Effect of H2 evolution on 15N2 fixation, C2H2 reduction and relative efficiency of leguminous symbionts

The (C2H4+ H2(C2H2))/15N2 ratios of 15 clover‐Rhizobium symbionts. soybean, and black medick symbionts were measured. Relative efficiency based on the C2H4 production and on 15N2 incorporation were compared, and in most symbionts there was little difference between the two measures of relative efficiency. Total measurable electron flux through nitrogenase during acetylene reduction and 15N2 incorporation were nearly equal for most symbionts studied. The relative efficiency and the (C2H4+ H2(C2H2))/15N2 ratio showed an inverse correlation. Use of this ratio appears preferable to use of the ratio of C2H2 reduction/N2 reduction. Some evolution of H2 was observed in the presence of C2H2.