Soil organic carbon stock

SOC (soil organic carbon) stock is calculated value that can be expressed in different units across various spatial scales. It can be as in total mass per region (global, country) or as density per area unit (Mg ha-1, kg m-2). In large-scale studies SOC stock is most commonly calculated and harmonized to fixed depth as 0-20 cm, 0-30 cm, 0-100 cm or 0-200 cm. In some cases site/soil-specific depth is used. You need carefully consider all these aspects in comparing results of different studies.

Common SOC stock units across different spatial scales
ScaleUnitExamples
GlobalPg (petagram = billion tons)

680 Pg (0-30 cm), map

Region, countryPg, Tg (teragram = million tons), ...

17.6 Pg in EU agricultural soils (0-30 cm)

242 Tg  in Estonian mineral soils (topsoil)

Landscape, farm, field, soil or forest type, etc

Tg, Mg (megagram = ton)

Mg ha-1 = t ha-1

176 Tg in Estonian forest soils (topsoil)

77 Mg ha-1 (topsoil weighted mean for all forest site types)

Plot, soil profileMg ha-1, kg m-2

48.5 Mg ha-1 or 4.85 kg m-2 (arable soil in field experiment, 0-25 cm)

SOC stock estimates are related to high uncertainties, especially when generalized and upscaled to larger regional and global scales. In recent years several guidelines and updated global SOC map has been composed by country-driven mapping approach (read more).

Plot and soil profile level measurements are typically basis for SOC stock calculations with following general formula:

SOC stock = SOC concentration * soil bulk density * coefficient of fine fraction * soil volume (layer depth*area)

SOC concentration (%) estimation can be made by several classical (wet digestion, dry combustion) or spectroscopic methods (read more).

Soil bulk density (g cm-3) is weight of dry soil divided by the total soil volume. Its measured typically by core method or estimated as function of SOM/SOC concentration (read more).

Coefficient of fine fraction (>0...1) shows proportion of soil particles smaller than 2 mm in diameter from total soil volume. If coarse fragments (gravel, stones) are 15% of soil volume then factor 0.85 must be used in SOC stock calculation formula.

SOC stock is always calculate value and every part in formula includes uncertainties starting from field work (sampling methods, depth etc), laboratory analysis of SOC concentration up to modelling and up-scaling approcahes if generalized to landscape wider scales. The distribution of SOC is very heterogeneous and is strongly dependant on soil type, land use and climatic conditions.