Previous studies have determined that introduced plant species can alter microbial community dynamics. This studied evaluated the impacts of a severe infestation of an invasive species, Alliaria petiolata, on microbial community dynamics in a forest soil. We sampled areas of infestation vs. non-infestation within a sand prairie pine plantation in central Illinois using several techniques to evaluate microbial community structure and function. In situ and laboratory incubations of showed increased CO₂ efflux for soils collected from garlic mustard infested sites. Laboratory incubations of soils collected from non-infested sites showed increased CO₂ efflux when soils were amended with garlic mustard tissues or with mustard seed meal (chemically similar to garlic mustard). In a separate study, soils from infested or non-infested sites were exposed to 12 different carbon/nitrogen treatments over 20 days. This resulted in a 2-fold increase in the output of CO₂ for infested soils treated with glucose and a nitrogen source. Nitrogen mineralization rates were evaluated using thirty-two day incubations. There was a significant increase in nitrogen turnover in infested soils. Microbial community dynamics were further evaluated by inoculating soil slurries on non-selective nutrient agar plates and nitrogen free selective agar plates. Direct counts of colonies on non-selective plates revealed an almost ten-fold increase in bacteria for infested soils. These studies suggest that Alliaria petiolata infestations shift microbial community structure by increasing the population of free-living nitrogen fixing bacteria. As garlic mustard is currently invading forests at high rates there is a concern that changes in microbial community structure will alter nutrient availability or competitive dynamics of aboveground biomass and alter nutrient dynamics of North American forests.