The soil carbon balance

Soil organic matter consists largely of carbon, referred to as soil organic carbon (SOC). On a global scale, the top 30 cm of soils contain more carbon than the atmosphere and vegetation combined.

Carbon enters soils mainly through plant growth. Through photosynthesis, plants absorb carbon dioxide from the atmosphere and convert it into organic compounds that form leaves, stems, and roots. When plant residues or roots enter the soil, this carbon becomes part of the soil system.

Soil organisms then transform these materials. Microbes and soil animals break down organic matter and use part of it as energy. During this process, some carbon returns to the atmosphere as carbon dioxide. In environments where oxygen is limited (such as wetlands), part of the carbon may be released as methane.

However, not all carbon is lost. A portion becomes stabilised in the soil for decades to centuries — much of this stable carbon originates from microbial residues that bind to mineral surfaces or become protected within soil aggregates.