Mn(II)-Mediated Abiotic Humification via Synergistic Interactions with Molecular Oxygen

Terrestrial organic carbon (OC) dynamics critically influence the global carbon budget. Humification is a key pathway for stabilizing OC by transforming labile compounds into macromolecular polymers. High-valent manganese oxides (e.g., Mn(IV)O2) are well-known as efficient catalysts of abiotic humification, whereas the role of Mn(II) in OC transformation remains poorly understood. Here, dissolved Mn2+ drives oxidative polymerization and nitrogen-enriched humic-like product formation in the presence of common soil precursors─catechol, glycine, and glucose─through a synergistic interaction with molecular oxygen (O2). Mechanistically, Mn2+ mediates sequential single-electron transfers from catechol to O2, forming reactive intermediates that potentially couple with Maillard precursors to yield nitrogen-enriched macromolecules. FT-ICR-MS reveals that CHON species in lignin-like and highly condensed aromatic regions collectively account for 75.6% of total product relative abundance. This process is accompanied by the generation of reactive oxygen species (ROS), which could further enhance molecular diversity. Across 14 soil and freshwater DOM samples, 13.1-54.7% of assigned product formulas overlap with those in the corresponding DOM samples. Collectively, these shared formulas account for 78% of the total relative abundance of product formulas, indicating molecular-formula-level compositional similarity. These findings provide mechanistic and molecular-level insight into the underexplored role of Mn2+ in abiotic humification.