Why we have a nitrogen cycle
About 78% of the air is nitrogen gas, yet plants and animals cannot use this N₂ directly because it is very unreactive. The nitrogen cycle is the natural process that changes nitrogen from one form to another so that it can be used by living things and then returned to the air. Nitrogen is needed to build proteins, which every living cell depends on for growth and repair.
The main steps
- Nitrogen fixation changes nitrogen gas into nitrogen compounds. This is done by nitrogen-fixing bacteria such as Rhizobium in root nodules, by lightning, and industrially by the Haber process.
- Nitrification is carried out by nitrifying bacteria, which change ammonium compounds into nitrates.
- Assimilation is when plants absorb nitrates through their roots and use them to make proteins. Animals get nitrogen by eating plants.
- Ammonification (decay) occurs when decomposers break down dead organisms and waste, releasing ammonia.
- Denitrification is carried out by denitrifying bacteria, which change nitrates back into nitrogen gas, returning it to the air.
Key idea
Bacteria drive the cycle at almost every stage — fixing, nitrifying, decaying and denitrifying — which is why microorganisms are essential for keeping soil fertile.
Why it matters
Without the nitrogen cycle, the nitrogen locked in the air would never reach plants, and the soil would lose its fertility. Farmers add nitrogen fertiliser and grow legumes to keep enough usable nitrogen in the soil. The cycle also shows why microorganisms matter so much: the same bacteria that make nitrogen available can, through denitrification, also remove it, so a balance is constantly maintained in nature.
Example
A Malaysian farmer plants groundnuts (a legume) between crops. Rhizobium bacteria in their root nodules fix nitrogen, naturally enriching the soil before the next planting.