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 influence the outcome of seed inoculation with selected rhizobial strains. A greenhouse study was made to compare the effect of inoculating an effective Bradyrhizobium japonicum strain onto seed (s) or into the bulk soil (b) with treatments containing an additional inoculation (with the same strain, or an ineffective B. japonicum strain) onto seed or into soil. Nodulation, N2 fixation and growth of soybean were measured, using the N-15 isotope dilution method to quantify the N fixed. The 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, more uniform distribution of nodules on the root, and greater nitrogen fixation. The influence of the ineffective strain (in combinations containing both strains) on nodulation and N2 fixation depended on where each or both strains were inoculated. Generally, highest N2 fixation was achieved by inoculating the effective strain into soil. N2 fixation by seed-inoculated effective strain was significantly depressed in the presence of the soil-inoculated ineffective strain (Es + Ib) but not when both strains were inoculated onto seed (Es + Is), or when both strains were inoculated together into soil (Eb + Ib). The trend for %Ndfa was generally similar to total nitrogen fixed. Our results therefore indicate that there can still be substantial N2 fixation in legumes by highly effective strains even in the presence of less effective strains if the effective strains are fairly well distributed in soil.