Methods assessment for organic and inorganic carbon quantification in calcareous soils of the Mediterranean region

In calcareous soils, two methods for measuring soil organic carbon (SOC) have been traditionally used: 1) wet oxidation, and 2) dry combustion in which SOC is measured either directly by removing soil inorganic carbon (SIC) from samples through an acid pretreatment, or indirectly by subtracting SIC measured by a calcimeter from total soil C. These methods have well documented experimental limitations that may result in erroneous quantification of C stocks and cycling in calcareous Mediterranean soils. An alternative approach is ramped combustion, which takes advantage of the large difference in the temperatures at which SOC and SIC combust or decompose. Our aim in this study was to test the capability of a continuous, ramped combustion technique combining differential scanning calorimetry and evolved gas analysis to quantify SIC and SOC in calcareous soil samples, and compare this technique with conventional techniques. We also sought to use thermal analysis to test the efficacy of acid fumigation for carbonate removal, and to evaluate the effects of acid fumigation on SOC quality. SIC and SOC concentrations from a set of 23 soil samples differing in carbonate concentrations were well correlated among conventional and ramped combustion methods, indicating that ramped combustion is an effective method to simultaneously quantify SIC and SOC in calcareous soil samples. Ramped combustion was also effective in demonstrating incomplete carbonate removal in several samples, though quantification was made difficult by sample mass issues due to hydrophilicity after fumigation. Ramped combustion also revealed substantial qualitative changes in the thermal lability of residual organic matter, as evidenced by changes in CO₂-EGA thermogram shape and peak temperatures. As a result, ramped combustion appears to be a preferable single-sample method for the simultaneous quantification of SOC and SIC in calcareous soil, providing a relatively rapid, cost- and labor-effective method that precludes the use of hazardous reactants such as those used in wet oxidation and acid fumigation.