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. Display omitted •Invasive C4 grasses modify SOM decomposition in temperate deciduous forests in the USA.•SOM decomposition hinged on mycorrhizal association of the dominant trees.•In situ measurements revealed enhanced SOM decomposition in ectomycorrhizal systems.•In arbuscular-dominated systems, the invasive grass reduced SOM decomposition.•Invader effects depend on dominant trees and edaphic characteristics of the stand.