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 layer(1). Agricultural soil is the largest source of N2O (ref. 2). Soybean is a globally important leguminous crop, and hosts symbiotic nitrogen-fixing soil bacteria (rhizobia) that can also produce N2O (ref. 3). In agricultural soil, N2O is emitted from fertilizer and soil nitrogen. In soybean ecosystems, N2O is also emitted from the degradation of the root nodules(4). 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 N-2 by N2O reductase (N2OR), which is encoded by the nosZ gene. Pure culture and vermiculite pot experiments showed lower N2O emission by nosZ+ strains(5) and nosZ++ strains (mutants with increased N2OR activity)(6) of Bradyrhizobium japonicum than by nosZ-strains. A pot experiment using soil confirmed these results(7). Although enhancing N2OR activity has been suggested as a N2O 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.