Nitrogen Cycling in Soybean Rhizosphere: Sources and Sinks of Nitrous Oxide (NO).

Nitrous oxide (NO) 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 NO 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 NO. Soybean root nodules are the host of dinitrogen (N)-fixing bacteria from the genus . Nodule decomposition is the main source of NO in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce NO. This NO is either emitted into the atmosphere or further reduced to N by the bradyrhizobial NO reductase (NOR), encoded by the gene cluster. The dominance of indigenous populations of soybean bradyrhizobia results in the emission of NO into the atmosphere. Hence, inoculation with or (mutants with enhanced NOR activity) bradyrhizobia has proved to be promising strategies to reduce NO 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 NO emission from soils.