Effect of Elevated Atmospheric Carbon Dioxide Levels on Nodule Relative Efficiency in White Clover

Plant growth, total nitrogenase activity and hydrogen evolution rates were determined for 39-, 47- and 54-day-old white clover (Trifo/ium repe11s L.) plants growing at 350 or 700 ppm CO2 with day/night temperatures of 18/13°C or 28/23°C. The three plant parameters increased with plant age, temperature and CO2 concentration. Nitrogenase activity/g plant dry matter (DM) and hydrogen evolved/ g DM decreased as plants developed, presumably as nitrogen was cycled internally and plants became less reliant on recently fixed nitrogen. Nodule relative efficiency - an expression of the energy flux to nitrogenase relative to hydrogen production - was significantly lower initially in plants grown at 700 ppm CO2 and l8/13°C, but otherwise did not vary between treatments, It is proposed that legume­based agriculture will contribute more ozone-damaging hydrogen to the stratosphere under future atmospheric CO2 levels and that the efficiency of nitrogen fixation will be reduced at lower temperatures.