Invasive plant species affect a range of ecosystem processes but their impact on belowground carbon (C) pools is relatively unexplored. This is particularly true for grass invasions of forested ecosystems. Such invasions may alter both the quantity and quality of forest floor inputs. Dependent on both, two theories, ‘priming’ and ‘preferential substrate utilization’, suggest these changes may decrease, increase, or leave unchanged native plant-derived soil C. Decreases are expected under ‘priming’ theory due to increased soil microbial activity. Under ‘preferential substrate utilization’, either an increase or no change is expected because the invasive plant's inputs are used by the microbial community instead of soil C. Here, we examine how Microstegium vimineum affects belowground C-cycling in a southeastern US forest. Following predictions of priming theory, M. vimineum's presence is associated with decreases in native-derived, C pools. For example, in September 2006 M. vimineum is associated with 24%, 34%, 36%, and 72% declines in total organic, particulate organic matter, mineralizable (a measure of microbially-available C), and microbial biomass C, respectively. Soil C derived from M. vimineum does not compensate for these decreases, meaning that the sum of native- plus invasive-derived C pools is smaller than native-derived pools in uninvaded plots. Supporting our inferences that C-cycling accelerates under invasion, the microbial community is more active per unit biomass: added 13C-glucose is respired more rapidly in invaded plots. Our work suggests that this invader may accelerate C-cycling in forest soils and deplete C stocks. The paucity of studies investigating impacts of grass invasion on C-cycling in forests highlights the need to study further M. vimineum and other invasive grasses to assess their impacts on C sink strength and forest fertility.