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

Abstract

Will changes in nutritive values and N-relations offset initial gains in forage productivity under potential climate change observed for grass-legume pastures of south-eastern Australia? Herbage nutritive value and symbiotic nitrogen fixation were investigated for pure and mixed swards of subterranean clover (Trifolium subterraneum) and phalaris (Phalaris aquatica) in field tunnels at ambient and 690 μmol mol-1 CO2 concentrations and at ambient and warmed (+3.4°C) air temperatures. Elevated CO2 increased the nonstructural carbohydrate content of herbage whereas warming tended to decrease it. These effects were mainly on soluble carbohydrates in phalaris and starch in clover herbage. The N concentration of both species was decreased by elevated CO2 but unaffected by warming. The proportion of clover-N derived from N2 fixation was increased by 12% under elevated CO2 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 soluble carbohydrate content. Longer term effects on nutritive value and N-dynamics via species persistence and competition require further study.