The Nitrogen Cycle

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Nitrogen makes up 78% of air but plants can't use it directly. Nitrogen-fixing bacteria in soil and legume root nodules convert N₂ to ammonia; nitrifying bacteria change ammonia to nitrites and nitrates, which plants absorb to make protein. Animals get nitrogen by eating. Decomposers return it as ammonia, and denitrifying bacteria turn some nitrate back into nitrogen gas.

Nitrogen is needed to build amino acids, proteins and DNA, so shortage limits plant growth in many soils. The key steps are nitrogen fixation (also done by lightning), nitrification, assimilation by plants, ammonification by decomposers and denitrification in waterlogged soil. Each step is carried out by specific microorganisms.

Farmers use this knowledge. Growing clover, peas or beans and ploughing them in adds fixed nitrogen, which is why crop rotation with legumes improves yields. Fertiliser provides nitrates directly, but waterlogged fields lose nitrate through denitrification and runoff.

Excess nitrate washing into rivers causes eutrophication, linking this cycle to pollution. A typical misconception is that plants absorb nitrogen gas from the air through their leaves; they take up nitrate and ammonium ions through the roots. Another is that bacteria are all harmful, when in fact these bacteria keep soils fertile.