Humic Substances Extracted by Alkali Are Invalid Proxies for the Dynamics and Functions of Organic Matter in Terrestrial and Aquatic Ecosystems

The conceptual rigor of using organic materials extracted from soil by alkali, called humic substances, 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.