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 f ixation activity, even in the presence of moderate to high amounts of soil nitrate, may result in larger inputs of N into the soil-plant sys tem. We report experiments from a 6-year study (1988-93) to evaluate y ield, N-2 fixation and effects on subsequent cereal crops of intermedi ate 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 a t two sites in southern Queensland (Norwin and Gatton) in blocks ferti lized with N (40 and 180 kg ha(-1)) or left unfertilized. Seed was ino culated at sowing with commercial peat inoculant containing effective Bradyrhizobium japonicum CB1809. The supernodulating genotypes produce d 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 t he xylem ureide technique, varied with site, season, genotype and fert ilizer N. In some instances, relative ureide-N values of the supernodu lators were significantly higher than for commercial cultivars. Result s over all experiments indicated that the supernodulators and cv. Mana rk were similar with values 13-21% above Centaur. In two of three expe riments to evaluate the effects of fertilizer N on N-2 fixation, the s upernodulators showed higher activity in the N fertilized plots than t he commercial cultivars, including Manark. There was good agreement (r (2) = 0.81) between assessments of N-2 fixation using ureide and natur al N-15 abundance techniques. Grain yield of the supernodulators, aver aged 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) g reater after the supernodulators than after commercial cultivars. At N orwin in 1990 and 1991, significant benefits on barley shoot dry matte r and grain were recorded for the 2 x nodulation phenotypes only.