Hydrogen evolved by nitrogenase may be recycled by a hydrogenase present in some legume nodules. Anoka and Portage cultivars of soybeans were inoculated with each of 8 and 24 strains, respectively, of Rhizobium japonicum and surveyed for H2 evolution and C2H2 reduction rates nodule weight, and plant dry weight. Six of the strains (3Ilb 110, USDA 122, USDA 136, 3Ilb 6, 3Ilb 142, and 3Ilb 143) which exhibited no H2 evolution in air were shown to take up H2. The relative efficiencies of nitrogenase energy utilization based on C2H2 reduction rates of nodules relative efficiences of nitrogenase energy utilization based on C2H2 reduction rates of nodules ranged from 0.96 to 1.0 for the six strains. Nodules formed by strain WA 5099-1-1 evolved small amounts of H2 in air and had a relative efficiency of 0.92. Nodules formed by the remaining 25 strains had relative efficiencies ranging from 0.41 to 0.80. A H2-evolving (3Ilb 123) and non-H2-evolving (3Ilb 143) strain were tested on seven soybean cultivars to determine the effect on the expression of hydrogenase. Nodules formed by strain 3Ilb 143 exhibited an efficiency of 1.0 on the following cultivars: Amsoy 71, Anoka, Bonus, Clark 63, Kent, Peking, and Portage. Relative efficiencies from 0.63 to 0.77 were determined for the five cultivars nodulated by strain 3Ilb 123. From the experiments with these cultivars, the capacity to recycle H2 produced from the nitrogenase system appears to be determined by the R. japonicum strain.

Red alder (Alnus rubra Bong.) was grown in sand culture in the greenhouse to obtain data on the effects of cadmium (Cd) on a symbiotic nitrogen (N) fixation system which contributes to the fertility of forest soils. Treatment of red alder seedlings for 11 weeks with 0.545 to 136 µM CdCl2 in N‐free nutrient solution decreased in situ nitrogenase activity 25 to 89%, N fixation 23 to 98%, and the number of nodules per plant 29 to 74%, when compared to plants receiving no Cd. Similar reductions were observed in plant growth. Treatment with 0.054 to 0.545 µM CdCl2 decreased nitrogenase activity 6 to 31%. When non‐nodulated A. rubra seedlings were inoculated in the presence of 0.054 and 0.109 µM CdCl2, N fixation decreased 52 and 89%, respectively. Plant growth decreased in a similar fashion. When non‐nodulated seedlings were given 0.545 µM CdCl2 and 6 mM NH4NO3 or Ca(NO3)2, nitrate reductase activity decreased 22 and 24%, respectively, when compared to plants receiving no CdCl2. Nitrogen accretion and plant dry weights were decreased by treatments in which plants were supplied with Ca(NO3)2 and 0.273 and 0.545 µM CdCl2, when compared to controls without CdCl2. These results suggest that Cd can inhibit nitrogenase activity and therefore N fixation. Growth, nodulation, and nitrate reductase activity are also inhibited by Cd.