Rhizobia as sinks or sources for N2O

The recent report by the Intergovernmental Panel on Climate Change (IPCC) foresees rising emissions of the greenhouse gas N2O throughout this century, unless novel mitigation options are developed. Denitrifying bacteria in agricultural soils produce most of the emitted N2O, but they can also act as net sinks. Stimulating N2O-reducing bacterial populations in situ seems unrealistic. Instead, N2O-reducing organisms could be introduced to the soil using suitable vectors. One such possibility is to develop inoculants for legume crops that are both efficient N2O reducers and N2-fixers. Rhizobia are found both free-living in soil, and as N2-fixing bacteroids inside nodules. The signaling leading to nodule formation and development of bacteroids is complex, and not all combinations of bacterial strain and plant variety lead to efficient N2 fixation. Currently, there is only a limited variation of inoculants on the market, and there is demand for high-quality inoculants compatible with specific crops. We investigated a large number of Bradyrhizobium and Ensifer strains, isolated from nodules of various legumes. Most of the strains could perform two or more of the denitrification steps. While 50% of the Bradyrhizobium strains could reduce N2O to harmless N2, only 6% of the Ensifer strains showed this capacity. Rhizobia with a complete denitrification pathway carry a periplasmic NO3- reductase (NapAB) but lack membrane-bound NarG; a Cu containing NO2- reductase (NirK); a cytochrome c-dependent NO reductase (NorCB); and a clade I N2O reductase (Nos). All N2O-reducing bradyrhizobia in our collection exhibited strong preference for N2O reduction over NO3- reduction, likely because the electron transport pathway to N2O reductase competes very efficiently with NO3- reduction for electrons. This preferential N2O reduction was retained during starvation. Greenhouse experiments revealed that several of the N2O reducing strains were also efficient N2-fixers in combination with various economically important legumes. Our findings provide novel information about the transcriptional and metabolic control of denitrification in these ecologically and economically significant groups of bacteria and demonstrate that full-fledged denitrifying rhizobia can be strong sinks for N2O. This highlights the need to consider N2O reduction, in addition to N2-fixation efficacy, when developing new inoculants for legume production and responds to IPCC’s demand for a switch to a more plant-based diet and reduced use of mineral fertilizers.