Inoculation with Bradyrhizobium ottawaense Effectively Reduces N₂O Emissions from Degraded Soybean Nodules in Andosols Dominated by nosZ-lacking Bradyrhizobia

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Nitrous oxide (N₂O) is a potent greenhouse gas, and its largest anthropogenic source is agricultural soils. In these soils, N₂O emissions are produced from nitrogen fertilization and nitrogen-rich crop residues, such as soybean nodules. Previous studies demonstrated that inoculation with Bradyrhizobium diazoefficiens possessing the N₂O reductase gene (nosZ) effectively mitigated N₂O emissions from soybean fields at harvest. More recently, Bradyrhizobium ottawaense, which exhibits higher nosZ expression, reduced N₂O emissions more effectively than B. diazoefficiens during the degradation of soybean nodules formed under sterile vermiculite conditions. Although the effects of inoculation with B. ottawaense have subsequently been exami­ned under natural soil conditions, it remains unclear whether these effects are maintained across different soil types and soybean cultivars. Therefore, we herein investigated the effects of inoculation with B. ottawaense on its competitiveness and N₂O emissions in pot experiments using two field soils (Andosol and Fluvisol) and three Japanese commercial soybean cultivars (‘Hatayutaka’, ‘Yukihomare R’, and ‘Satonohohoemi’). The results demonstrated that inoculation with B. ottawaense significantly increased higher nodule occupancy of the inoculant strain compared with the non-inoculated controls in the two field soils. Moreover, B. ottawaense tended to reduce N₂O emissions in Andosol dominated by nosZ-lacking bradyrhizobia, regardless of the soybean cultivar. No significant N₂O-reducing effects were observed in Fluvisol dominated by nosZ-possessing bradyrhizobia. Collectively, these results suggest the applicability of inoculation with B. ottawaense to diverse soybean cultivars and its effectiveness in mitigating N₂O emissions from soybean fields in which indigenous nosZ-lacking bradyrhizobial populations are dominant, such as Andosols.