A model system developed to produce N
2O emissions from degrading soybean nodules in the laboratory was used to clarify the mechanism of N
2O emission from soybean fields. Soybean plants inoculated with
nosZ-defective strains of
Bradyrhizobium japonicum USDA110 (Δ
nosZ, lacking N
2O 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. N
2O was emitted only with DS treatment. Thus, both soil microbes and nodule degradation are required for the emission of N
2O from the soybean rhizosphere. The N
2O flux peaked 15 days after DS treatment. Nitrate addition markedly enhanced N
2O emission. A
15N tracer experiment indicated that N
2O was derived from N fixed in the nodules. To evaluate the contribution of bradyrhizobia, N
2O emission was compared between a
nirK mutant (Δ
nirKΔ
nosZ, lacking nitrite reductase) and Δ
nosZ. The N
2O flux from the Δ
nirKΔ
nosZ rhizosphere was significantly lower than that from Δ
nosZ, but was still 40% to 60% of that of Δ
nosZ, suggesting that N
2O emission is due to both
B. japonicum and other soil microorganisms. Only
nosZ-competent
B. japonicum (
nosZ+ strain) could take up N
2O. Therefore, during nodule degradation, both
B. japonicum and other soil microorganisms release N
2O from nodule N via their denitrification processes (N
2O source), whereas
nosZ-competent
B. japonicum exclusively takes up N
2O (N
2O sink). Net N
2O flux from soybean rhizosphere is likely determined by the balance of N
2O source and sink.
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