The main processes in the nitrogen cycle. N₂ is reduced to NH₃ via biological or industrial nitrogen fixation, providing nitrogen fertilizer for plants. 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, the remainder being metabolized by soil microbes in processes with detrimental environmental impacts. The first of these processes, nitrification, refers to the biological oxidation of NH₃ to NO₂^- and NO₃^-, which have low retention in soil and pollute the waterways, leading to downstream eutrophication and ultimately the 'dead zones' (low oxygen zones) in coastal waters, for example the Gulf of Mexico. In a second process, denitrification, NO₂^- and NO₃^- undergo stepwise reduction to N₂O and N₂. Significant amounts of the N₂O produced in this process escape into the atmosphere, contributing to climate change and ozone destruction.

Adapted from:
Lehnert, N.; Coruzzi, G.; Hegg, E.; Seefeldt, L.; Stein, L.
"NSF Workshop Report: Feeding the World in the 21st Century: Grand Challenges in the Nitrogen Cycle"
National Science Foundation: Arlington, VA, 2016