Effect of Postemergence, Supplemental Inoculation on Nodulation and Symbiotic Performance of Soybean (Glycine max (L.) Merrill) at Three Levels of Soil Nitrogen

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The influence of a supplementary bradyrhizobial inoculation after an initial seed slurry inoculation with the same strain on nodulation and N2 fixation in soybeans was examined in the greenhouse. The plants were grown in a Typic Eutrocrepts soil: sand mixture containing 25, 65, or 83 mg of N per kg (i.e., native soil N plus 15N-labeled ammonium sulfate). Harvests were made at early flowering and physiological maturity. The supplementary inoculations which were made 14 or 21 days after planting (DAP) caused formation of substantially more nodules than the single slurry inoculation did. Autoregulation was therefore not completely successful in preventing subsequent infections. For the slurry-inoculated plants, at both harvests the proportion of N derived from fixation was greatest in the soil containing the least N, and only slight increases in N2 fixation resulted from a second inoculation. The inhibition of N2 fixation at the higher N levels was significantly reduced by a second inoculation at 21 DAP; this treatment resulted in at least a doubling of both the percentage and total amount of N2 fixed by the single slurry inoculation at physiological maturity. The N2 fixation increases resulting from the supplementary inoculation at 14 DAP were less pronounced and not significant. Greater N2 fixation was frequently not reflected by increased total N or dry matter yield, suggesting that the major benefit of the increased fixation was a decreased dependence of plants on soil N for growth.

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