Mitigation of nitrous oxide emissions from soils by Bradyrhizobium japonicum inoculation

Nitrous oxide (N₂O) is a greenhouse gas that is also capable of destroying the ozone layer¹. Agricultural soil is the largest source of N₂O (ref.). Soybean is a globally important leguminous crop, and hosts symbiotic nitrogen-fixing soil bacteria (rhizobia) that can also produce N₂O (ref.). In agricultural soil, N₂O is emitted from fertilizer and soil nitrogen. In soybean ecosystems, N₂O is also emitted from the degradation of the root nodules⁴. 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. Pure culture and vermiculite pot experiments showed lower N₂O emission by nosZ+ strains⁵ and nosZ++ strains (mutants with increased N₂OR activity)⁶ of Bradyrhizobium japonicum than by nosZ-strains. A pot experiment using soil confirmed these results⁷. Although enhancing N₂OR activity has been suggested as a N ₂O mitigation option^8,9, this has never been tested in the field. Here, we show that post-harvest N₂O emission from soybean ecosystems due to degradation of nodules can be mitigated by inoculation of nosZ+ and non-genetically modified organism nosZ++ strains of B. japonicum at a field scale.