Symbiotic Bradyrhizobium japonicum reduces N2O surrounding the soybean root system via nitrous oxide reductase

N 2 O reductase activity in soybean nodules formed with Bradyrhizobium japonicum was evaluated from N 2 O uptake and conversion of 15 N-N 2 O into 15 N-N 2 . Free-living cells of USDA110 showed N 2 O reductase activity, whereas a nosZ mutant did not. Complementation of the nosZ mutant with two cosmids containing the nosRZDFYLX genes of B. japonicum USDA110 restored the N 2 O reductase activity. When detached soybean nodules formed with USDA110 were fed with 15 N-N 2 O, they rapidly emitted 15 N-N 2 outside the nodules at a ratio of 98.5% of 15 N-N 2 O uptake, but nodules inoculated with the nosZ mutant did not. Surprisingly, N 2 O uptake by soybean roots nodulated with USDA110 was observed even in ambient air containing a low concentration of N 2 O (0.34 ppm). These results indicate that the conversion of N 2 O to N 2 depends exclusively on the respiratory N 2 O reductase and that soybean roots nodulated with B. japonicum carrying the nos genes are able to remove very low concentrations of N 2 O.