Reviews and syntheses: The mechanisms underlying carbon storage in soil

International audienceScientific research in the 21st century has considerably improved our knowledge of soil organic matter and its dynamics, particularly under the pressure of the global disruption of the carbon cycle. This paper reviews the processes that control C dynamics in soil, the representation of these processes over time, and their dependence on variations in major biotic and abiotic factors. The most recent advances in soil organic matter knowledge are: – Most organic matter is composed of small molecules, derived from living organisms, without transformation via additional abiotic organic polymerization. – Microbial compounds are predominant in the long term. – Primary belowground production contributes more to organic matter than aboveground inputs. – Contribution of less biodegradable compounds to soil organic matter is low in the long term. – Two major factors determine the soil organic carbon production yield from the initial substrates: the yield of carbon used by microorganisms and the association with minerals, particularly poorly crystallized minerals, which stabilize microbial compounds. – Interactions between plants and microorganisms and between microbial communities affect or even regulate carbon residence times, and therefore carbon stocks. Farming practices therefore affect soil C stocks not only through carbon inputs but also via their effect on microbial and organomineral interactions