Hydrogen evolution from root nodules of Trifolium repens and Medicago sativa plants grown under elevated atmospheric CO 2

Abstract Nitrogenase activity and hydrogen (H 2 ) evolution from nodules of Trifolium repens L. and Medicago sativa L. were measured on plants grown under 700 or 350 μl/1 atmospheric CO 2 and day/night temperatures of 18/13°C or 28/23°C. Assays were done after 39, 47, and 54 days' exposure to the treatments. In Trifolium , nitrogenase activity/plant was stimulated by elevated CO 2 and higher temperatures but in Medicago only temperature had an effect. Hydrogen emission/plant was greater in Trifolium plants grown at 700 °l/1 CO 2 than in plants at 350 °l/1 CO 2 , but in Medicago , H 2 emission rates did not respond to elevated CO 2 . Elevated CO 2 reduced nodule relative efficiency (RE) in 39‐day‐old Trifolium plants growing at 18/13°C, but not under other conditions. It is concluded that predicted future CO 2 concentration will lead to a greater contribution from legume nitrogen (N) fixation to global H 2 sources. The magnitude of the increase will be influenced by the legume species involved and temperature.