Abstract. Black carbon (BC) plays an important role in terrestrial carbon storage and can improve sustainably soil fertility. Nevertheless, the accurate quantification of BC remains a critical issue to fully unravel the functions and dynamics of BC in soil. In this work, we explored the potential of differential scanning calorimetry (DSC) to identify, characterize and quantify charcoal in the soil of pre-industrial charcoal kiln sites (CKS) from a diversity of forest and cropland soils in Belgium and Germany. Pre-industrial charcoals and uncharred soil organic matter (SOM) demonstrated a distinct thermal signature that allowed their discrimination, with charcoal being more thermally stable than SOM. The DSC pattern of charcoals is characterized by one to three specific exotherms varying in size and position depending on soil conditions. From our data, we assume that the thermal moieties within charcoal depend on the strength of chemical bonds of C atoms (increasing with the degree of aromatic condensation and decreasing with weathering) and on the activation energy required to onset the combustion. Despite the specific thermal features of charcoal, its decomposition spans over a wide range of temperatures that overlaps with the thermal signature of uncharred SOM. This stresses the challenge of BC quantification in soil and hinders the use of cut-off temperatures to accurately quantify charcoal in soil. Therefore, charcoal-C content was estimated from the relative height of exotherms attributed either to the combustion of charcoal or SOM. For a selection of 45 soil samples, charcoal-C content estimated by DSC was compared to benzene polycarboxylic acids (BPCA) pattern, a widely used method to quantify BC in soil. The two methods correlated strongly (R² = 0.97), with BPCA-C representing about one fifth of DSC-derived charcoal-C. This remind us that operationally-defined BC content has an absolute quantitative value only if the recovery rate is controlled, which is very complicated for many case studies. Overall, our results demonstrate that dynamic thermal analysis is largely under-exploited despite providing rapidly and at low cost quantitatively interpretable information all over the continuum of soil organic matter.

中文翻译：

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差示扫描量热法和 BPCA 标记法对土壤中木炭的热特征和定量

摘要。黑碳 (BC) 在陆地碳储存中起着重要作用，可以可持续地提高土壤肥力。然而，BC 的准确量化仍然是充分揭示 BC 在土壤中的功能和动态的关键问题。在这项工作中，我们探索了差示扫描量热法 (DSC) 在比利时和德国的各种森林和农田土壤中识别、表征和量化工业化前炭窑场地 (CKS) 土壤中木炭的潜力。工业化前的木炭和未烧焦的土壤有机质 (SOM) 表现出独特的热特征，可以区分它们，木炭比 SOM 更热稳定。木炭的 DSC 模式的特点是一到三个特定的放热，其大小和位置因土壤条件而异。根据我们的数据，我们假设木炭中的热部分取决于 C 原子化学键的强度（随着芳烃缩合程度的增加而随着风化而降低）以及开始燃烧所需的活化能。尽管木炭具有特定的热特性，但其分解跨越的温度范围很广，与未烧焦的 SOM 的热特征重叠。这强调了土壤中 BC 量化的挑战，并阻碍了使用截止温度来准确量化土壤中的木炭。因此，木炭-C 含量是从归因于木炭或 SOM 燃烧的放热的相对高度估计的。对于选择的 45 个土壤样品，将 DSC 估计的木炭-C 含量与苯多羧酸 (BPCA) 模式进行比较，一种广泛使用的方法来量化土壤中的 BC。两种方法有很强的相关性（R² = 0.97)，BPCA-C 约占 DSC 衍生的木炭-C 的五分之一。这提醒我们，操作定义的 BC 含量只有在回收率得到控制的情况下才具有绝对定量值，这对于许多案例研究来说非常复杂。总体而言，我们的结果表明，尽管在整个土壤有机质的连续体中提供了快速且低成本的定量可解释信息，但动态热分析在很大程度上并未得到充分利用。