Nitrogen Cycle IGCSE Biology 0610

The Nitrogen Cycle: Recycling Nitrogen Through Ecosystems

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

The nitrogen cycle converts atmospheric nitrogen into forms usable by plants (nitrates), which are then passed through food chains and returned to the soil by decomposition. Key processes include nitrogen fixation, nitrification, and denitrification, each carried out by specific bacteria.

The nitrogen cycle is a Supplement-only topic that involves four types of bacteria and multiple chemical conversions. Students must know which bacteria carry out which conversion and explain why nitrogen gas cannot be used directly by most organisms.

Why organisms need nitrogen

Nitrogen is essential for making amino acids (the building blocks of proteins) and nucleic acids (DNA and RNA). Although the atmosphere is 78% nitrogen gas (N₂), most organisms cannot use nitrogen in this form because the triple bond between the two nitrogen atoms is very strong and difficult to break.

Plants absorb nitrogen in the form of nitrate ions (NO₃^-) from the soil through their roots by active transport. Animals obtain nitrogen by eating plants or other animals that contain proteins.

Nitrogen fixation

Nitrogen fixation converts atmospheric nitrogen gas (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). This is carried out by nitrogen-fixing bacteria, some of which live freely in the soil and others in root nodules of leguminous plants (such as peas, beans and clover).

The bacteria in root nodules have a mutualistic relationship with the plant: the bacteria fix nitrogen into a usable form for the plant, and the plant provides the bacteria with sugars from photosynthesis. Lightning can also fix small amounts of nitrogen.

Nitrification and denitrification

Nitrification is carried out by nitrifying bacteria in the soil. They convert ammonium ions (NH₄⁺) first into nitrite ions (NO₂^-) and then into nitrate ions (NO₃^-), which plants can absorb. Nitrifying bacteria are aerobic and work best in well-aerated, moist soils.

Denitrification is the reverse process, carried out by denitrifying bacteria in anaerobic (waterlogged) conditions. They convert nitrates back into nitrogen gas, which is released into the atmosphere. Denitrification is why waterlogged soils lose fertility.

Decomposition and the role of decomposers

When organisms die or produce waste (urine, faeces), decomposers (bacteria and fungi) break down the proteins and other nitrogen-containing compounds into ammonium ions. This process is called ammonification or putrefaction.

The ammonium ions can then be converted into nitrates by nitrifying bacteria (nitrification), completing the cycle. Farmers add nitrogen to the soil by applying fertilisers (artificial nitrates) or manure (which decomposers convert into ammonium and then nitrates), or by growing leguminous crops that host nitrogen-fixing bacteria.

Try a focused question (3 marks)

Explain the role of nitrogen-fixing bacteria in the nitrogen cycle.

Show the answer and marking guidance

Nitrogen-fixing bacteria convert atmospheric nitrogen gas (N2) into ammonia or ammonium ions.
Some live freely in the soil; others live in root nodules of leguminous plants.
This makes nitrogen available in a form that can be converted to nitrates and absorbed by plants for making amino acids and proteins.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Saying plants absorb nitrogen gas directly from the atmosphere.
Confusing nitrifying bacteria with nitrogen-fixing bacteria.
Forgetting that denitrification occurs in anaerobic conditions.
Not knowing that nitrates are the form of nitrogen absorbed by plant roots.