Overview
Although the figure is frequently being revised upwards with new discoveries, over 2,700 Gigatonnes (Gt) of carbon is stored in soils worldwide, which is well above the combined total of atmosphere (780 Gt) or biomass (575 Gt), most of which is wood. Carbon is taken out of the atmosphere by plant photosynthesis; about 60 Gt annually is incorporated into various types of soil organic matter including surface litter; about 60 Gt annually is respired or oxidized from soil.
Soil carbon is the last major pool of the carbon cycle. The carbon that is fixed by plants is transferred to the soil via dead plant matter including dead roots, leaves and fruiting bodies. This dead organic matter creates a substrate which decomposes and respires back to the atmosphere as carbon dioxide or methane depending on the availability of oxygen in the soil. Soil carbon is also oxidized by combustion and returned to the atmosphere as carbon dioxide.
Soil carbon is primarily composed of biomass and non-biomass sources. Biomass carbon includes various bacteria and fungi. Non-biomass carbon sources or substrates reflect the chemical composition of plant biomass and primarily include cellulose, starch, lignin and other diverse organic carbon compounds. Some of the substrate carbon binds to the mineral soil, becoming encapsulated in soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexional.
Biomass feeds on the substrate carbon compounds at different rates.
Some of the carbon compounds are easily digested and respired by the microbes resulting in a relatively short residence time. Others, like lignin, humic acid or substrate encapsulated in soil aggregates are very difficult for the biomass to absorb and have long residence times.
Read more about this topic: Soil Carbon