The Nitrogen cycle—another natural cycle we've altered
Nitrogen is an essential element for all life, as it's a major component of amino acids (proteins) and nucleic acids (DNA and RNA). The main pool of nitrogen is the atmosphere, comprising 78% of the atmosphere as nitrogen gas (N 2 ). Unfortunately, plants cannot use N 2 directly. It must be converted to either ammonium (NH 3 ) or nitrate (NO 3 - ) before plants can absorb it and incorporate it into amino acids and nucleic acids. In nature, most nitrogen conversion is done through a process known as nitrification wherein N 2 is converted first to NH 3 by several genera of nitrogen-fixing bacteria and cyanobacteria, then to NO 2 - (nitrite) by bacteria in the genera Nitrosomonas or Nitrococcus , and finally to NO 3 - by bacteria in the Nitrobacter genus. Nitrate that isn't absorbed by plants is returned to the atmosphere by the process of denitrification, wherein nitrate is converted to N 2 gas by denitrifying bacteria.