The resistance of centennial soil charcoal to the "Walkley-Black" oxidation
The resistance of centennial soil charcoal to the "Walkley-Black" oxidation
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DOI：
10.1016/j.geoderma.2017.05.001
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发表时间：
2017-10-01
期刊：
影响因子：
6.1
通讯作者：
Dufey, J. E.
中科院分区：
文献类型：
--
作者：
Hardy, B.;Dufey, J. E.
重铬酸盐氧化法是测定土壤有机碳含量的常规方法。重铬酸盐氧化也被用来量化黑碳（BC）在土壤和沉积物中的基础上，更大的阻力，重铬酸盐氧化比未炭化的SOC。本研究的目的是（i）评估是否重铬酸盐为基础的SOC量化方法区分未炭化的SOC和木炭残留物在前工业木炭窑遗址的表土和（ii）测试老化对通过重铬酸盐氧化回收木炭的影响。我们选择了40个表土样品，从20个前工业木炭窑网站瓦隆，比利时，和邻近的参考土壤不受木炭生产。样品包括森林和农田的土壤，具有多样性的土壤类型和质地类。通过原始Walkley-Black程序（结果乘以传统的校正因子1.32）和通过包括外部加热消化混合物以克服未炭化的SOC的不完全氧化并摆脱校正因子1.32的适应方法来估计SOC含量。结果进行了比较的总有机碳（TOC）的含量，干燃烧测量，并在土壤中的木炭-C的含量，估计通过差示扫描量热法。目前的活性炭窑址的土壤也进行了分析，并作为参考有限老化。23.6%的木炭-C回收的Walkley-Black程序在目前活跃的窑址的土壤中对65%的前工业窑址的土壤，这表明，木炭的抗重铬酸钾氧化随着老化而降低。在煮沸消化混合物后，木炭的回收率增加到90%，这提供了热催化木炭氧化的证据。重铬酸盐对木炭的大量氧化以及根据木炭的改变程度和反应条件的回收率的可变性支持了以下观点：基于其耐化学性对BC进行定量具有挑战性，并且如果校准不适合感兴趣样品的BC质量，则可能会受到重要偏倚的影响。由于通过Walkley-Black方法恢复BC是不完整的，因此经常受火灾影响的土壤中存在大量BC可能是Walkley-Black方法在区域和全球数据库中低估SOC的重要原因。
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.