The nitrogen cycle

Nitrogen is essential for life—many of the biomolecules that make up organisms contain nitrogen atoms, including nucleic acids and proteins.

The majority of nitrogen on Earth is found in the atmosphere, with nitrogen gas $(\mathrm{N}{\phantom{A}}_2)$‍ making up about $78\mathrm{\%}$‍ of air. However, many organisms (such as plants and animals) cannot directly use nitrogen gas. To obtain nitrogen, these organisms rely on the nitrogen cycle. The nitrogen cycle describes how nitrogen moves between living organisms and the environment. Through this cycle, nitrogen takes on different chemical forms.

The nitrogen cycle involves four main processes: nitrogen fixation, ammonification (also called mineralization), nitrification, and denitrification. Most of the nitrogen cycle is driven by microorganisms, such as bacteria.

Nitrogen enters the biosphere through bacteria and other prokaryotes, which convert atmospheric nitrogen $(\mathrm{N}{\phantom{A}}_2)$‍ into biologically usable forms through a process called nitrogen fixation. Some of these microorganisms live in the soil, and others are found in the roots of plants such as peanuts and peas. A small amount of atmospheric nitrogen is also fixed through lightning.

Nitrogen-fixing microorganisms capture atmospheric nitrogen by converting it to ammonia $(\mathrm{NH}{\phantom{A}}_3)$‍, which can be used by producers (such as plants) to make biomolecules. These biomolecules are then passed along to consumers through food chains and food webs. Nitrogen-containing biomolecules are incorporated into an organism’s cells or are broken down and leave the organism as waste.

As dead organisms and waste decompose, ammonium $(\mathrm{NH}{\phantom{A}}_4{\phantom{A}}^{+})$‍ is returned to the environment through ammonification. This process is carried out by certain bacteria and fungi.

Through nitrification, microorganisms convert ammonia and ammonium to nitrites $(\mathrm{NO}{\phantom{A}}_2{\phantom{A}}^{-})$‍ and nitrates $(\mathrm{NO}{\phantom{A}}_3{\phantom{A}}^{-})$‍ which can be used by producers (via assimilation). Some nitrites and nitrates are also transformed back into nitrogen gas through denitrification (by denitrifying microorganisms), returning nitrogen to the atmosphere.