IMS-8:

N₂O emission from soybean rhizosphere and its mitigation by Bradyrhizobium diazoefficiens inoculation

Nitrous oxide (N₂O) is a greenhouse gas that is also capable of destroying the ozone layer. Agricultural soil is the largest source of N₂O. In soybean rhizosphere, N₂O is mainly emitted with the degradation of the root nodules. Organic nitrogen inside the nodules is mineralized to NH₄⁺, followed by nitrification and denitrification that produce N₂O. N₂O is then emitted into the atmosphere or is further reduced to N₂ by N₂O reductase (N₂OR), which is encoded by the nosZ gene. The amount of N₂O emission from soybean rhizosphere was decided by balance between denitrification (N₂O source) and N₂O reduction (N₂O sink). Bradyrhizobium diazoefficiens strains with increased N₂OR activity (Nos++) were generated using a proofreading-deficient mutant of B. diazoefficiens USDA110. The N₂OR activity of the Nos⁺⁺ strains was significantly higher than that of USDA110 in free-living and bacteroid cells. In vermiculite pot experiments, N₂O emission from the degraded nodules with Nos⁺⁺ strains was significantly lower than from degraded nodules with USDA110. We simulated field conditions in a pot experiment with an Andosol soil, dominated by nosZ⁻ rhizobium. N₂O emission decreased significantly compared with emission by native strains (nosZ⁻ dominant) as a result of inoculation with the Nos⁺⁺ strain. These results suggested that Nos⁺⁺ strain could reduce N₂O emission from soybean field. Actually, the post-harvest N₂O emission from soybean rhizosphere was reduced 43% by inoculation of Nos⁺⁺ strain at field scale. SNPs analysis indicates that bll4572 gene was commonly mutated in Nos⁺⁺ strains. bll4572 gene showed similarity to nasS gene, which encodes nitrate sensor protein of two-component regulatory system NasST for nitrate assimilation genes. We demonstrated that nosZ gene is regulated by NasST system and higher N₂OR activity is induced by nasS mutation in B. diazoefficiens.

keywords:Nitrous oxide,Greenhouse gas,Bradyrhizobium diazoefficiens,Nitrous oxide reductase,Soybean rhizosphere