Impaired synthesis of N2O reductase leads to high emissions from acidic soils

Nitrous oxide (N2O) is the third most important greenhouse gas, but it has until now received less attention than carbon dioxide (CO2) and methane (CH4). Emissions of N2O are rising steadily since the start of industrialization, and the Intergovernmental Panel on Climate Change (IPCC) predicts that they, unlike CO2 emissions, will continue to escalate unless novel mitigation options are implemented. N2O can be produced and liberated to the atmosphere by several human-induced processes, but the dominate source of the anthropogenic N2O emissions is microbial processes in agricultural soils. The enzyme N2O reductase (NOS), which is a multi-copper enzyme, is only found in some types of bacteria and in some archaea and is the only known biological sink for N2O. It reduces N2O to harmless N2-gas, the main constituent of the air in our biosphere. One of the major environmental controllers of N2O emissions is pH, and acidic soils are known to release larger amounts of N2O than neutral soils. This is a global problem since, in addition to naturally occurring acidic soils, many soils are acidified due to excessive use of synthetic fertilizers. Although the negative correlation between pH and N2O/N2 product ratios (the fraction of added N released as N2O vs N2) has been known for decades, the biological mechanisms leading to this have not been understood. Our studies of denitrifying bacteria in pure culture showed that transcription of the gene nosZ (encoding NOS) did take place under acidic conditions (pH 5.7-6.1) during periods of active denitrification but with negligible or strongly delayed N2O reduction, and we also showed that the NOS apo-protein was transported to the periplasm. If the organisms were instead allowed to synthesize the protein at neutral pH they readily reduced N2O at acidic pH, but at a lower rate1. Studies of complex soil bacterial communities corroborated several of these findings and showed that the phenomenon is general to a wide range of bacteria2,3,4. Recent metagenome- and metatranscriptome analyses of soils of pH 4 and 7 showed similar abundance of nosZ genes and transcripts (nosZ clades I + II), thus lending little support for a direct, causal relationship between nosZ abundance and N2O emissions. The results also demonstrated that other accessory genes in the nos operon were transcribed. These included nosR encoding an enzyme involved in activation of transcription and in electron transport to NOS; nosL encoding an enzyme delivering Cu to NOS; and the ORF nosDFY encoding NosD, suggested to be involved in NosZ maturation, and the ABC-transporter NosFY. Taken together, our findings point to one or more post-transcriptional mechanisms affecting the maturation of the NOS apo-protein after transport to the periplasm. Further studies will dive deeper into the NOS structure after synthesis at different pH levels. We will also try to understand why the NOS function is restored after prolonged incubation under denitrifying conditions, suggesting that successful NOS maturation does take place stochastically at low pH, resulting in growth of some organisms.