AUTHOR=Sánchez Cristina , Minamisawa Kiwamu 

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

JOURNAL=Frontiers in Microbiology

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.01943

DOI=10.3389/fmicb.2019.01943

ISSN=1664-302X

ABSTRACT=<p>Nitrous oxide (N<sub>2</sub>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 N<sub>2</sub>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 N<sub>2</sub>O. Soybean root nodules are the host of dinitrogen (N<sub>2</sub>)-fixing bacteria from the genus <italic>Bradyrhizobium</italic>. Nodule decomposition is the main source of N<sub>2</sub>O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N<sub>2</sub>O. This N<sub>2</sub>O is either emitted into the atmosphere or further reduced to N<sub>2</sub> by the bradyrhizobial N<sub>2</sub>O reductase (N<sub>2</sub>OR), encoded by the <italic>nos</italic> gene cluster. The dominance of <italic>nos</italic><sup>–</sup> indigenous populations of soybean bradyrhizobia results in the emission of N<sub>2</sub>O into the atmosphere. Hence, inoculation with <italic>nos</italic><sup>+</sup> or <italic>nos</italic><sup>++</sup> (mutants with enhanced N<sub>2</sub>OR activity) bradyrhizobia has proved to be promising strategies to reduce N<sub>2</sub>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 N<sub>2</sub>O emission from soils.</p>