Reversing nitrogen fixation

The nitrogen cycle is one of the most important biogeochemical cycles on Earth because nitrogen is an essential nutrient for all life forms. To supplement natural nitrogen fixation, farmers add large amounts of nitrogen-containing fertilizer to their soils such that nitrogen never becomes a limiting nutrient for plant growth. However, of the nitrogen added to fields — most of which is in the form of NH3 and NO3− — only 30–50% is taken up by plants, while the remainder is metabolized by soil microorganisms in processes with detrimental environmental impacts. The first of these processes, that is, nitrification, refers to the biological oxidation of NH3 to NO2− and NO3−, which have low retention in soil and pollute waterways, leading to downstream eutrophication and ultimately ‘dead zones’ (low oxygen zones) in coastal waters, for example, the Gulf of Mexico. In a second process, namely, denitrification, NO3− and NO2− undergo stepwise reduction to N2O and N2. Substantial amounts of the N2O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction. Recent results suggest that nitrification also affords N2O. This Review describes the enzymes involved in NH3 oxidation and N2O production and degradation in the nitrogen cycle. We pay particular attention to the active site structures, the associated coordination chemistry that enables the chemical transformations and the reaction mechanisms. Nitrification and denitrification are responsible for the processing of ammonia fertilizer, ultimately leading to the generation of environmental pollutants that accumulate in waterways and the atmosphere. This Review describes the enzymes involved in these processes, which fascinate with their unusual active sites and the surprising reactions that they catalyse.