The Nitrogen Cycle Explained: The Diagram Examiners Love

Quick answer: The nitrogen cycle moves nitrogen between the air, the soil and living things. Four processes do the work: nitrogen fixation (N2 gas into ammonia and nitrates, by nitrogen-fixing bacteria and lightning), nitrification (ammonium into nitrates, by nitrifying bacteria), assimilation (plants take up nitrates to build proteins; animals get theirs by eating), and denitrification (nitrates back into N2 gas, by denitrifying bacteria in waterlogged soil). Decomposers return nitrogen in dead matter to the soil as ammonium.

The nitrogen cycle was the first diagram I genuinely feared — a plate of spaghetti, arrows everywhere and four sorts of bacteria that all blur together. Then a teacher told me examiners don't want the spaghetti; they want four clean moves and the bacteria that drive each one. Once I learned it as a story — nitrogen leaving the air, looping through living things, then going home — the marks stopped hiding.

Why nitrogen matters

• The air is about 78% nitrogen gas (N2) — more of it above your head than anything else.
• But N2 is almost unreactive — the two atoms are held by a strong triple bond, so plants and animals cannot use it straight from the air. This one fact is why the whole cycle exists.
• Living things need nitrogen to make amino acids (which build proteins) and DNA — no usable nitrogen, no growth.
• Plants can only take nitrogen in as nitrate ions (NO3-) dissolved in soil water, through their roots. Animals can't even do that — we get ours by eating plants or other animals.

The four processes examiners want

1. Nitrogen fixation (air to soil)

Turning unreactive N2 into useful nitrogen compounds (ammonia, then nitrates). Three routes:

• Nitrogen-fixing bacteria. Some live free in the soil; others (Rhizobium, in the CBSE and Cambridge detail) live inside the root nodules of legumes — peas, beans, clover. It's a mutualism: the plant feeds them sugars, they hand back nitrogen compounds. That is why farmers grow clover or beans and plough them in — free fertiliser.
• Lightning. The energy in a strike makes nitrogen and oxygen react, forming nitrogen oxides that dissolve in rain and reach the soil as nitrates.
• The Haber process. Factories fix N2 into ammonia for fertiliser.

2. Nitrification (ammonium to nitrate)

Nitrifying bacteria convert ammonium ions into nitrates in two oxidation steps: ammonium (NH4+) to nitrite (NO2-) to nitrate (NO3-). Because it is oxidation, these bacteria need oxygen — they only work in well-aerated soil. That is why farmers plough and drain fields: to let air in so nitrification keeps running.

3. Assimilation (nitrate to protein)

Plants absorb nitrate ions through their roots and use the nitrogen to build amino acids, then proteins and DNA. When an animal eats the plant, that nitrogen becomes animal protein — the nitrogen has been assimilated into a living body.

4. Denitrification (soil back to air)

Denitrifying bacteria convert nitrates back into nitrogen gas, which escapes to the atmosphere. Crucially, they are anaerobic — they thrive when there is no oxygen, meaning waterlogged, compacted soil. For a farmer they are the villain, stripping nitrates and lowering fertility. It is the exact opposite condition to nitrification — a favourite contrast to test.

And the step that keeps the whole thing turning — decomposition. When plants and animals die, the nitrogen is locked in their proteins (and in waste like urea and faeces). Decomposers — bacteria and fungi — break these down and release the nitrogen back into the soil as ammonium. This step (ammonification) refills the soil so nitrification has something to work on.

The bacteria, sorted (the part people lose marks on)

Keep the four jobs straight:

• Nitrogen-fixing bacteria: N2 gas to ammonia and nitrogen compounds. In legume root nodules, or free in the soil.
• Decomposers (bacteria and fungi): dead matter and waste to ammonium.
• Nitrifying bacteria: ammonium to nitrite to nitrate. Need oxygen.
• Denitrifying bacteria: nitrate to N2 gas. Need NO oxygen — waterlogged soil.

The most common mix-up is thinking nitrogen-fixing and nitrifying bacteria do the same job. Fixing brings nitrogen in from the air; nitrifying only upgrades ammonium already in the soil into nitrate.

How much you memorise depends on your board:

• Cambridge IGCSE and most GCSE (Edexcel, OCR): name the types — nitrogen-fixing, nitrifying, denitrifying bacteria, plus decomposers — and the role of lightning. Genus names are not required.
• CBSE and Cambridge A-Level: go further, naming Rhizobium (fixing, in nodules) and often the two-step oxidation of nitrification.
• AQA GCSE leans on decomposition and the carbon cycle rather than the full nitrogen cycle, so check your spec before over-learning it.

Not sure which applies to you? Paste your syllabus point into Explain anything and ask it to pitch the answer at your board.

Worked example: trace the nitrogen (the classic 6-marker)

Question: "Nitrogen in a dead rabbit ends up as protein in a growing wheat plant. Describe how." Six marks means roughly six mark-bearing steps:

1. Decomposition. Decomposers (bacteria and fungi) break down the proteins in the dead rabbit and release the nitrogen as ammonium ions into the soil. (1 mark)
2. Nitrification, part one. Nitrifying bacteria oxidise the ammonium into nitrite. (1 mark)
3. Nitrification, part two. The nitrite is oxidised into nitrate ions, NO3-. (1 mark)
4. Condition. This needs oxygen, so it happens in well-aerated soil. (1 mark — the condition mark most people skip.)
5. Absorption. The wheat plant absorbs the nitrate ions from the soil through its roots. (1 mark)
6. Assimilation. The plant uses the nitrogen from the nitrates to make amino acids, joined together into proteins. (1 mark)

Notice what scores: naming the bacteria, the ions at each stage (ammonium, nitrite, nitrate), and the condition (oxygen). A vague "the nitrogen goes into the soil and the plant takes it up" earns maybe one mark of six. Want your own answer marked against your board's scheme? Drop it into Explain anything for line-by-line grading, or make more like it with the quiz maker.

Test yourself

1. A field floods and stays waterlogged for weeks. Explain what happens to the amount of nitrate in the soil, and name the type of bacteria responsible.
2. Why can't a plant use the nitrogen in the air directly, even though the air is 78% nitrogen?
3. Put these in the right order for nitrogen from a dead leaf to reach a plant as nitrate: nitrification, absorption, decomposition.

Quick answers:

1. The nitrate falls. Waterlogged soil is low in oxygen (anaerobic), which suits denitrifying bacteria; they convert nitrates back into nitrogen gas, lowering soil fertility.
2. Nitrogen gas (N2) is unreactive because of its strong triple bond, so plants cannot absorb or use it. They can only take it up as nitrate ions from the soil, so fixation must happen first.
3. Decomposition, then nitrification, then absorption.

Turn the four processes and their bacteria into flashcards, or sit a timed set on ecology with the quiz maker.

FAQ

What is the difference between nitrogen fixation and nitrification?

Fixation brings new nitrogen in from the air, turning N2 gas into ammonia and nitrogen compounds. Nitrification happens in the soil, converting ammonium already there into nitrite then nitrate. Different starting materials, different bacteria.

What is the role of decomposers in the nitrogen cycle?

Decomposers (bacteria and fungi) break down proteins in dead organisms and waste, releasing the nitrogen into the soil as ammonium. Without them, nitrogen stays locked in dead matter and the cycle stalls.

Which bacteria need oxygen, and which don't?

Nitrifying bacteria are aerobic — they need oxygen and work in well-aerated soil. Denitrifying bacteria are anaerobic — they thrive without oxygen, which is why waterlogged soil loses its nitrates.

Do I have to memorise names like Rhizobium?

It depends on your board. CBSE and Cambridge A-Level expect genus names; Cambridge IGCSE and most GCSE only want the type of bacteria and its job. If in doubt, learn the job first — a named bacterium on the wrong role scores nothing.

In short: The nitrogen cycle solves one problem — getting unreactive N2 out of the air into a form roots can use. Learn it as four moves (fixation, nitrification, assimilation, denitrification) plus decomposers refilling the soil; name the bacteria behind each, and the two conditions that flip it: oxygen for nitrification, none for denitrification.