Key points • Carbon occurs in different forms in soils: in living organisms, dead organic matter with particulate organic matter, organic molecules, and as inorganic carbon • Soil carbon stock and soil carbon storage are different concepts: a carbon stock is quantified at a given date while its storage is quantified over a period of time • The carbon stock distribution across the planet is heterogeneous, and complex biotic and abiotic processes underlie its dynamics • For a given soil and climate, the largest stocks are found in forests and grasslands. In comparison, cultivated soils are carbon depleted • Land-use changes and agricultural practices have a major impact on soil carbon stocks: they may have negative, positive or diverse effects on carbon storage, depending on climate and soil conditions • Soil carbon storage provides socioeconomic benefits as targeted by five Sustainable Development Goals