The resistance of centennial soil charcoal to the “Walkley-Black” oxidation

The “Walkley-Black” oxidation with dichromate is a routine protocol for the estimation of soil organic carbon (SOC) content. Dichromate oxidation is also used to quantify black carbon (BC) in soil and sediments based on the larger resistance of BC to dichromate oxidation than uncharred SOC. The aim of this study was (i) to evaluate whether dichromate-based SOC quantification methods discriminate between uncharred SOC and charcoal residues in the topsoil of pre-industrial charcoal kiln sites and (ii) to test the influence of aging on the recovery of charcoal by dichromate oxidation. We selected 40 topsoil samples from 20 pre-industrial charcoal kiln sites of Wallonia, Belgium, and adjacent reference soils unaffected by charcoal production. Samples included soils from forest and cropland, with a diversity of soil types and textural classes. SOC content was estimated by the original Walkley-Black procedure (results were multiplied by the traditional correction factor of 1.32), and by an adapted method that includes external heating of the digestion mixture to overcome incomplete oxidation of uncharred SOC and get rid of the correction factor of 1.32. Results were compared to the amount of total organic carbon (TOC) content, measured by dry combustion, and to the content of charcoal-C in soil, estimated by differential scanning calorimetry. The soil of a currently active charcoal kiln site was also analyzed and used as a reference subject to limited aging. 23.6% of charcoal-C was recovered by the Walkley-Black procedure in the soil of the currently active kiln site against 65% for the soil of pre-industrial kiln sites, which indicates that the resistance of charcoal to dichromate oxidation decreases with aging. The recovery of charcoal increased to 90% after boiling of the digestion mixture, providing evidence that heat catalyzes the oxidation of charcoal. The substantial oxidation of charcoal by dichromate and the variability of recovery according to the degree of alteration of charcoal and conditions of reaction support the idea that the quantification of BC based on its chemical resistance is challenging and can be subject to important biases if calibration is not adapted to the quality of BC of the sample of interest. Because the recovery of BC by the Walkley-Black method is incomplete, the presence of large amounts of BC in soils frequently affected by fire might be a significant cause of underestimation of SOC by the Walkley-Black method in regional and global databases.