Measurement and Monitoring Soil Carbon

Laboratory analysis

Total carbon and organic carbon concentration (g kg-1) are determined on soil reference samples only, by thermal oxidation (Skjemstad and Baldock, 2008) using a carbon analyzer according to Standard ISO 10694: Soil quality - Determination of organic and total carbon after dry combustion (elementary analysis). These reference soil carbon measurements are calibrated to soil infrared spectral measurements and the calibrations then used to predict carbon values for all samples. Soil reference carbon is determined on total and acidified samples, i.e. fumigated with hydrochloric acid to remove inorganic carbon (carbonate) (Harris et al., 2001). Inorganic carbon is estimated as the difference between unacidified and acidified carbon.

The World Agroforestry Centre’s standard operating procedure for carbon analysis in soils and plants uses the Thermal Scientific FlashEA 1112. It is based on the flash dynamic combustion method, which produces complete combustion of the sample within a high temperature reactor, followed by an accurate and precise determination of the elemental gases produced using a thermal conductivity detector. A complete standard operating procedures for carbon analysis in soils and plants using the Thermal Scientific FlashEA 1112 can be found at www.africasoils.net

Soil often has greater spatial variability that demands more sampling efforts. In some cases the cost of demonstrating the change in carbon stocks in soils to the required accuracy and precision may exceed the benefits that accrue from the increase in stocks (IPCC, 2003; MacDicken, 1997). Therefore developing locally calibrated models that can use easily collected data can minimize the cost of demonstrating a change in soil organic carbon stock (IPCC, 2003). Thus, developing alternative cheaper and repeatable measures is a research priority.

Infrared spectroscopy offers promise for a rapid, reliable and cost effective measurement of soil organic carbon.

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Harris, D., Horwath, W.R., van Kessel, C. 2001. Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon13 isotopic analysis. Soil Science Society of America Journal 63:1853-1856.

IPCC. 2003. Good Practice Guidance for Land Use, Land-Use Change and Forestry, In Penman, J., et al., eds. Institute for Global Environmental Strategies (IGES), Japan.

MacDicken, K.G. 1997. A Guide to Monitoring Carbon Storage in Forestry and Agroforestry Projects. Winrock International, Arlington, Virginia.

Skjemstad, J., Baldock, J.A. 2008. Total and organic carbon, p. 225-238, In Carter, M. R. and Gregorich, E. G., eds. Soil sampling and methods of analysis. Soil Science Society of Canada.