The nitrogen cycle diagram at right outlines the main steps by which nitrogen is converted between its various chemical forms in the environment (click on the link beneath the diagram to enlarge it).
Nitrogen occurs naturally in many chemical compounds, the simplest of which are nitrogen gas (N₂), ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻). The nitrogen present in living things, and in the matter produced by the decomposition of living things, is known as "organic nitrogen." The processes of the nitrogen cycle create these various molecules from each other by a straightforward recycling process.
The utilization of gaseous nitrogen, however, is not such a simple matter. Nitrogen in its gaseous form (N₂) makes up the great majority (~78%) of the earth's atmosphere. Nitrogen is also essential to all life since it is present in all amino acids. But atmospheric nitrogen cannot be assimilated by most organisms. It must first be "fixed." The fixation of nitrogen is a natural process of conversion by which certain prokaryotes (bacteria) convert gaseous nitrogen into forms that other organisms can use. In the diagram, note that this is the essential first step that makes all of the subsequent steps possible. Without it, complex life could not exist on earth.
The prokaryotes that fix atmospheric nitrogen as organic nitrogen (e.g., Rhizobium) often live in association with legumes. But certain free-living prokaryotes can also carry out this conversion (for example, those in the genus Azotobacter). Denitrifying prokaryotes (shown at right in the diagram) complete the cycle by converting organic nitrogen back into nitrogen gas.