Abiotic Catalysis of the Maillard and Polyphenol-Maillard Humification Pathways by Soil Clays from Temperate and Tropical Environments
The Maillard reaction and integrated polyphenol-Maillard reaction are regarded as important pathways in natural humification. Little is known about the abiotic catalysis of these humification pathways by naturally occurring soils and sediments. Therefore, the objective of this study was to investigate the abiotic catalysis of the Maillard reaction and integrated polyphenol-Maillard reaction by two contrasting soil clays from a temperate (Canadian Prairies) and tropical (Northeastern South Africa) region. Treatments containing an equimolar amount of glucose and glycine (Maillard reaction) or catechol, glucose and glycine (polyphenol-Maillard reaction) in the presence of temperate Mollisol and tropical Oxisol clays, were conducted under environmentally relevant conditions, i.e., pH 7.0 and 45 °C, for a period of 15 days under sterile conditions. The nature of the humification products were examined through C and Al K-edge, and Fe and Mn L-edge NEXAFS spectroscopy. The Oxisol clay enhanced humification in the Maillard and integrated catechol-Maillard systems to a greater extent than the Mollisol clay, which is attributable to its high content of sesquioxides, particularly poorly-crystalline Mn oxides. The humic substances produced in the Oxisol- and Mollisol-catalyzed polyphenol-Maillard reaction systems were chemically distinct from one another. Changes were observed in the Al coordination and Fe and Mn oxidation states in the soil clays after humification. The Oxisol clay showed a much greater accumulation of organic C compared to the Mollisol clay in the Maillard and catechol-Maillard systems. These results reveal the important role of Fe(III) and Mn(III, IV) oxides present in soils in catalyzing the Maillard reaction and polyphenol-Maillard humification pathways. The findings of this study are of fundamental importance in understanding the role of soil clays from temperate and tropical regions in abiotic humification pathways and C stabilization in natural environments.
KeywordsAbiotic humification Biomolecules C, Al, Fe and Mn NEXAFS Polyphenol-Maillard reaction Soil clays
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