Positive feedbacks on M. vimineum growth through alteration of nitrogen cycling
Understanding the mechanisms by which invasive plants maintain monodominance through positive plant-soil feedbacks is essential to the long-term goal of restoring native species. To date, most research has focused on the role of plant alteration of microbial symbionts and pathogens, with limited consideration of how invasives influence microbially-mediated nutrient transformations and nutrient availability. We quantified the degree to which the invasive grass M. vimineum alters soil N cycling in two separate field experiments, and then manipulated N form and availability in a mesocosm experiment to examine whether such alterations contribute to the success of this prolific invader. Across the growing season, net nitrification rates were 124% greater in plots with M. vimineum in the understory of a mixed hardwood forest than in adjacent reference plots (p = 0.097), and 64% greater in common garden plots seeded with M. vimineum relative to plots seeded with native perennials (p = 0.001). In the mesocosms, M. vimineum productivity was enhanced in pots receiving equivalent amounts of N in the form of nitrate relative to ammonium (p = 0.013), in contrast to native perennials which showed no preference for N form. Our results demonstrate that persistence of M. vimineum monodominance in the understory of eastern forests may result, in part, from positive feedbacks associated with this species’ enhancement of nitrification rates and preferential utilization of nitrate.