NODULATION, N2 FIXATION AND DRY-MATTER YIELD OF SOYBEAN [GLYCINE-MAX (L) MERRILL] INOCULATED WITH EFFECTIVE AND INEFFECTIVE BRADYRHIZOBIUM-JAPONICUM STRAINS

Many soils harbour rhizobia of varying effectiveness, and this could i nfluence the outcome of seed inoculation with selected rhizobial strai ns. A greenhouse study was made to compare the effect of inoculating a n effective Bradyrhizobium japonicum strain onto seed (s) or into the bulk soil (b) with treatments containing an additional inoculation (wi th the same strain, or an ineffective B. japonicum strain) onto seed o r into soil. Nodulation, N2 fixation and growth of soybean were measur ed, using the N-15 isotope dilution method to quantify the N fixed. Th e method of inoculation had significant effects on nodule distribution along soybean roots, nodule number and fresh weight, and N2 fixation. In general, soil inoculation resulted in more nodules being formed, m ore uniform distribution of nodules on the root, and greater nitrogen fixation. The influence of the ineffective strain (in combinations con taining both strains) on nodulation and N2 fixation depended on where each or both strains were inoculated. Generally, highest N2 fixation w as achieved by inoculating the effective strain into soil. N2 fixation by seed-inoculated effective strain was significantly depressed in th e presence of the soil-inoculated ineffective strain (Es + Ib) but not when both strains were inoculated onto seed (Es + Is), or when both s trains were inoculated together into soil (Eb + Ib). The trend for %Nd fa was generally similar to total nitrogen fixed. Our results therefor e indicate that there can still be substantial N2 fixation in legumes by highly effective strains even in the presence of less effective str ains if the effective strains are fairly well distributed in soil.