Invasive exotic plants may negatively impact terrestrial ecosystems by displacing native plants and the organisms that depend on them. Recent evidence indicates that some exotic plants are successful invaders because of their impacts on soil microbial processes such as nutrient cycling, but the mechanisms driving these effects are poorly understood. Our objective was to determine the effects of garlic mustard (Alliaria petiolata), an herbaceous forest understory plant currently invading many regions of the USA, on soil nitrogen (N) and phosphorus (P) availability, microbial biomass, and the activities of the microbial enzymes that drive decomposition and soil nutrient cycling, in different locations across Ohio. We hypothesized that garlic mustard alters soil microbial growth, and carbon and nutrient cycling, potentially creating more favorable conditions for itself, and less favorable conditions for native plants.
Results/Conclusions

Analyzing our data based on the presence or absence of garlic mustard, we found that the presence of garlic mustard was often associated with significant differences soil edaphic properties, nutrient pools, microbial biomass, and/or microbial enzyme activities, but that these effects were not consistent, varying across sampling locations and dates. We also found significant negative correlations between these variables and garlic mustard numbers; with low garlic mustard counts, nutrients, microbial biomass, and enzyme activity were high or low, but were almost always low with high garlic mustard counts. Thus it appears that the effects of garlic mustard on soil nutrients and microbes are density dependent.