Nitrogen cycling in soybean rhizosphere: Sources and sinks of nitrous oxide (N₂O)

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.