While it has long been held that invasive plants alter ecosystem processes, the magnitude and direction of these effects have rarely been quantified in situ. We measured the effects of an invasive C₄ grass (Microstegium vimineum) on soil organic matter (SOM) decomposition in a deciduous forest in south-central Indiana, USA. The unique ¹³C signature of the C₄ grass relative to the C₃ trees allowed us to partition soil CO₂ fluxes and estimate M. vimineum effects on decomposition. The magnitude and direction of priming effects hinged on the soil characteristics, which related to the mycorrhizal association of dominant trees. In forest plots dominated by ectomycorrhizal trees, with low nitrogen availability and most SOM in particulate (i.e., unprotected) forms, M. vimineum increased SOM decomposition by 58%. In contrast, in plots dominated by arbuscular mycorrhizal trees, characterized by high nitrogen availability and most SOM in mineral-associated (i.e., protected) forms, M. vimineum decreased decomposition by 14%. Collectively, our results demonstrate that invasive species can play a large role in altering ecosystem processes and suggest that the magnitude and direction of such effects depend on the dominant trees and edaphic characteristics of the stand.