Yield and N Content of Soybean Seed as Influenced by Rhizobium japonicum Inoculants Possessing the Hydrogenase Characteristic1

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Abstract
Rhizobium japonicum strains possessing the hydrogenase characteristic (Hup+) are capable of forming nodules that recycle the H2 produced during N2 fixation. Experiments were conducted to compare seed yield and N content of soybcan (Glycine max (L.) Merr.) inoculated with Hup+ and Hup‐ strains under field conditions. Hup+ wild‐type strains (USDA 143, USDA 6, USDA 122, USDA 110, USDA 136, and SR), H2 uptake negative (Hup−) wild‐type strains (USDA 117, USDA 135, USDA 120, and USDA 16) and Hup− mutants (SR2 and SR3, derived from Hup+ SR) were used as inoculants. Chehalis silt loam (mesic Cumulic Ultic Haploxeroll) and Ephrata loamy fine sand (mesic Xerollic Camborthid) relatively free of indigenous R. japonicum were used in the experiments. Bacteroid H2 uptake rates and H2 evolution from nodules from inoculated plots indicated no appreciable mixing among treatments. The inoculants applied to these relatively high N soils did not significantly affect seed yield, however, plants inoculated with the Hup+ strains appeared greener, matured later, and in all four experiments produced seed with significantly higher N percentage than plants inoculated with groups of Hup− strains. Average data from all four exepriments showed that Hup+ inoculants increased crude seed protein 8.9%. These results provide further evidence that R. japonicum inoculants for soybeans should possess the capacity to oxidize H2 produced as a by‐product of N2 fixation.

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