Nutrient cycles

Recycling of materials keeps ecosystems running. Three cycles dominate at GCSE: water, carbon and nitrogen.

The water cycle

1. Evaporation — water from oceans/lakes vaporises (driven by Sun).
2. Transpiration — water vapour from leaves of plants.
3. Condensation — vapour rises, cools, forms clouds.
4. Precipitation — rain, snow, hail return water to land or sea.
5. Surface run-off / percolation — water flows back to rivers and oceans.

The carbon cycle

CO2 in the atmosphere is the central reservoir.

• Photosynthesis removes CO2: CO2 + H2O to give glucose + O2 (uses light).
• Respiration by all organisms returns CO2: glucose + O2 to give CO2 + H2O.
• Decomposition by bacteria and fungi releases CO2 from dead matter.
• Combustion of fuels (wood, coal, oil, gas) releases CO2.
• Sequestration in fossil fuels and carbonate rocks locks carbon away over geological time.

Burning fossil fuels at faster rates than photosynthesis can absorb is increasing atmospheric CO2 and driving climate change.

The nitrogen cycle

Plants need nitrogen for amino acids and proteins. Atmospheric N2 (78%) is unreactive and unusable directly.

Key processes:

• Nitrogen fixation — Rhizobium (in legume root nodules) and lightning convert N2 to ammonium / nitrate compounds.
• Nitrification — nitrifying bacteria convert ammonium ions (NH4+) to nitrites then nitrates (NO3-).
• Plant uptake — roots absorb nitrate ions to make amino acids and proteins.
• Decomposition — decomposers break down dead organisms / waste, releasing ammonium ions.
• Denitrification — denitrifying bacteria (in waterlogged soil) convert nitrates back to N2 gas.

WJEC exam tip

Always name the bacteria type when describing the nitrogen cycle: nitrogen-fixing, nitrifying, decomposing, denitrifying. Generic "bacteria" loses the marking-point B1.