The nitrogen cycle — Biology, 14–17 years
Nitrogen is needed to build proteins and DNA, but most atmospheric nitrogen cannot be used directly by plants. The nitrogen cycle moves it through air, soil, organisms and decomposers, making life possible while linking ecosystems to human activity.
Nitrogen changes form as it moves
Nitrogen travels through an ecosystem in several chemical forms. Bacteria can change nitrogen gas into compounds plants absorb, plants build it into proteins and DNA, and animals obtain it by eating plants or other animals. When organisms excrete waste or die, decomposers return nitrogen compounds to the soil; other bacteria eventually return nitrogen gas to the atmosphere.
Why nitrogen needs a cycle
Living things need nitrogen to build important molecules, but atmospheric nitrogen is held in a form most plants cannot use. Without a way to convert and recycle it, growth would soon be limited by nitrogen in the soil. The cycle explains both the natural supply of this essential element and why adding too much fertiliser can disturb waterways and ecosystems.
From air to a tomato
Follow one nitrogen atom. First, nitrogen-fixing bacteria change nitrogen gas in the soil into ammonium compounds. Second, other bacteria convert these into nitrates, which tomato roots absorb. Third, the plant uses the nitrogen to make amino acids and proteins in its leaves and fruit. Finally, when the plant dies, decomposers return nitrogen compounds to the soil for another cycle.
Plants cannot use most air nitrogen
It seems reasonable to think plants should absorb nitrogen gas directly, because their leaves are surrounded by air and nitrogen is abundant there. The difficulty is chemical: nitrogen gas has very strong bonds between its atoms. Most plants need bacteria, lightning or industrial processes to turn it into usable compounds. Air can therefore be nitrogen-rich while soil still lacks plant-available nitrogen.
Fertilisers and polluted water
Farmers add nitrogen fertiliser when crops cannot obtain enough usable nitrogen from the soil. But plants cannot absorb all of it. Rain can carry extra nitrates into rivers and lakes, where algae grow rapidly; when algae die, decomposers use oxygen, sometimes leaving too little for fish. Understanding the cycle helps people choose amounts and methods that support crops without overloading waterways.
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