Consider the following: 1) Photosynthesis 2) Respiration 3) Decay of organic matter 4) Volcanic action Which of the above add carbon dioxide to the carbon cycle on Earth?

Think of the carbon cycle as a ledger: some processes take CO2 out of the atmosphere, others put it back. Photosynthesis is the main sink here. Plants fix atmospheric CO2 into sugars, so statement 1 removes carbon rather than adding it. That rules out options A and D straight away.

• Respiration (2): Plants, animals and microbes release CO2 as they oxidise glucose for energy. This returns the carbon that photosynthesis had fixed.
• Decay of organic matter (3): Bacteria and fungi break down dead matter and release CO2 back into the air and soil.
• Volcanic action (4): Volcanoes release CO2 from deep carbon stores in the mantle and crust. It is a slow geological source, but it is a real source.

Core concept: carbon reservoirs (atmosphere, biosphere, oceans, lithosphere) and the fluxes between them. Mains angle: fossil fuel burning has pushed human emissions far above these natural fluxes, which connects to climate change questions in GS-III.

Trap: Candidates pick D by assuming photosynthesis adds carbon because it is a carbon-cycle process. Always ask the direction of flow: is CO2 moving out of the air or into it?