Iron- and Manganese-Catalyzed Maillard Chemistry: Molecular Characteristics, Evolution Pathways, and Implications for Soil Organic Carbon Formation.

Mineral-catalyzed Maillard reactions have been proposed as plausible abiotic pathways for the formation of organic carbon in soil. However, the molecular characteristics of the resulting Maillard products and their linkages to real soil organic matter (SOM) remain poorly understood. Here, we conducted abiotic incubation experiments using glucose and glycine as model soil precursors with ferrihydrite (Fh) and birnessite (δ-MnO2) catalysis under environmentally relevant conditions. FT-ICR-MS analysis revealed that both minerals significantly accelerated the polymerization, yielding products that were predominantly categorized as recalcitrant lignin-like compounds including C15H18O5N2, C16H23O6N3, C18H26O5N2, C19H28O6N2, and C23H33O4N3. Molecular reaction network analysis showed that glucose addition reaction (+C6H10O5), glycine addition reaction (+C2H3NO), Strecker degradation (-CO2), and retro-aldol reactions (-C3H6O3 or -C4H8O4) played key roles in molecular evolution. The catalytic mechanisms of Fh and δ-MnO2 were found to be consistent, involving electron acceptance from reducing sugars and cation bridges via dissolved metal ions. Notably, 52.2-84.8% Maillard-like molecules in real soil dissolved organic matter samples, suggesting that the Fh- and δ-MnO2-catalyzed Maillard reactions may represent a significant while previously underappreciated contributor to SOM formation. Together, these findings offer new insights into the mineralogy-mediated pathways that govern the molecular polymerization and persistence of organic carbon in soil systems.