Molecular signatures of electrochemically reduced peatland dissolved organic matter resolved by liquid chromatography Fourier transform ion cyclotron resonance mass spectrometry.

Dissolved organic matter (DOM) plays a central role in peatland carbon cycling, where anoxic conditions, redox processes, and thermodynamic factors impact carbon preservation. DOM mediates electron transfer reactions through quinone-like and other redox-active moieties, substantially influencing electron and carbon balances. However, the molecular basis of its electron-accepting and electron-donating capacities remains poorly resolved. Here, DOM from three peatlands was subjected to mild, direct electrochemical reduction (DER; - 0.59 V vs. Ag/AgCl) and analyzed by liquid chromatography coupled to Fourier transform ion cyclotron resonance mass spectrometry (LC-FT-ICR MS). Dissolved organic carbon concentrations measured before and after DER indicated only marginal loss of carbon during reduction. In contrast, electrochemical reduction induced pronounced decreases in FT-ICR MS-derived total ion counts across peat samples, reaching up to 30%, relative to unreduced DOM, while the overall molecular formula space was largely conserved. Changes in signal intensity were negatively associated with double bond equivalents (DBE), indicating that more unsaturated molecular formulas were preferentially affected by DER. In contrast, DBE-O (DBE minus number of oxygen) and modified aromaticity index showed weaker relationships with intensity changes, suggesting that the DBE dependence reflects a broader unsaturation-related response rather than direct reduction of specific functional groups. Across peat DOM, we found average changes in DBE between -0.23 and -0.62 that related to bulk electron-accepting capacity (1000 to 1600 µmol e⁻ (gC)⁻