Farmed soils contribute substantially to global warming by emitting N₂O (ref. ¹), and mitigation has proved difficult². Several microbial nitrogen transformations produce N₂O, but the only biological sink for N₂O is the enzyme NosZ, catalysing the reduction of N₂O to N₂ (ref. ³). Although strengthening the NosZ activity in soils would reduce N₂O 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 N₂O-respiring bacteria selected for their capacity to thrive in soil^6-8. Here we have analysed the biokinetics of N₂O reduction by our most promising N₂O-respiring bacterium, Cloacibacterium sp. CB-01, its survival in soil and its effect on N₂O emissions in field experiments. Fertilization with waste from biogas production, in which CB-01 had grown aerobically to about 6 × 10⁹ cells per millilitre, reduced N₂O 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 N₂O-respiring bacteria. Scaling our data up to the European level, we find that national anthropogenic N₂O emissions could be reduced by 5-20%, and more if including other organic wastes. This opens an avenue for cost-effective reduction of N₂O emissions for which other mitigation options are lacking at present.

Elisabeth Gautefall Hiis

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