Mitigation of soil N(2)O emission by inoculation with a mixed culture of indigenous Bradyrhizobium diazoefficiens

Agricultural soil is the largest source of nitrous oxide (N(2)O), a greenhouse gas. Soybean is an important leguminous crop worldwide. Soybean hosts symbiotic nitrogen-fixing soil bacteria (rhizobia) in root nodules. In soybean ecosystems, N(2)O emissions often increase during decomposition of the root nodules. Our previous study showed that N(2)O reductase can be used to mitigate N(2)O emission from soybean fields during nodule decomposition by inoculation with nosZ++ strains [mutants with increased N(2)O reductase (N(2)OR) activity] of Bradyrhizobium diazoefficiens. Here, we show that N(2)O emission can be reduced at the field scale by inoculation with a mixed culture of indigenous nosZ+ strains of B. diazoefficiens USDA110 group isolated from Japanese agricultural fields. Our results also suggested that nodule nitrogen is the main source of N(2)O production during nodule decomposition. Isolating nosZ+ strains from local soybean fields would be more applicable and feasible for many soybean-producing countries than generating mutants.