The Nitrogen (N) Cycle

What are the major processes in the nitrogen cycle?

The nitrogen cycle describes how the different forms of nitrogen in the air, soil, water and living organisms are interconnected. It is described as a cycle because the nitrogen is never lost completely; it just changes form and is held in different places. There are 5 major processes involved:

1. Ammonification

Ammonification is the conversion of organic forms of nitrogen (for example, nitrogen in proteins in dead plants and soil animals) to ammonium. Ammonium is an inorganic form of nitrogen that has the symbol NH₄⁺. A wide range of soil organisms carry out the ammonification process. Many different types of bacteria and fungi are involved. The organisms get carbon and energy from the breakdown of organic matter, while nitrogen is released at the same time.

2. Nitrification

Nitrification is the process where ammonium is converted to nitrite (NO₂^-) and then to nitrate (NO₃^-). The organisms involved in nitrification are a relatively small number of species in soil. They are autotrophic, which means they get energy from the chemical transformation of NH₄⁺ to NO₂^-, while any needed carbon is obtained from CO₂. One group of organisms converts ammonium to nitrite and another group of organisms converts nitrite to nitrate.

3. Denitrification

Denitrification is the process where oxides of nitrogen (e.g. nitrate and nitrite) are converted into gaseous nitrogen and are removed from the soil system. Examples are the gas nitrous oxide (NO₂), and nitrogen gas (N₂), which is the form of nitrogen most common in the atmosphere. Many species of microorganisms are capable of denitrification of soil. Denitrification occurs mainly when there is little or no oxygen in the soil, such as when the soil is waterlogged. However, if the waterlogging is only temporary, the denitrification process will stop when the soil dries.

4. Nitrogen fixation

Nitrogen fixation is the conversion of nitrogen gas (N₂) to ammonium (NH₄⁺)- either by free living bacteria in soil or water, or by bacteria in symbiotic association with plants (eg legume symbiosis).

5. Nitrogen immobilization

This is the process whereby nitrogen is incorporated into the microbial cells and ‘tied-up’ in the 'microbial pool' of nitrogen. Immobilisation occurs in parallel with ammonification.

Refer to “Soils are Alive’ newsletter Volume 2 Number 1: ‘Bacteria transform ammonium to nitrate in soil’ by Dr Nui Milton