CARBON-ISOTOPE COMPOSITION OF N-2-FIXING AND N-FERTILIZED LEGUMES ALONG AN ELEVATIONAL GRADIENT

Dinitrogen-fixing legumes are frequently assumed to be less water-use efficient than plants utilizing soil mineral N, because of the high re spiratory requirements for driving N-2 fixation. However, since respir ation is assumed not to discriminate against C-13, any differences in water-use efficiency exclusively due to respiration should not be appa rent in carbon isotope discrimination (a) values. Our objective was to determine if the source of N (N-2 fixation versus soil N) had any eff ect on a of field-grown grain legumes grown at different elevations. F our legume species, Glycine max, Phaseolus lunatus, P. vulgar's, and V igna unguiculata, were grown on five field sites spanning a 633 m elev ational gradient on the island of Maul, Hawaii. The legumes were eithe r inoculated with a mixture of three effective strains of rhizobia or fertilized weekly with urea at 100 kg N ha(-1) in an attempt to comple tely suppress symbiotic Ne-fixing activity. In 14 of 20 analyses of st over and 12 of 15 analyses of seed Delta values were significantly hig her (p=0.10) in the inoculated plants than the N-fertilized plants. Ni trogen concentrations were generally higher in the fertilized treatmen ts than the inoculated treatments. The different a values obtained dep ending on N-source may have implications in using a as an indicator of water-use efficiency or yield potential of legumes.