Measurement and Monitoring Soil Carbon

Cumulative mass soil

Soil organic carbon stock can be expressed on an equal mass or equal volume basis. To express changes in soil carbon stocks on an equal mass basis requires that the change in the soil bulk density. Estimates of soil carbon stocks to a fixed depth using single depth bulk density are mostly biased due to the spatial and temporal variability in bulk density (Lee et al., 2009; VandenBygaart and Angers, 2006). Despite the high carbon concentration in the top soil, (20 cm) the carbon density is often less than in the sub-soil due to lower soil mass (bulk density) in the top soil than in deeper soil layers. The variability in bulk density with depth can be addressed by establishing relationship between cumulative soil mass and volume. It is likely that projects designed to enhance soil organic carbon (e.g. afforestation) will also cause the soil bulk density to decrease. A management that leads to a decrease in bulk density will under estimates soil organic carbon stock and vice versa (Ellert and Bettany, 1995).

If it is expected that the soil bulk density will change significantly during the course of the project, it is recommended to assess the impact of expressing the changes in soil carbon on an equal mass or equal volume basis on the total projected change in soil carbon stocks.

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Ellert, B.H., & Bettany, J.R. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75:529–538.

Lee, J., Wopmans,, , J.W., Rolston, D.E., Baer, S.G., & Six,, J. 2009.Determining soil carbon stock changes: Simple bulk density corrections fail. Agriculture. Ecosystems and Environment 134:251-256.

Vandenbygaart, A.J. & Angers, D.A. 2006. Towards accurate measurements of soil organic carbon stock change in agroecosystems. Canadian Journal of Soil Scince, 86(3): 465-471.