Nitrogen Cycling in Soybean Rhizosphere: Sources and Sinks of Nitrous Oxide (N₂O)

Nitrous oxide (N₂O) 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 N₂O 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 N₂O. Soybean root nodules are the host of dinitrogen (N₂)-fixing bacteria from the genus Bradyrhizobium. Nodule decomposition is the main source of N₂O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N₂O. This N₂O is either emitted into the atmosphere or further reduced to N₂ by the bradyrhizobial N₂O reductase (N₂OR), encoded by the nos gene cluster. The dominance of nos ^- indigenous populations of soybean bradyrhizobia results in the emission of N₂O into the atmosphere. Hence, inoculation with nos ⁺ or nos ⁺⁺ (mutants with enhanced N₂OR activity) bradyrhizobia has proved to be promising strategies to reduce N₂O 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 N₂O emission from soils.