Using rhizobia to reduce N2O emissions

Many countries in Europe and elsewhere intend to increase their domestic production of plant-based proteins through extended cultivation of legume crops. The capacity of legumes to fix atmospheric N2 through their symbiosis with bacteria, so called rhizobia, decreases the need for chemical fertilizers and thus the climate footprint from the agricultural sector. Rhizobia are found both as free-living cells in soil, and inside legume nodules where they differentiate into N2-fixing bacteroids. The signalling 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. Therefore, legumes are often inoculated with rhizobia before sowing. Currently, there is only a limited variation of inoculants on the market, and there is increasing interest in producing high-quality inoculants compatible with specific crops. Many rhizobia can also denitrify, enabling them to respire N-oxides when oxygen becomes scarce. Screening of different collections of rhizobia showed that 30-50% of the Bradyrhizobium strains and only 6% of the Ensifer strains were able to reduce N2O to harmless N2, while the others were net producers of this greenhouse gas. The bradyrhizobia exhibited a strong preference for N2O reduction over nitrate reduction, and we recently demonstrated that this is because the electron transport pathway to N2O reductase competes very efficiently with nitrate reduction for electrons. Greenhouse experiments revealed that several of the N2O reducing strains were also efficient N2-fixers with various legumes. Our findings provide novel information about transcriptional and metabolic control of denitrification in this ecologically and economically significant group of bacteria, and highlight the need to consider N2O reduction as an important trait, in addition to N2-fixation efficacy, when developing new inoculants for legume production.