N₂O Emission from Degraded Soybean Nodules Depends on Denitrification by Bradyrhizobium japonicum and Other Microbes in the Rhizosphere

抄録

A model system developed to produce N₂O emissions from degrading soybean nodules in the laboratory was used to clarify the mechanism of N₂O emission from soybean fields. Soybean plants inoculated with nosZ-defective strains of Bradyrhizobium japonicum USDA110 (ΔnosZ, lacking N₂O reductase) were grown in aseptic jars. After 30 days, shoot decapitation (D, to promote nodule degradation), soil addition (S, to supply soil microbes), or both (DS) were applied. N₂O was emitted only with DS treatment. Thus, both soil microbes and nodule degradation are required for the emission of N₂O from the soybean rhizosphere. The N₂O flux peaked 15 days after DS treatment. Nitrate addition markedly enhanced N₂O emission. A ¹⁵N tracer experiment indicated that N₂O was derived from N fixed in the nodules. To evaluate the contribution of bradyrhizobia, N₂O emission was compared between a nirK mutant (ΔnirKΔnosZ, lacking nitrite reductase) and ΔnosZ. The N₂O flux from the ΔnirKΔnosZ rhizosphere was significantly lower than that from ΔnosZ, but was still 40% to 60% of that of ΔnosZ, suggesting that N₂O emission is due to both B. japonicum and other soil microorganisms. Only nosZ-competent B. japonicum (nosZ+ strain) could take up N₂O. Therefore, during nodule degradation, both B. japonicum and other soil microorganisms release N₂O from nodule N via their denitrification processes (N₂O source), whereas nosZ-competent B. japonicum exclusively takes up N₂O (N₂O sink). Net N₂O flux from soybean rhizosphere is likely determined by the balance of N₂O source and sink.