Carbonate removal by acid fumigation for measuring the δ13C of soil organic carbon

Ramnarine, R., Voroney, R. P., Wagner-Riddle, C. and Dunfield. K. E. 2011. Carbonate removal by acid fumigation for measuring the delta C-13 of soil organic carbon. Can. J. Soil Sci. 91: 247-250. Complete removal of carbonates from calcareous soil samples is critical for accurate measurement of the quantity and isotopic signature (delta C-13) of soil organic carbon (SOC). Carbonates confound SOC and delta C-13 measurements because they have delta C-13 values ranging from -10 parts per thousand to +2 parts per thousand, whereas those of soil organic carbon range from -27 parts per thousand to -13 parts per thousand, depending on the source of plant residues. Commonly used methods for removing carbonates involve treatment with acid followed by repeated water washings; however, these methods are time consuming, labour-intensive and lead to losses of acid- and water-soluble organic carbon. Fumigation of soil samples with HCl was evaluated as an alternative method, and the time required for complete carbonate removal was determined in this study. Moistened soil samples, taken from 0- to 10-cm and 30- to 50-cm depths, were exposed to HCl vapours for periods of 0, 6, 12, 24, 48, 72, and 96 h, followed by measurements of total C and delta C-13 using coupled elemental analyzer-isotope ratio mass spectrometry. The minimum time required to remove all carbonates was ca. 30 h and 56 h for surface and subsurface soils containing 0.80 and 1.94% inorganic C, respectively. Therefore, the fumigation period required is dependent on the total carbonate content of the sample and the nature of the carbonate (pedogenic vs lithogenic). In our study, the rate of removal of inorganic carbon was 0.08-0.10 mg h(-1) for soil samples sizes with 2.4 to 5.8 mg of carbonate-C, a rate similar to previous studies on acid fumigation. A correction factor was used to account for a change in sample mass due to fumigation and is necessary for accurate determination of SOC concentration using our proposed methodology.