FIELD ASSESSMENT OF SUPERNODULATING GENOTYPES OF SOYBEAN FOR YIELD, N-2 FIXATION AND BENEFIT TO SUBSEQUENT CROPS

Development and commercial use of cultivars of soybean with high N-2 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, N-2 fixation and effects on subsequent cereal crops of intermediate supernodulating (designated 2 x nodulation phenotype) and extreme supernodulating mutants of Bragg soybean (6 x nodulation phenotype), a non-nodulating mutant of Bragg (0 x 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 x 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 N-2 fixation, the supernodulators showed higher activity in the N fertilized plots than the commercial cultivars, including Manark. There was good agreement (r(2) = 0.81) between assessments of N-2 fixation using ureide and natural N-15 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 x nodulation phenotypes only.