N2O emission from soybean rhizosphere and its mitigation by Bradyrhizobium diazoefficiens inoculation
Graduate School of Life Sciences, Tohoku University
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