Unlocking bacterial potential to reduce farmland NO emissions.

Farmed soils contribute substantially to global warming by emitting NO (ref. ), and mitigation has proved difficult. Several microbial nitrogen transformations produce NO, but the only biological sink for NO is the enzyme NosZ, catalysing the reduction of NO to N (ref. ). Although strengthening the NosZ activity in soils would reduce NO emissions, such bioengineering of the soil microbiota is considered challenging. However, we have developed a technology to achieve this, using organic waste as a substrate and vector for NO-respiring bacteria selected for their capacity to thrive in soil. Here we have analysed the biokinetics of NO reduction by our most promising NO-respiring bacterium, Cloacibacterium sp. CB-01, its survival in soil and its effect on NO emissions in field experiments. Fertilization with waste from biogas production, in which CB-01 had grown aerobically to about 6 × 10 cells per millilitre, reduced NO 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 NO-respiring bacteria. Scaling our data up to the European level, we find that national anthropogenic NO emissions could be reduced by 5-20%, and more if including other organic wastes. This opens an avenue for cost-effective reduction of NO emissions for which other mitigation options are lacking at present.