N(2)O emission from degraded soybean nodules depends on denitrification by Bradyrhizobium japonicum and other microbes in the rhizosphere.

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

SUBMITTER: Inaba S 

PROVIDER: S-EPMC4103556 | biostudies-literature | 2012

REPOSITORIES: biostudies-literature