What is Soil Carbon: An Overview

Soil carbon is the solid carbon stored in global soils; existing in organic and inorganic forms and is the key component of a soil’s ‘health’, affecting the soil’s chemical, physical and biological properties [1].

Soil organic carbon (SOC) refers only to the carbon component of organic compounds and constitutes the largest carbon stock in terrestrial ecosystems and is considered a fundamental building block of life. It is introduced into soils through decay, and processing by a soil microbiome evolved to make use of all the nutrients contained within it. Soil inorganic carbon (SIC), however, consists of mineral forms of carbon, either from weathering of parent material, or from the reaction of soil minerals with atmospheric CO2 [2].

SIC is often considered more securely sequestered in soils, than SOC, which will wax and wane with the seasons, hopefully in an upward trend. However, the processes that increase SIC happen far below the surface and are dependent on factors harder to directly manage such as precipitation rates and weathering of bedrock [3]. On the other hand, for SOC, we are seeing numerous farming practices (such as cover cropping and mob grazing), being deployed to improve the levels of SOC.

Soil carbon is also a fundamental component of soil organic matter (SOM), and commonly recognized as one of the key parameters of soil quality [4], with SOC often considered an important indicator of soil health. The quantity and quality of SOC are linked to vital soil functions including nutrient mineralization, permeability to air, water infiltration, and flood control.

Citations:
[1] Rice, C.W. (2005) ‘Carbon cycle in Soils | Dynamics and management’, Encyclopedia of Soils in the Environment, pp. 164–170. doi:10.1016/b0-12-348530-4/00183-1. 
[2] Lorenz, K. and Lal, R. (2018) ‘Soil Carbon Stock’, Carbon Sequestration in Agricultural Ecosystems, pp. 39–136. doi:10.1007/978-3-319-92318-5_2. 
[3] Ferdush, J. and Paul, V. (2021) ‘A review on the possible factors influencing soil inorganic carbon under elevated CO2’, CATENA, 204, p. 105434. doi:10.1016/j.catena.2021.105434. 
[4] Blanco, J.A. (2017) ‘Managing Forest Soils for Carbon Sequestration: Insights From Modeling Forests Around the Globe’, Soil Management and Climate Change [Preprint]. doi:10.1016/b978-0-12-812128-3.00016-1. ​