Kleber Markus, Lehmann Johannes
J Environ Qual. 2019 Mar;48(2):207-216. doi: 10.2134/jeq2019.01.0036.
The conceptual rigor of using organic materials extracted from soil by alkali, called , as proxies for soil organic matter has been questioned for almost 180 years. However, the humic substances paradigm, i.e., the understanding that alkali-extracted humic substances are newly synthesized materials with unique properties that are distinct from non-humic organic matter, continues to be propagated throughout the literature. Here, we revisit the mechanistic background of the extraction process to show that alkaline extraction is unable to discriminate for the chemical history of a compound. For this reason, alkaline extraction cannot distinguish between (i) materials that have undergone secondary synthesis or humification ("humic" substances), and (ii) materials that are decorated with ionizable functional groups for other reasons, such as oxidative depolymerization ("non-humic" substances). While this mechanistic consideration alone renders invalid a distinction between humic substances and non-humic substances based on alkaline extraction, we further show that the evidence available to date does not support the assumption that processes of secondary synthesis create quantitatively significant proportions of "chemically reactive, yet recalcitrant" materials in natural environments. Any definition of humic substances that invokes both alkaline extraction and secondary synthesis is thus flawed on at least two accounts: (i) alkaline extraction is unable to achieve its purpose of separating humic from non-humic substances, and (ii) the assertion that the extracted materials have unique molecular properties as a consequence of secondary synthesis cannot be proven because alkaline extraction cannot separate materials created by secondary synthesis from other, ionizable organic compounds. Finally, we point out that since the definition of humic substances is tied to the alkaline-extraction procedure, neither the existence of operationally defined humic substances in the environment nor their chemical integrity during the course of alkaline extraction can be independently verified. We conclude that organic materials extracted by alkali require appropriate nomenclature and rigorous definition to merit consideration in teaching, research, and application.
使用碱从土壤中提取的有机物质（称为 ）作为土壤有机质的替代物，其概念的严谨性在近180年里一直受到质疑。然而，腐殖物质范式，即认为碱提取的腐殖物质是新合成的材料，具有与非腐殖有机物质不同的独特性质，仍在整个文献中传播。在这里，我们重新审视提取过程的机理背景，以表明碱性提取无法区分化合物的化学历史。因此，碱性提取无法区分（i）经历过二次合成或腐殖化的物质（“腐殖”物质），以及（ii）因其他原因带有可电离官能团的物质，例如氧化解聚（“非腐殖”物质）。仅这一机理考量就使得基于碱性提取区分腐殖物质和非腐殖物质的做法无效，我们进一步表明，迄今为止可得的证据并不支持这样的假设，即在自然环境中二次合成过程会产生数量可观的“化学反应性强但难降解”的物质。因此，任何同时涉及碱性提取和二次合成的腐殖物质定义至少在两个方面存在缺陷：（i）碱性提取无法实现将腐殖物质与非腐殖物质分离的目的，（ii）由于碱性提取无法将二次合成产生的物质与其他可电离有机化合物分离，所以无法证明所提取的物质因二次合成而具有独特的分子性质。最后，我们指出，由于腐殖物质的定义与碱性提取程序相关，所以环境中操作定义的腐殖物质的存在及其在碱性提取过程中的化学完整性都无法独立验证。我们得出结论，碱提取的有机物质需要适当的命名和严格的定义，才能在教学、研究和应用中得到考虑。
