Plant-soil feedbacks contribute to the persistence of Bromus inermis in tallgrass prairie

As invasive plants become a greater threat to native ecosystems, we need to improve our understanding of the factors underlying their success and persis-tence. Over the past 30 years, the C3 nonnative plant Bromus inermis (smooth brome) has been spreading throughout the central grasslands in North America. Invasion by this grass has resulted in the local displacement of natives, including the tallgrass species Panicum virgatum (switchgrass). To determine if factors related to resource availability and plant–soil interactions were conferring a com-petitive advantage on smooth brome, field plots were set up under varying nitrogen (N) levels. Plots composed of a 1:1 ratio of smooth brome and switchgrass were located in a restored tallgrass prairie and were randomly assigned one of the following three N levels: (a) NH4NO3 added to in-crease available N, (b) sucrose added to reduce available N, and (c) no additions to serve as control. In addition, soil N status, soil respiration rates, plant growth, and litter decomposition rates were mon-itored. Results indicate that by the 2nd year, the addition of sucrose significantly reduced avail-able soil N and additions of NH4NO3 increased it. Further, smooth brome had greater tiller density, mass, and canopy interception of light on N-en-riched soils, whereas none of these characteristics were stimulated by added N in the case of switch-grass. This suggests that smooth brome may have a competitive advantage on higher-N soils. Smooth-brome plant tissue also had a lower carbon–nitro-gen (C:N) ratio and a higher decomposition rate than switchgrass and thus may cycle N more rap-idly in the plant–soil system. These differences suggest a possible mechanism for the persistence of smooth brome in the tallgrass prairie: Efficient recycling of nutrient-rich litter under patches of smooth brome may confer a competitive advantage that enables it to persist in remnant or restored prairies. Increased N deposition associated with human activity and changing land use may play a critical role in the persistence of smooth brome and other N-philic exotic species. Key words: invasion; tallgrass prairie; plant–soil feedback; nitrogen; smooth brome; Bromus inermis; carbon addition