Nitrogen dynamics in a forest understory following invasion by Microstegium vimineum
Direct resource competition and plant-soil feedbacks have been independently proposed as causative mechanisms explaining how invasive plant species impact native community composition and ecosystem processes. Yet, microbes mediate many nutrient transformations and may influence plant nutrient supply. We evaluated whether an invasive plant influences native communities indirectly by altering plant-microbe competition for nitrogen (N). Using a 15N tracer, we quantified differences in N uptake and allocation in communities with and without Microstegium vimineum, a shade-tolerant, C4 grass that is rapidly invading eastern deciduous forests. We also investigated whether plants or the microbial biomass exhibited preferences for certain N forms (glycine, nitrate, and ammonium) to gain insight about N partitioning in invaded communities. Contrary to expectations, native plants took up slightly more N than M. vimineum, but allocated it primarily to roots, whereas M. vimineum allocated most N to shoots. Nitrogen uptake by the microbial biomass did not vary with invasion status. However, invaded plots had 50% less microbial biomass and the ratio of microbial biomass N to plant biomass N was significantly lower (P < 0.001), suggesting stronger plant-microbe competition for N post-invasion. The microbial biomass exhibited a strong preference for ammonium over glycine and nitrate, regardless of invasion status. By comparison, native plants showed no N preferences and M. vimineum preferred inorganic N species. We interpret our findings as evidence that invasion by M. vimineum leads to substrate changes that ultimately shift microbial community structure and alter rates of microbially mediated N transformations. These changes do not appear to negatively impact native communities, however, and may in fact enhance N acquisition by native species.