Integration of the polyphenol and Maillard reactions into a unified abiotic pathway for humification in nature: the role of δ-MnO2

The Maillard reaction involving condensation reactions between sugars and amino acids is considered to be an important pathway in natural humification processes. Polyphenols in terrestrial and aquatic environments are regarded as important precursors in the formation of humic substances. In nature it is most likely that these two processes do not occur separately but rather interact with each other. We report that the ubiquitous soil mineral δ-MnO2 significantly accelerated humification processes in a system containing glucose, glycine, and catechol, at temperatures and a pH typical of natural environments. The data obtained indicate the significance of linking the polyphenol and Maillard reactions as promoted by δ-MnO2 into an integrated humification pathway.