Evaluation of methods for measuring soil organic carbon in West African soils

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 which are also suitable for use by most 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 (often less than 5 g kg-1 soil). 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 diffused reflectance infrared spectroscopy that has 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. Diffused reflectance spectroscopy using either the near- or midinfrared 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.