Nitrous oxide (N2 O) is the third most important greenhouse gas after carbon dioxide and methane, and a prominent ozone-depleting substance. Agricultural soils are the primary anthropogenic source of N2 O because of the constant increase in the use of industrial nitrogen (N) fertilizers. The soybean crop is grown on 6% of the world’s arable land, and its production is expected to increase rapidly in the future. In this review, we summarize the current knowledge on N-cycle in the rhizosphere of soybean plants, particularly sources and sinks of N2 O. Soybean root nodules are the host of dinitrogen (N2 )-fixing bacteria from the genus Bradyrhizobium. Nodule decomposition is the main source of N2 O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N2 O. This N2 O is either emitted into the atmosphere or further reduced to N2 by the bradyrhizobial N2 O reductase (N2 OR), encoded by the nos gene cluster. The dominance of nos− indigenous populations of soybean bradyrhizobia results in the emission of N2 O into the atmosphere. Hence, inoculation with nos+ or nos++ (mutants with enhanced N2 OR activity) bradyrhizobia has proved to be promising strategies to reduce N2 O emission in the field. We discussed these strategies, the molecular mechanisms underlying them, and the future perspectives to develop better options for global mitigation of N2 O emission from soils.
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