The cycle has four main steps. In nitrogen fixation, specialized soil bacteria break the strong triple bond in N2 gas and convert it into ammonia, NH3, and ammonium ions, NH4+. Plants absorb these forms and use them to build proteins and nucleic acids.
Nitrogenase, the enzyme that performs fixation, works only in bacteria and a few related microbes, never in plants or animals directly. In nitrification, other soil bacteria oxidize ammonium first into nitrite, NO2-, and then into nitrate, NO3-, which plants can also absorb.
When plants and animals die or release waste, decomposer bacteria break down nitrogen-containing molecules in a process called ammonification, releasing ammonia back into the soil. Finally, denitrifying bacteria convert nitrate back into N2 gas, completing the loop. This cycle matters because nitrogen is often the nutrient that limits plant growth the most, even though nitrogen gas is everywhere in the air.
Human activity, especially the industrial production of ammonia through the Haber process for fertilizer, now fixes roughly as much nitrogen as all natural processes combined, which has significantly altered the natural nitrogen cycle.