SL and HL: Subtopic 5.1 Soil: Soil is a carbon store because it contains carbon at a particular time. It acts as a carbon sink when carbon inputs exceed outputs, but becomes a carbon source when outputs exceed inputs.
Carbon enters soil mainly through dead plant material, animal remains, waste and root secretions. Decomposers break down this organic matter, and some becomes relatively stable soil organic carbon, including carbon in humus.
Carbon leaves when decomposers carry out cellular respiration and release carbon dioxide. Carbon may also be lost through soil erosion, leaching, fire or removal of harvested biomass. The overall change can be represented as:
| Soil function | Carbon balance and conditions |
|---|---|
| Store | Soil contains a stock of carbon. This term describes the amount present, not whether it is increasing or decreasing. |
| Sink | Inputs exceed outputs, so the carbon store increases. Plant growth, litter accumulation, reduced tillage and restoration of wetlands can promote sequestration. |
| Source | Outputs exceed inputs, so the carbon store decreases. Deforestation, intensive ploughing, erosion, peat drainage and warming can accelerate carbon losses. |
For example, Indonesian peatlands contain very large carbon stores. When peat is drained for agriculture, oxygen enters the soil and decomposition accelerates; fires can then release additional carbon. The soil changes from a long-term carbon sink or store into a major carbon source, affecting the atmospheric carbon system and nearby communities through smoke pollution.
A common misconception is that a carbon store is automatically a sink. A store may remain constant or even shrink; sink specifically means that its carbon stock is increasing. At HL, also recognize that soils may release carbon as carbon dioxide or methane, depending partly on oxygen availability.
Exam technique: For an “explain” question, state the carbon balance, identify specific inputs and outputs, and link management to changes in decomposition. Use precise systems terminology: soil is a storage, while decomposition and carbon sequestration are flows.