Comment on “Humic Substances Extracted by Alkali Are Invalid Proxies for the Dynamics and Functions of Organic Matter in Terrestrial and Aquatic Ecosystems,” by Kleber and Lehmann (2019)

Abstract

787 Dear Editor, I very much appreciate the special section dedicated by the Journal of Environmental Quality to “Viewpoints on the Future of Humic Substances Research” (March 2019 issue), which has given space to the current debate on the novel and classic type of approaches to investigate the nature of soil organic matter. Indeed, as remarked by Henry Janzen (2019) in his preface, this special section provides scientists with a clear account of the supporting arguments from both sides. I am writing to you to comment on the paper of Kleber and Lehmann (2019) in this special section. Their criticism of the “humic substances paradigm” is, for a substantial part of the article, discussed on the basis of definitions by Achard (1786; cited four times) and Liebig (1840) and experiments by Braconnot (1819; cited twice) and Waksman (1936; cited seven times). The reviewed literature leaves out most of the research performed during the 20th century; save for a few pre–World War II citations, it carries on to the years between 2000 and 2010 and contains only three references to work published after 2010 (one of them a self-citation, another a textbook). The article proceeds (“The Mechanism of Alkaline Extraction,” p. 208) with a chemistry lesson on the fact that more polar organic compounds are increasingly soluble in water (Table 1) and become even more soluble at alkaline pH and discloses in Table 2 that lipids and alcohols do not dissociate, but that organic acids do dissociate depending on pH. The authors quickly dismiss, without comment, the fact that scientists were able to obtain humic-like substances by microbial transformations (Claus et al., 1999) and by reactions catalyzed by enzymes and soil minerals (see extensive review by Huang and Hardie, 2009), to stress simply that they were asserted based on their solubility in alkaline reactions. Humic acids are acids not only because the name says so but because they are “decorated” with titratable carboxyl groups, which can be spotted easily by any type of suitable spectroscopy. The scheme in their Fig. 1 is incomplete. It states that “decomposition creates functionalized organic fragments Comment on “Humic Substances Extracted by Alkali Are Invalid Proxies for the Dynamics and Functions of Organic Matter in Terrestrial and Aquatic Ecosystems,” by Kleber and Lehmann (2019)