasslands differ in their 1) activity (respiration and extracellular enzyme activities), 2) abundance (microbial biomass), and 3) community composition. In addition, we examined 4) how the microbial communities associated with two soil types (C4 and C3) respond to different litter inputs. We collected soil derived from C4 and C3 species in two successional grasslands and evaluated them for differences in initial soil microbial activity, abundance and composition. Then, we added litter treatments representing a tissue quality gradient from labile to recalcitrant (sucrose, C3 leaves, C4 leaves, oak leaves, and wood) to both soil types from each grassland. Soil respiration rates and microbial community composition (using molecular-based techniques) of both the soil and litter were measured repeatedly for 252 days. Results/Conclusions Although one grassland (TM) showed no significant difference in respiration rates between soil types, the other site (MF) showed consistently higher respiration in C4-dervied soils than C3-derived soils, regardless of litter treatment. Soil respiration responses to litter treatments were similar across sites and soil type. Sucrose additions drastically increased respiration rates within the first week, but the effect was gone by day 46. C4 and C3 litter additions had significantly higher respiration rates than control and wood treatments by day 46, and thereafter remained higher than the control. The oak treatment was intermediate between the grasses and the control. Wood treatments responded slowest, but respiration rates were significantly higher than the control by day 84 (p<0.002). There were few initial differences in enzyme activities at MF except for phenol oxidase, which was significantly higher in C3-derived soils (p=0.003) at both sites. At TM, several enzymes had higher activity in C4-dervied than C3-derived soils, suggesting a priori differences in soil microbial activity. Additional analyses will examine how litter type affected both enzyme activity and microbial community composition through time, and whether the microbial communities in the two soil types differ in their response to litter quality. These results improve our understanding of functional responses to plant community change by focusing on the microbial communities that mediate soil processes.