The processes that move carbon and nitrogen through the biosphere, atmosphere, hydrosphere and lithosphere, including photosynthesis, respiration, nitrogen fixation, nitrification, denitrification and decomposition.

What this dot point is asking

AQA wants you to describe the carbon cycle and the nitrogen cycle, name and explain the key processes (photosynthesis, respiration, combustion, nitrogen fixation, nitrification, denitrification and decomposition), and explain how human activity disturbs each cycle. The most-rewarded skill is stating the direction of each conversion correctly, since this is where most marks are lost.

The carbon cycle

Carbon moves between several stores through clearly defined fluxes:

• Photosynthesis removes carbon dioxide from the atmosphere and fixes it into organic compounds (glucose, then biomass) in plants and algae.
• Respiration by all organisms (plants, animals, decomposers) breaks down organic compounds and returns carbon dioxide to the atmosphere.
• Decomposition of dead matter releases carbon dioxide as decomposers respire.
• Combustion of fossil fuels, wood and other biomass releases carbon dioxide.
• Ocean exchange: carbon dioxide dissolves in seawater and is used by marine organisms; some forms carbonate shells.
• Long-term stores include fossil fuels, carbonate rocks such as limestone, and the deep oceans, which hold carbon for millions of years.

The nitrogen cycle

Atmospheric nitrogen gas ($N_2$) makes up about 78 percent of the air but is very unreactive because of its strong triple bond, so it must be converted into usable forms before organisms can use it to build proteins and DNA.

Decomposition (ammonification) by decomposers releases nitrogen from dead organisms and waste as ammonium, recycling it back into the soil. Plants absorb nitrate (and some ammonium) through their roots, build it into amino acids and proteins, and pass it along food chains. The cycle therefore links the atmosphere, soil, microbes, plants and animals, and depends heavily on bacteria at almost every step.

Human disruption of the cycles

• Burning fossil fuels and clearing forests raise atmospheric carbon dioxide, enhancing the greenhouse effect and driving climate change; deforestation also removes a carbon sink.
• Heavy use of nitrogen fertilisers adds large amounts of reactive nitrogen to soils; runoff into rivers and lakes causes eutrophication, algal blooms and oxygen depletion.
• The Haber process and combustion engines add reactive nitrogen compounds (including nitrogen oxides) to the environment, contributing to acid rain and smog.
• Drainage and ploughing can speed decomposition of soil organic matter, releasing stored carbon.

Try this

Q1. Name the process that converts atmospheric nitrogen into ammonium and give two ways it occurs. [3 marks]

• Cue. Nitrogen fixation, by nitrogen-fixing bacteria (free-living or in legume nodules), lightning, or the Haber process (any two).

Q2. Explain how burning fossil fuels disrupts the carbon cycle and why this raises atmospheric carbon dioxide. [3 marks]

• Cue. It moves carbon from long-term geological stores to the atmosphere faster than photosynthesis and ocean uptake remove it, so the input exceeds removal and concentration rises.

Q3. Explain why growing a legume crop in a rotation can reduce the need for nitrogen fertiliser. [2 marks]

• Cue. Legumes host nitrogen-fixing Rhizobium in root nodules, adding fixed nitrogen to the soil and replenishing fertility naturally.