Invasive species are one of the major threats to the integrity of ecosystems. Aggressive plant invaders such as Typha x glauca threaten wetland ecosystems, because they often cause shifts in ecosystem functioning such as modification of biogeochemical cycles and nutrient budgets. Increases in biologically-available nutrients have been shown to accelerate losses of plant diversity. Cheboygan Marsh on the coast of Lake Huron is currently undergoing invasion by Typha. Soils in Typha-dominated areas of the marsh have revealed nitrogen concentrations significantly higher than in areas dominated by native species. In this study, we investigated whether higher concentrations of soil nitrogen could be explained by increased rates of soil microbial nitrogen fixation in Typha-dominated areas.
During the 2007 growing season, we measured rates of nitrogen fixation in Cheboygan Marsh (using the acetylene reduction method) in uninvaded areas of the marsh, recently-invaded areas, and long-established, dense Typha monotypes. We found significantly higher nitrogen fixation rates in Typha soils (p<0.001) than in native or newly-invaded soils, agreeing with our expectations. We believe high production of litter by Typha and their carbon-rich root exudates may fuel nitrogen fixing bacteria and enhance fixation, resulting in a positive feedback loop that benefits the invader.