How Is Carbon Released From Soil (Methane; Carbon Dioxide)?

Carbon is released from soil mainly as carbon dioxide during aerobic respiration and decomposition, or as methane when organic matter decomposes under anaerobic conditions. This content belongs to IB ESS Subtopic 5.1 Soil and is HL only.

Soil contains a carbon store made up mainly of dead organic matter, humus, living organisms and plant roots. Whether soil acts as a carbon sink or carbon source depends on the balance between carbon inputs, such as leaf litter, and carbon outputs through decomposition and respiration.

Microorganisms decompose dead organic matter and use some of its carbon compounds in cellular respiration. The gas produced depends strongly on oxygen availability.

Temperature and moisture affect the rate of these transformations. Warming generally increases microbial activity and decomposition, accelerating $\text{CO}_2$ release if sufficient oxygen and moisture are available. Waterlogging restricts oxygen diffusion, encouraging anaerobic conditions and increasing $\text{CH}_4$ production.

For example, thawing Arctic permafrost exposes previously frozen organic matter to decomposition. This can release both $\text{CO}_2$ and $\text{CH}_4$, strengthening a positive feedback loop: warming increases decomposition, greenhouse gas emissions increase, and further warming occurs. The impacts also have an environmental justice dimension because Arctic Indigenous communities may experience rapid ecosystem and livelihood changes despite contributing relatively little to global emissions.

A common misconception is that methane and carbon dioxide are released interchangeably. Instead, oxygen availability largely determines which gas dominates.

Exam technique: For an “explain” question, link the soil condition to the process and then the gas: well-drained soil leads toaerobic respiration produces$\text{CO}_2$; waterlogged soil leads toanaerobic decomposition produces$\text{CH}_4$.