Submitted to: African Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 30, 2009
Publication Date: August 10, 2010
Citation: McCarty, G.W., Reeves, J.B., Yost, R., Doraiswamy, P.C., Doumbia, M. 2010. Evaluation of methods for measuring soil organic carbon in West African soils. African Journal of Agricultural Research. 5(16):2169-2177.
Interpretive Summary: The increasing atmospheric carbon dioxide (CO2) content has stimulated research to assess the role of terrestrial ecosystems in the global carbon (C) cycle. There is recognition that C sequestration in developing countries in sub-Saharan Africa could play a vital role in economic development. In addition, food security within the African Sahel is not only adversely affected by climate change associated with decreased rainfall but also but also by decreased soil fertility and soil C loss. Increased C sequestration can help reverse the decline in soil fertility in the region and thereby add to food security. There is need for good methods for measuring soil carbon by soil analytical laboratories in the sub-Saharan region. In the developing world, most soil analytical laboratories rely on secondary methods of analysis which only provide indirect measurements of soil C and until recently this has also generally held for laboratories in developed countries. With use of such methods there is considerable risk for analytical drift as methods evolve as well as general inaccuracy which can provide poor baseline soil C datasets and uncertainty in C sequestration. Our evaluation found that both dichromate oxidation and mass loss on ignition methods can lead to substantial error for soils in Mali which generally have low organic matter content whereas spectroscopic methods hold considerable promise if calibrations are well maintained. With routine use of any indirect method, constant attention is required to check for drift and accuracy by referencing against a primary method of analysis. The terrestrial biosphere is an important component of global C budget, but estimates of C sequestration in terrestrial ecosystems are partly constrained by the limited ability to assess changes in soil C storage.
Increased interest in implementing projects in the Sahel region of Africa for sequestration of atmospheric carbon dioxide in soil organic matter has intensified the need for methods that accurately measure soil C but are also suitable for use by often resource limited soil analysis laboratories in the region. The challenge is amplified by the low carbon content of the degraded soils found on agricultural lands in the region. Various indirect methods have been recommended for estimating soil C which can be readily implemented in a limited resource environment but they require more evaluation in terms of accuracy. This study compares soil carbon results obtained by direct elemental analysis using dry combustion to estimates obtained by indirect methods such as dichromate oxidation, mass loss-on-ignition (LOI), and diffuse reflectance infrared spectroscopy that hold advantages in being simple, inexpensive, and/or rapid. The results showed that both dichromate oxidation and loss-on-ignition had substantial limitations when used to analyze the low carbon soils within the region. Diffuse reflectance spectroscopy using either the near- infrared or mid-infrared spectral regions performed well for predicting soil organic carbon as well as sand, silt and clay content. These results indicate that once calibrated infrared spectroscopy holds great promise for quantifying soil properties in the Sahel region and that regional laboratories could adopt the technology if instrumentation were made available.