Quantification methods of pyrogenic carbon in soil with soil as a complex matrix: comparing the CTO-375 and Cr2O7 methods

Pyrogenic carbon (PyC) or black carbon (BC), the residue of grassland and forest fires and fossil fuel combustion, plays an important role in carbon storage and pollutant adsorption in soil. To understand its roles in soil, it is necessary to accurately assess the PyC fraction. Soil is, however, a complex mixture of inorganic and organic matter, usually containing humic substances, i.e., non-PyC organic matter, in concentrations one to two orders of magnitude higher than PyC. This engenders difficulty in determining the PyC content in soils; thus, various quantitative methods have been proposed for different research purposes. In this study, the two widely used methods of chemo-thermal oxidation at 375 degrees C (CTO-375) and acid dichromate oxidation (Cr2O7) method were reconsidered in terms of the suitability for determining the PyC content of soil by using model soil. The CTO-375 method was simpler, but the quantified PyC was suggested to be the combustion residue of non-PyC organic matter. By contrast, 29 to 40% of PyC content was determined as non-PyC organic matter by the Cr2O7 method. Therefore, we proposed an improved Cr2O7 method by increasing acidic K2Cr2O7 concentration threefold for quantification accuracy: although time-consuming, it resulted in the lowest overestimation on the PyC content, that is, 16 to 23% from non-PyC organic matter. The application of the improved Cr2O7 method to natural soil was confirmed with 99 to 103% certainty (recovery rate).