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Common buckthorn (Rhamnus cathartica) is an important invasive shrub in North American forests, where it is thought that the plant’s retention of green leaves during autumn canopy senescence helps it succeed in deciduous understory habitat. This trait results in loss of nitrogen (N) in N-rich leaf litter, which has led some workers to suggest buckthorn may foster associative N-fixation in soil. We examined this possibility in an oak woodland understory in eastern Minnesota using the acetylene reduction assay to compare apparent nitrogenase activity in soils collected from beneath buckthorn individuals with soils collected from a canopy species (Quercus spp.), an important understory shrub (Prunus serotina), and non-vegetated areas. Buckthorn and non-buckthorn soils differed in variability of acetylene reduction (AR) rates, with buckthorn values covering a range 10× the range of non-buckthorn soils. Mean AR also differed between buckthorn and non-buckthorn soils, but the direction and magnitude of the difference varied with sampling location. Estimates of N inputs calculated from our data suggest that AR values at the high end of the buckthorn-associated range are biologically significant. Our results represent the first measurement of AR activity associated with common buckthorn, and are consistent with the hypothesis that this plant supports associative N fixation under some conditions. Suggestions for follow-up studies are provided.