The carbon and nitrogen cycles
How carbon and nitrogen are recycled through ecosystems.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 19.2.1Describe the carbon cycle and the role of decomposers.
- 19.2.2Describe the nitrogen cycle and the roles of the bacteria involved.Supplement
6 minute read
The carbon and nitrogen cycles
Energy flows through an ecosystem and is lost as heat. Nutrients are different — they cycle, being used repeatedly.
The carbon cycle
Carbon removed from the atmosphere:
- Photosynthesis — the only significant route, converting CO₂ into glucose in plants.
Carbon returned to the atmosphere:
- Respiration by plants, animals and decomposers.
- Combustion of fuels, including fossil fuels and wood.
- Decomposition — decomposers respire as they break down dead material.
Carbon passes along food chains when animals eat plants, and is locked away long-term as fossil fuels when dead organisms do not fully decompose. The modern imbalance: combustion of fossil fuels releases carbon stored for millions of years faster than photosynthesis removes it, raising atmospheric CO₂.
Decomposers
Bacteria and fungi break down dead organisms and waste, releasing mineral ions back into the soil for plants to reuse. Without them, nutrients would stay locked in dead material and ecosystems would collapse — which is why they appear in every cycle.
The nitrogen cycle
Nitrogen gas is 78% of the air but unreactive, so most organisms cannot use it directly. Four groups of bacteria do the work:
- Nitrogen-fixing bacteria — convert atmospheric N₂ into nitrogen compounds. Some live free in soil, some in root nodules of legumes.
- Decomposers — break down proteins in dead matter into ammonium compounds.
- Nitrifying bacteria — convert ammonium into nitrites then nitrates, the form plants absorb.
- Denitrifying bacteria — convert nitrates back into nitrogen gas, reducing soil fertility. They thrive in waterlogged, low-oxygen soils, which is why good drainage matters.
Plants absorb nitrates to make amino acids and proteins; animals obtain nitrogen by eating plants. Lightning also fixes small amounts of nitrogen.
Think of it like this
Nutrients are library books and energy is the electricity bill: the books circulate endlessly between borrowers, while the electricity is consumed and gone. That is why one cycles and the other must be constantly resupplied by the Sun.
Worked examples
Method, step by step
Explain why waterlogged soil becomes less fertile for crop growth.
- 1Waterlogged soil contains little oxygen.
- 2Denitrifying bacteria thrive in low-oxygen conditions.
- 3They convert nitrates in the soil back into nitrogen gas, which plants cannot absorb.
- 4With fewer nitrates, plants cannot make enough amino acids and proteins, so growth is poor.
Low oxygen favours denitrifying bacteria, which convert useful nitrates back into unusable nitrogen gas, so plants have less nitrate for making proteins.
Common misconceptions
- Saying energy is recycled. Energy flows one way and is lost as heat; only nutrients cycle.
- Thinking plants absorb nitrogen gas from the air. They absorb **nitrates** from the soil.
- Confusing nitrifying with denitrifying bacteria. Nitrifying makes nitrates (helpful); denitrifying destroys them (harmful to fertility).
In the exam
- Name the specific bacteria group in nitrogen cycle answers — "bacteria" alone rarely earns the mark.
- For the carbon cycle, remember photosynthesis is the only major process removing CO₂, while three processes return it.