Nitrogen makes up 78% of the atmosphere, but most organisms cannot use N₂ gas directly. The nitrogen cycle describes how nitrogen is converted into usable forms — nitrates and ammonia — by microorganisms, absorbed by plants, passed through food chains, and returned to the atmosphere by denitrifying bacteria. At GCSE, you need the four main processes.
Why is nitrogen essential for living organisms?
Nitrogen is a component of amino acids (the building blocks of proteins), DNA, RNA, and ATP. Without a reliable supply of nitrogen in a usable form, organisms cannot make these vital molecules.
The problem is that atmospheric nitrogen (N₂) consists of two nitrogen atoms held together by a very strong triple bond (N≡N), which requires a great deal of energy to break. Most organisms lack the biochemical machinery to do this. Instead, they depend on specialised bacteria to convert N₂ into reactive forms they can absorb — primarily ammonium ions (NH₄⁺) and nitrate ions (NO₃⁻).
What are the four key processes in the GCSE nitrogen cycle?
The GCSE nitrogen cycle involves four interconnected processes, each carried out primarily by specific microorganisms.
1. Nitrogen fixation
Nitrogen fixation converts atmospheric N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺). There are two main routes:
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Biological fixation: Certain bacteria possess the enzyme nitrogenase, which catalyses the reduction of N₂ to NH₃. Key bacteria include:
- Rhizobium — lives in root nodules of leguminous plants (clover, peas, beans, lentils) in a mutualistic relationship. The plant provides sugars; the bacteria fix nitrogen for the plant.
- Free-living soil bacteria (e.g. Azotobacter) fix nitrogen independently in the soil.
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Industrial fixation: The Haber process (N₂ + 3H₂ → 2NH₃) fixes nitrogen industrially for fertilisers, but this is not part of the biological nitrogen cycle.
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Lightning: High-energy lightning converts N₂ to NO, which then dissolves in rain as dilute nitric acid, adding nitrate to the soil. This contributes only a small fraction of total fixed nitrogen.
2. Nitrification
Nitrification is the oxidation of ammonium ions to nitrite and then to nitrate by nitrifying bacteria in the soil. This occurs in two steps, each carried out by different bacteria:
- Step 1: Ammonium → Nitrite (NO₂⁻) — carried out by bacteria such as Nitrosomonas
- Step 2: Nitrite → Nitrate (NO₃⁻) — carried out by bacteria such as Nitrobacter
Nitrate (NO₃⁻) is the form most easily absorbed by plant roots. Nitrification requires aerobic conditions (oxygen) — it is suppressed in waterlogged, oxygen-poor soil.
3. Decomposition (ammonification)
Decomposition returns nitrogen to the soil from dead organisms and animal waste. Decomposers (bacteria and fungi that are saprotrophs) break down proteins and nucleic acids in dead organic matter, releasing nitrogen as ammonium ions (NH₄⁺) — a process also called ammonification.
Without decomposition, nitrogen would become permanently locked in dead organic material and organisms would run short of nitrogen compounds in the soil.
Earthworms and other detritivores assist by breaking large pieces of dead material into smaller fragments, increasing the surface area available for microbial decomposition.
4. Denitrification
Denitrification is the reduction of nitrate (NO₃⁻) back to nitrogen gas (N₂), which is released into the atmosphere. It is carried out by denitrifying bacteria in anaerobic (oxygen-poor) conditions, such as waterlogged or heavily compacted soil.
From the perspective of agriculture, denitrification is undesirable — it removes nitrate from the soil and reduces its fertility. Waterlogged fields encourage denitrification, which is why drainage and aeration improve crop yield.
A summary table of the nitrogen cycle processes
| Process | Transformation | Carried out by | Conditions |
|---|---|---|---|
| Nitrogen fixation | N₂ → NH₃ / NH₄⁺ | Rhizobium, Azotobacter, lightning | Anaerobic (inside root nodules) or aerobic (free-living) |
| Nitrification | NH₄⁺ → NO₂⁻ → NO₃⁻ | Nitrosomonas, Nitrobacter | Aerobic |
| Decomposition | Organic N → NH₄⁺ | Decomposing bacteria and fungi | Aerobic or anaerobic |
| Denitrification | NO₃⁻ → N₂ | Denitrifying bacteria | Anaerobic |
How do animals obtain nitrogen?
Animals cannot absorb nitrogen directly from the soil or atmosphere. They obtain nitrogen by eating proteins — from plants (which have absorbed nitrate from soil) or from other animals. The nitrogen in food proteins is used to build the animal's own proteins and DNA. Nitrogen from excess amino acids is excreted as urea (in mammals), which decomposes to NH₄⁺ in the soil and re-enters the cycle.
What is the role of leguminous plants in agriculture?
Farmers exploit the mutualistic relationship between legumes and Rhizobium bacteria by crop rotation — alternating cereal crops (which deplete soil nitrate) with nitrogen-fixing legume crops such as clover, field beans, or peas. The legume roots replenish soil ammonium/nitrate without artificial fertiliser. This traditional practice reduces the cost of fertilisers and prevents nitrate leaching (which causes eutrophication in waterways).
How does the nitrogen cycle connect to eutrophication?
If excess nitrate fertiliser is applied to fields, rain can wash it into rivers and lakes — a process called leaching. High nitrate concentrations stimulate rapid algal growth (algal bloom), blocking light from submerged plants. When the algae die, aerobic decomposers proliferate and consume dissolved oxygen, making the water hypoxic. Fish and other aquatic organisms suffocate. This cascade is eutrophication — a topic that links the nitrogen cycle to water pollution.
Frequently asked questions
Why can plants not use atmospheric nitrogen directly?
Atmospheric nitrogen (N₂) contains a triple bond (N≡N) with a bond energy of approximately 945 kJ/mol — one of the strongest bonds in chemistry. Breaking this bond to release reactive nitrogen requires specialised enzyme systems (nitrogenase) found only in nitrogen-fixing bacteria. Plants have evolved to absorb the products of nitrogen fixation — ammonium ions and nitrates — from the soil rather than attempting to fix atmospheric nitrogen themselves.
What is the difference between nitrification and nitrogen fixation?
Nitrogen fixation converts atmospheric N₂ (inert gas) into ammonium compounds (NH₃/NH₄⁺) — it is the entry point for new nitrogen into the biological cycle. Nitrification converts ammonium compounds already in the soil into nitrite and then nitrate — it is a transformation within the cycle, not an entry of new nitrogen. Both require bacterial action but in different conditions and using different bacteria.
Why does waterlogged soil have lower nitrogen availability for plants?
Waterlogged soil is oxygen-depleted. Two effects reduce nitrogen availability: (1) nitrifying bacteria (which need aerobic conditions) cannot oxidise ammonium to nitrate, so plants cannot access nitrate from that pool; (2) denitrifying bacteria thrive in anaerobic conditions and convert nitrate back to N₂ gas, removing it from the soil. Drainage and aeration restore aerobic conditions, allowing nitrification to proceed and suppressing denitrification.
How does the nitrogen cycle differ from the carbon cycle at GCSE level?
Both cycles involve microorganisms as key agents and connect producers, consumers, and decomposers. The key difference is that the nitrogen cycle includes a unique entry-point step — nitrogen fixation, which converts inert atmospheric gas into biologically usable compounds. The carbon cycle's entry-point is photosynthesis, which uses a carbon compound (CO₂) that is already reactive. Nitrogen also has an additional return pathway (denitrification back to N₂) that has no direct parallel in the carbon cycle.
For Socratic GCSE biology with Professor Darwin — tracing atoms through the living systems and microbial communities that cycle them — visit aitutors.me.