Manganese Oxides Drive Divergent Fates of Litter-Derived DOM via pH-Regulated Oxidative Decomposition and Polymerization

Manganese (Mn) redox cycling critically regulates soil organic carbon (SOC) turnover, especially in the forest litter layer. However, the molecular-level mechanisms underlying the Mn oxide-mediated decomposition and polymerization of dissolved organic matter (DOM) from litter remain poorly understood. Integrating excitation-emission matrix spectroscopy (EEM) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we investigated molecular mechanisms of δ-MnO₂-driven transformations of natural leaf litter-derived DOM samples across an ambient pH gradient (4, 6, 8). Results reveal that δ-MnO₂ simultaneously drives decomposition and polymerization of DOM, with pH modulating their relative contributions. At higher pH (pH 8), δ-MnO₂ facilitates aromatic condensation and coupling involving Mn(IV) → Mn(III)-mediated single-electron transfer, forming highly aromatic humic-like polymers and reducing electron-donating capacity, thereby enhancing oxidative resistance. Under lower pH (pH 4), δ-MnO₂ drives depolymerization and aromatic ring-opening by selectively cleaving oxygen-rich aromatic species. This process is dominated by multielectron transfer, with Mn(IV) reduced to Mn(II) via successive electron donation from DOM. Additionally, initial DOM composition impacts its transformation pathway. Specifically, high-aromaticity, high-molecular-weight fractions preferentially decompose, whereas low-aromaticity DOM tends to polymerize. These findings highlight the dual roles of Mn oxides in stabilizing organic matter, driving both the breakdown and reassembly through pH-regulated redox reactions.