Soybean nodule hydrogen metabolism affects soil hydrogen uptake and growth of rotation crops

To test the beneficial effect on the following crop of hydrogen released by Hup − soybean nodules, soybean was inoculated with either a Hup − (JH47) or a Hup + (JH) strain of Bradyrhizobium japonicum. These isogenic strains differ only in that JH47 has a Tn5 inserted in the gene coding for the small hydrogenase subunit which eliminates hydrogenase activity; thus when present in soybean nodules, hydrogen is released into the rhizosphere. Inoculated alfalfa plants were used as the positive control as no hydrogenase activity has ever been found in alfalfa nodules. Soil adjacent to hydrogen releasing (Hup − strain) legume nodules had a significantly higher hydrogen uptake rate than that around the nodules containing the Hup + strain. Barley grown following soybean inoculated with the Hup − strain exhibited an increased grain yield under field conditions.