Soil carbon

Soil carbon includes both inorganic carbon as carbonate minerals, and as soil organic matter. Soil carbon plays a key role in the carbon cycle, and thus it is important in global climate models. Soil carbon includes both inorganic carbon as carbonate minerals, and as soil organic matter. Soil carbon plays a key role in the carbon cycle, and thus it is important in global climate models. Soil carbon is present in two forms: inorganic and organic. Soil inorganic carbon consists of mineral forms of C, either from weathering of parent material, or from reaction of soil minerals with atmospheric CO2. Carbonate minerals are the dominant form of soil carbon in desert climates. Soil organic carbon is present as soil organic matter. It includes relatively available C as fresh plant remains and relatively inert C in materials derived from plant remains: humus and charcoal. Although exact quantities are difficult to measure, human activities have caused massive losses of soil organic carbon. Of the 2,700 Gt of C stored in soils worldwide, 1550 GtC is organic and 950 GtC is inorganic carbon, which is approximately three times greater than the current atmospheric C and 240 times higher compared with the current annual fossil fuel emission. The balance of soil carbon is held in peat and wetlands (150 GtC), and in plant litter at the soil surface (50 GtC). This compares to 780 GtC in the atmosphere, and 600 GtC in all living organisms. The oceanic pool holds 38,200 GtC. About 60 GtC/yr is added to soil. This 60 GtC/yr is the balance of 120 GtC/yr taken out of the atmosphere by terrestrial plant photosynthesis reduced by 60 GtC/yr plant respiration. An equivalent 60 GtC/yr is respired from soil, joining the 60G tC/yr plant respiration to return to the atmosphere. Soil organic carbon is divided between living soil biota and dead biotic material derived from biomass. Together these comprise the soil food web, with the living component sustained by the biotic material component. Soil biota includes earthworms, nematodes, protozoa, fungi, bacteria and different arthropods. Detritus resulting from plant senescence is the major source of soil organic carbon. Plant materials, with cell walls high in cellulose and lignin, are decomposed and the not-respired carbon is retained as humus. Cellulose and starches are easily degraded, resulting in relatively short residence times. More persistent forms of organic C include lignin, humus, organic matter encapsulated in soil aggregates, and charcoal. These resist alteration and have long residence times. Soil organic carbon tends to be concentrated in the topsoil. Topsoil ranges from 0.5% to 3.0% organic C for most upland soils. Soils with less than 0.5% organic C are mostly limited to desert areas. Soils containing greater than 12 - 18% organic C are generally classified as organic soils. High levels of organic C develop in soils supporting wetland ecology, flood deposition, fire ecology, and human activity. Fire derived forms of carbon are present in most soils as unweathered charcoal and weathered black carbon. Soil organic C is typically 5 - 50% derived from char, with levels above 50% encountered in mollisol, chernozem, and terra preta soils. Root exudates are another source of soil carbon. 5 - 20% of the total plant carbon fixed during photosynthesis is supplied as root exudates in support of rhizospheric mutualistic biota. Microbial populations are typically higher in the rhizosphere than in adjacent bulk soil.