Effects of Different Grass Species on Nitrogen Fixation by White Clover Under Conditions of Elevated Carbon Dioxide and Temperature

Growing clover in binary mixtures reduced clover shoot dry matter (DM) and nitrogenase activity compared with clover in monoculture. Nitrogenase activity/g clover shoot OM increased in the mixtures to a similar extent with 5 different companion grass species. Elevated atmospheric 002 increased nitrogenase activity/g clover OM in the mixtures but specific nitrogenase activity was not changed with increased temperature. Using nitrogenase activity/unit grass OM as a competitive index showed that elevated CO2 reduced competitive stress on white clover. Two tropical grasses (Pennisetum clandestinum and Paspalum dilatatum) were less competitive to white clover at 18/13°C than 3 temperate species (Agrostis capilaris, Lolium perenne, Festuca arrudinacea). At 28/23°C, P. clandestinum, P. dllatamm, and F. arundinacea were more competitive than A. capillaris or L. perenne.