Which of the following processes takes place in the carbon cycle?

The carbon cycle describes how carbon moves between the atmosphere, living organisms, the oceans, and the earth's crust. The key processes in the carbon cycle include photosynthesis, respiration, decomposition, combustion, and fossilisation.

Combustion (burning) releases carbon dioxide into the atmosphere when fossil fuels (coal, oil, natural gas) or organic matter (wood, biomass) are burned. This is a major pathway by which stored carbon re-enters the atmospheric pool as CO₂. It is therefore a direct component of the carbon cycle.

The other options belong to different nutrient cycles or involve water, not carbon:

• Nitrification is a process in the nitrogen cycle, in which nitrifying bacteria convert ammonium ions (NH₄⁺) to nitrites and then nitrates in the soil.
• Evaporation is a phase of the water cycle (hydrological cycle), where liquid water changes to water vapour.
• Transpiration is also part of the water cycle. It involves the loss of water vapour from the aerial parts of plants, mainly through stomata.

Exam tip: When asked about processes in a specific nutrient cycle, focus on the element being cycled. Carbon cycle processes involve movement of carbon-containing compounds: photosynthesis, respiration, decomposition, combustion, and fossilisation.