IB Biology Topic C4.2.18: Ecosystems as Carbon Sinks and Carbon Sources Notes

Whether an ecosystem is a carbon sink or source depends on its carbon balance

1. Carbon enters an ecosystem mainly as carbon dioxide fixed in photosynthesis and leaves it mainly through respiration.
2. A carbon sink absorbs more carbon than it releases, so its carbon store grows.
3. A carbon source releases more carbon than it absorbs, so its carbon store shrinks.

Growing forests, peatlands and oceans act as carbon sinks

1. A growing forest is a sink because photosynthesis locks carbon into new biomass faster than respiration returns it.
2. The oceans act as a sink by dissolving carbon dioxide and storing it as carbonate.
3. In waterlogged, acidic and anaerobic habitats such as bogs, decomposition is slowed so carbon builds up as peat.
4. A peatland is a long-term sink because dead organic matter accumulates instead of being fully decomposed.

Deforestation, drained peatland and fire turn ecosystems into carbon sources

1. After deforestation, less carbon is fixed while decomposition and burning of the felled trees release stored carbon.
2. When a peatland is drained, oxygen reaches the peat and decomposition releases the stored carbon as carbon dioxide.
3. During a fire, carbon stored in living organisms and organic matter is released rapidly as carbon dioxide.
4. This makes the ecosystem a carbon source until regrowth restores photosynthesis.