Unlocking bacterial potential to reduce farmland N 2 O emissions.

Farmed soils contribute substantially to global warming by emitting N 2 O (ref.  1 ), and mitigation has proved difficult 2 . Several microbial nitrogen transformations produce N 2 O, but the only biological sink for N 2 O is the enzyme NosZ, catalysing the reduction of N 2 O to N 2 (ref.  3 ). Although strengthening the NosZ activity in soils would reduce N 2 O emissions, such bioengineering of the soil microbiota is considered challenging 4,5 . However, we have developed a technology to achieve this, using organic waste as a substrate and vector for N 2 O-respiring bacteria selected for their capacity to thrive in soil 6-8 . Here we have analysed the biokinetics of N 2 O reduction by our most promising N 2 O-respiring bacterium, Cloacibacterium sp. CB-01, its survival in soil and its effect on N 2 O emissions in field experiments. Fertilization with waste from biogas production, in which CB-01 had grown aerobically to about 6 × 10 9 cells per millilitre, reduced N 2 O emissions by 50-95%, depending on soil type. The strong and long-lasting effect of CB-01 is ascribed to its tenacity in soil, rather than its biokinetic parameters, which were inferior to those of other strains of N 2 O-respiring bacteria. Scaling our data up to the European level, we find that national anthropogenic N 2 O emissions could be reduced by 5-20%, and more if including other organic wastes. This opens an avenue for cost-effective reduction of N 2 O emissions for which other mitigation options are lacking at present.