Field assessment of supernodulating genotypes of soybean for yield, N2 fixation and benefit to subsequent crops

Development and commercial use of cultivars of soybean with high N2 fixation activity, even in the presence of moderate to high amounts of soil nitrate, may result in larger inputs of N into the soil-plant system. We report experiments from a 6-year study (1988–93) to evaluate yield, N2 fixation and effects on subsequent cereal crops of intermediate supernodulating (designated 2 × nodulation phenotype) and extreme supernodulating mutants of Bragg soybean (6 × nodulation phenotype), a non-nodulating mutant of Bragg (0 × nodulation phenotype), genotypes derived from the mutants and commercial cultivars. Plants were grown at two sites in southern Queensland (Norwin and Gatton) in blocks fertilized with N (40 and 180 kg ha−1) or left unfertilized. Seed was inoculated at sowing with commercial peat inoculant containing effective Bradyrhizobium japonicum CB1809. The supernodulating genotypes produced up to 5 times the number of nodules and 8 times the nodule weight of cv. Centaur. Nitrogen fixation, assessed in five site × years using the xylem ureide technique, varied with site, season, genotype and fertilizer N. In some instances, relative ureide-N values of the supernodulators were significantly higher than for commercial cultivars. Results over all experiments indicated that the supernodulators and cv. Manark were similar with values 13–21% above Centaur. In two of three experiments to evaluate the effects of fertilizer N on N2 fixation, the supernodulators showed higher activity in the N fertilized plots than the commercial cultivars, including Manark. There was good agreement (r2=0.81) between assessments of N2 fixation using ureide and natural 15N abundance techniques. grain yield of the supernodulators, averaged over sites and years, were either the same or up to 25% less than Centaur and Bragg. At the Gatton site in 1989, the dry matter of oats, sown immediately after soybean harvest, was significantly (P < 0.05) greater after the supernodulators than after commercial cultivars. At Norwin in 1990 and 1991, significant benefits on barley shoot dry matter and grain were recorded for the 2 × nodulation phenotypes only.