Carbon Cycle

Quick Check: A scientist measures the net change in CO2 in a sealed container holding a green plant over 24 hours. During the 12 hours of daylight, CO2 levels fall. During the 12 hours of darkness, CO2 levels rise. Explain these observations in terms of the carbon cycle processes occurring in the plant.

During daylight: The plant carries out both photosynthesis and respiration simultaneously. However, the rate of photosynthesis exceeds the rate of respiration during the day. Photosynthesis removes CO2 from the air to produce glucose; respiration releases CO2 but at a lower rate. The net effect is CO2 removal from the container, so levels fall. During darkness: The plant can no longer carry out photosynthesis (no light energy available). However, it continues to respire at all times, breaking down glucose to release energy and producing CO2 as a waste product. With no photosynthesis to compensate, CO2 accumulates in the container and levels rise. This illustrates that plants are net carbon sinks only when the light is bright enough for photosynthesis to exceed respiration (the light compensation point).

Quick Check: Peat bogs form when dead plant material accumulates in waterlogged, oxygen-poor conditions over thousands of years. Farmers sometimes drain peat bogs to create agricultural land. Explain why draining peat bogs increases atmospheric CO2 concentration.

In a waterlogged peat bog, decomposer bacteria cannot function effectively because most require oxygen for aerobic respiration. Without decomposition, dead plant material — and the carbon locked within it — accumulates over thousands of years, forming peat (a major carbon store). When the bog is drained, oxygen enters the previously waterlogged soil. Aerobic decomposers (bacteria and fungi) can now break down the accumulated organic matter through respiration, releasing CO2 as a waste product. Decades of stored organic carbon are mineralised relatively quickly, releasing large amounts of CO2 into the atmosphere and converting a long-term carbon sink into a carbon source. This accelerates the rise in atmospheric CO2 and contributes to the enhanced greenhouse effect.

Quick Check: A student argues: "If we stopped burning fossil fuels tomorrow, CO2 levels in the atmosphere would immediately start falling." Evaluate this argument using your knowledge of the carbon cycle.

The argument is partially correct but oversimplified. Stopping fossil fuel combustion would remove a major source of CO2 emissions. However, CO2 levels would not fall immediately. CO2 already in the atmosphere would continue to drive photosynthesis and dissolution in oceans, but these processes would only gradually reduce atmospheric CO2 over decades. Respiration by all organisms (animals, plants, decomposers) would continue to release CO2. Additionally, positive feedbacks could delay or prevent a fall — for example, warming already caused by elevated CO2 could release methane from thawing permafrost (another greenhouse gas), or accelerate decomposition of organic matter, releasing more CO2. A rapid fall would require both ceasing emissions AND actively removing CO2 from the atmosphere (e.g., through reforestation or carbon capture technology).