A common mechanism for efficient N₂ O reduction in diverse isolates of nodule-forming bradyrhizobia

Bradyrhizobia are abundant soil bacteria, which can form nitrogen-fixing symbioses with leguminous plants, including important crops such as soybean, cowpea and peanut. Many bradyrhizobia can denitrify, but studies have hitherto focused on a few model organisms. We screened 39 diverse Bradyrhizobium strains, isolated from legume nodules. Half of them were unable to reduce N₂ O, making them sources of this greenhouse gas. Most others could denitrify NO₃ ^- to N₂ . Time-resolved gas kinetics and transcription analyses during transition to anaerobic respiration revealed a common regulation of nirK, norCB and nosZ (encoding NO₂ ^- , NO and N₂ O reductases), and differing regulation of napAB (encoding periplasmic NO₃ ^- reductase). A prominent feature in all N₂ -producing strains was a virtually complete hampering of NO₃ ^- reduction in the presence of N₂ O. In-depth analyses suggest that this was due to a competition between electron transport pathways, strongly favouring N₂ O over NO₃ ^- reduction. In a natural context, bacteria with this feature would preferentially reduce available N₂ O, produced by themselves or other soil bacteria, making them powerful sinks for this greenhouse gas. One way to augment such populations in agricultural soils is to develop inoculants for legume crops with dual capabilities of efficient N₂ -fixation and efficient N₂ O reduction.