Nearly four fifths of the air is nitrogen, and almost nothing alive can use it. Plants need it in a completely different chemical form, and the only organisms that can make that conversion are bacteria. Once you accept that bacteria run this cycle, the four processes fall into place.
| Form | Where it is found |
|---|---|
| Organic nitrogen | In proteins, DNA and other compounds inside living organisms, and in dead organic matter. After death, decomposition returns it to the soil |
| Inorganic nitrogen | Nitrogen gas in the atmosphere, plus ammonia, nitrites and nitrates in soils and water |
The atmosphere holds the largest store, mostly as nitrogen gas (N2), making up around 78% of the air. It is also the least useful store, because the triple bond in N2 is very hard to break.
Bacteria such as Rhizobium convert atmospheric nitrogen (N2) into ammonia (NH3), which plants can use. This happens either free-living in the soil or through symbiotic relationships with plants such as legumes.
Fixation can also happen through lightning. The energy breaks nitrogen molecules in the atmosphere, allowing nitrogen atoms to combine with oxygen to form nitrogen oxides (NO and NO2). These dissolve in rain and fall to the ground as nitrates, a usable form for plants.
Nitrifying bacteria convert ammonia (NH3) into nitrites (NO2−), and then nitrites into nitrates (NO3−). Plants absorb these nitrates through their roots. Two steps, one direction — that is all you need.
Denitrifying bacteria convert nitrates back into nitrogen gas, which returns to the atmosphere. This happens in anaerobic (low oxygen) conditions, such as waterlogged soils.
Denitrification reduces nitrogen availability for plants, which is why plants in waterlogged ground often grow poorly — they cannot absorb enough nitrogen, and nitrogen is essential for growth. Some insectivorous plants such as pitcher plants and sundews have adapted to exactly these environments by obtaining nitrogen from insects instead.
When plants and animals die, decomposing bacteria break down their nitrogenous compounds into ammonium (NH4+), returning nitrogen to the soil where nitrification can pick it up again.
Just as with carbon, ESS separates movement from chemical change.
| Type | Process | What happens |
|---|---|---|
| Transfer | Mineral uptake | Plants absorb nitrates from the soil |
| Transfer | Consumption | Animals eat plants or other animals, moving nitrogen along the food chain as protein |
| Transfer | Excretion | Animals release nitrogen in urea, ammonia and faeces |
| Transfer | Death and decomposition | Dead plants and animals add nitrogen back to the soil |
| Transformation | Nitrogen fixation | Nitrogen gas becomes ammonia |
| Transformation | Nitrification | Ammonia becomes nitrites, then nitrates |
| Transformation | Ammonification | Organic nitrogen in proteins becomes ammonium |
| Transformation | Denitrification | Nitrates become nitrogen gas |
Plants cannot use atmospheric nitrogen directly. Some, though — legumes such as peas, beans and clover — form mutualistic relationships with nitrogen-fixing bacteria. The bacteria live in root nodules and convert nitrogen gas into ammonia.
This gives legumes a real competitive advantage in nitrogen-poor soils, because they can access nitrogen from the atmosphere that other plants cannot reach.
A field is repeatedly waterlogged after heavy rain. Explain why crop growth is poor, using the nitrogen cycle.
A farmer plants clover in a field for one season before returning to wheat. Explain how this benefits the wheat crop.
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