Soil organic matter represents a reservoir of stored (sequestered) carbon that slowly cycles between the earth and the atmosphere. It is predominately composed of humus stabilized by clay particles, with the remainder being less stable decaying plant and animal matter. Globally, soil holds roughly twice as much carbon as is in atmospheric carbon dioxide (CO₂), so there is potentially a lot of carbon to lose from soil. On the other hand, soil with the right properties can also still remove CO₂ from the atmosphere and sequester it. See also: Atmosphere; Atmospheric chemistry; Biogeochemistry; Carbon dioxide; Clay; Humus; Organic geochemistry; Soil; Soil chemistry

Storing organic matter in soil is important for modulating the Earth’s climate and improving soil properties for supporting plant life, including crops for agriculture. As a result, understanding the best conditions and practices for stabilizing carbon in soil is important, as is modeling the influence of factors such as a warming climate. For example, C. Vogel and coworkers found that it is not simply the amount of clay in soil but the type of clay minerals (ones having rough surfaces rather than smooth) that influences its carbon sequestration potential. See also: Clay minerals; Global climate change; Soil ecology

In general, if you can get organic matter in the ground and not disturb it, it will stick around. Physical disruption, such as tillage (overturning soil) and erosion, promotes the oxidation of organic matter through microbial respiration, sending CO₂ to the atmosphere. Waterlogged soils, such as wetland and peatlands, prevent oxidation and may therefore contain up to 20 percent organic matter, making restoration of wetlands an effective means of carbon sequestration. Likewise, reforestation and restoration of grassland ecosystems are effective for sequestering carbon in soils. See also: Agricultural soil and crop practices; Bog; Forest soil; Origin of grasslands; Oxidation-reduction; Peat; Respiration; Soil conservation; Soil degradation; Soil microbiology; Swamp, marsh, and bog; Wetlands