Definition · Plain-language
Nitrogen cycle
The nitrogen cycle is the series of processes that convert nitrogen between its forms as it moves through the air, soil and living organisms.
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Why nitrogen must be converted
Nitrogen is essential to all life — it is a key part of proteins and DNA. The atmosphere is about four-fifths nitrogen gas, yet this enormous supply is almost useless to most organisms, because the nitrogen molecule is bound by an extremely strong triple bond that plants and animals cannot break. The nitrogen cycle solves this problem by converting nitrogen between forms that organisms can and cannot use. Microbes do most of the work, transforming inert nitrogen gas into compounds that plants can absorb, and eventually returning it to the air, so the element is continually recycled.
The main stages
The cycle runs through several microbe-driven steps. In nitrogen fixation, specialised bacteria — many living in the roots of legumes — convert atmospheric nitrogen gas into ammonia; lightning fixes a smaller amount. In nitrification, other bacteria turn ammonia into nitrites and then nitrates, the form plants take up most readily. In assimilation, plant roots absorb nitrates and build them into proteins, and animals get their nitrogen by eating plants. In ammonification, decomposers break down dead organisms and waste back into ammonia. Finally, in denitrification, bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.
Humans and the nitrogen cycle
People have dramatically altered the nitrogen cycle. The industrial production of nitrogen fertiliser converts atmospheric nitrogen into plant-usable forms on a vast scale, boosting food production but adding far more reactive nitrogen to the environment than nature alone would. Excess fertiliser washing into rivers and seas causes nutrient pollution and oxygen-starved “dead zones”, while burning fossil fuels releases nitrogen oxides that contribute to smog and acid rain. Understanding the nitrogen cycle is therefore important not only for biology but for managing agriculture and protecting water and air quality.
Key facts
At a glance
- Definition: the movement of nitrogen between air, soil and living things
- The problem: air is ~78% nitrogen gas, but most life cannot use it directly
- Main stages: fixation, nitrification, assimilation, ammonification, denitrification
- Key players: nitrogen-fixing, nitrifying and denitrifying bacteria
- Why it matters: nitrogen is essential for proteins and DNA
- Human impact: fertiliser and pollution add excess reactive nitrogen
Common misconceptions
What people often get wrong
Often heard: Plants can take nitrogen straight from the air, since air is mostly nitrogen.
Actually: Most plants cannot use nitrogen gas directly. Bacteria must first “fix” it into ammonia and nitrates before plant roots can absorb it.
Often heard: The nitrogen cycle is driven mainly by plants and animals.
Actually: Bacteria do most of the essential work — fixing, nitrifying and denitrifying nitrogen. Plants and animals depend on those microbes to make nitrogen available and to recycle it.
Often heard: Adding more nitrogen fertiliser is simply good for the environment.
Actually: Fertiliser boosts crops but excess reactive nitrogen runs off into water, causing pollution and dead zones, and nitrogen oxides harm air quality. Too much disrupts the natural balance.
Going deeper
