5.1.21 (HL)—Carbon release from soils

Aerobic decomposition mainly releases CO₂; anaerobic conditions can produce CH₄, and warming or drainage can alter the balance and feedback.

Tillage can expose organic matter to oxygen and accelerate CO2 release; wetland drainage similarly increases aerobic decomposition, while saturated anaerobic soils can produce CH4. Warming may accelerate decomposition, although the response depends on moisture, oxygen and available substrate.

A reinforcing feedback occurs when warming increases soil greenhouse-gas release, which adds further warming and carbon loss. The syllabus also identifies a potential tipping point where rising temperature destabilizes methane clathrates in underlying geological structures, causing an additional methane release.

Treat a tipping point as a threshold risk, not a guaranteed outcome: the feedback depends on temperature, carbon form, water and oxygen conditions.

‘Wet soil always emits more greenhouse gas’ is too simple; distinguish CH₄, CO₂, oxygen and net radiative effect.