Nutritive value and the nitrogen dynamics of Trifolium subterraneum and Phalaris aquatica under warmer, high CO2 conditions

Will changes in nutritive values and N-relations offset initial gains in fo rage productivity under potential climate change observed for grass-legume pastures of south-eastern Australia? Herbage nutritive value and symbiotic nitrogen fixation were investigated f or pure and mixed swards of subterranean clover (Trifolium subterraneum) an d phalaris (Phalaris aquatica) in field tunnels at ambient and 690 mu mol m ol(-1) CO2 concentrations and at ambient and warmed (+3.4 degreesC) air tem peratures. Elevated CO2 increased the nonstructural carbohydrate content of herbage wh ereas warming tended to decrease it. These effects were mainly on soluble c arbohydrates in phalaris and starch in clover herbage. The N concentration of both species was decreased by elevated CO2 but unaffected by warming. Th e proportion of clover-N derived from N-2 fixation was increased by 12% und er elevated CO, but decreased by 6% under warming. Concurrent warming and high-CO2 conditions are expected to lead to improved herbage nutritive value for ruminants due to increased nonstructural solub le carbohydrate content. Longer term effects on nutritive value and N-dynam ics via species persistence and competition require further study.