Quantification of Organic Carbon in Soils: A Comparison of Methodologies and Assessment of the Carbon Content of Organic Matter

Renewed interest in temporal soil organic carbon (SOC) stock changes has stressed the importance of reliable methods for quantitative assessment of organic compound (OC) content. Particularly with the establishment of modern dry‐combustion analyzers, which are replacing the traditional wet‐oxidation methods, the need for correct relationships between both is of crucial importance for comparison of past and current SOC data in long‐term SOC stock change studies. Dry combustion with a Variomax CNS‐analyzer was the standard to evaluate three other methods for Belgian agricultural soils. Excellent linear relationships were found with the Walkey and Black method and the Springer and Klee method, whereas a Shimadzu TOC‐analyzer slightly underestimated the OC content. Precision of the investigated methods was comparable and tended to be dependent on the sample size used for measurement. The OC oxidation efficiency of the most widely applied method of Walkey and Black for the soils in this study was very close to the generally accepted 75%. Mass loss on ignition at 800°C could be very well related to the soil OC content and the clay content. The traditional factor of 1.724 used to convert OC measurements to organic matter percentages is not valid for the investigated soils, which demonstrates that rather regional‐specific factors (in this study 1.911) should be determined and adopted.