Nitrogen fixation and growth of soybean as influenced by varying the methods of inoculation with Bradyrhizobium japonicum

The effects of inoculating soil with a water suspension of Bradyrhizobium japonicum (i) at seeding, (ii) 7, or (iii) 14 days after planting (DAP), (iv) seed slurry inoculation and (v) seed slurry supplemented with postemergence inoculation of a water suspension of Bradyrhizobium at 7 or (vi) 14 DAP, on nodulation, N₂ fixation and yield of soybean (Glycine max. L.] Merrill) were compared in the greenhouse. The ¹⁵N isotope dilution technique was used to quantify N₂ fixed at flowering, early pod filling and physiological maturity stages (36, 52 and 70 DAP, respectively). On average, the water suspension inoculation formed the greatest number of nodules, and seed plus postemergence inoculation formed slightly more nodules than the seed-only inoculated plants (27, 19 and 12 nodules/plant respectively at physiological maturity). Seed slurry inoculation followed by postemergence inoculation at 14 DAP gave the highest nodule weight, with the plants fixing significantly more (P<0.05) N₂ (125 mg N plant⁻¹ or 56% N) than any other treatment (mean, 75 mg plant⁻¹ or 35% N). However, the higher N₂ fixation was not translated into higher N or dry matter yields. Estimates of N₂ fixed by the ostemergence Bradyrhizobium inoculations as well as plant yield were not significantly different from those of the seed slurry inoculation. Thus, delaying inoculation (e.g., by two weeks as in this study) did not reduce the symbiotic ability of soybean plants.