Heating-Induced Redox Property Dynamics of Peat Soil Dissolved Organic Matter in a Simulated Peat Fire: Electron Exchange Capacity and Molecular Characteristics

Peatlands store one-third of the world's soil organic carbon. Globally increased fires altered peat soil organic matter chemistry, yet the redox property and molecular dynamics of peat-dissolved organic matter (PDOM) during fires remain poorly characterized, limiting our understanding of postfire biogeochemical processes. Clarifying these dynamic changes is essential for effective peatland fire management. This study demonstrates temperature-dependent dynamic changes in the electron exchange capacity (EEC) of PDOM by simulating peat soil burning, significantly affecting microbial iron reduction. At low fire temperatures (200-250 °C), the EEC remains constant by releasing more phenolic moieties to enhance the electron-donating capacity (EDC). Higher temperatures (500 °C) diminish 90% of the EEC by consuming phenolic-quinone moieties. Pyrolytic PDOM (pyPDOM) contributes to 40% of the EEC of peat soil, with this contribution declining at higher temperatures. Phenolic-quinone moieties remain the primary redox-active moieties in pyPDOM. Fourier transform ion cyclotron resonance mass spectrometry analysis shows that postfire EDC depends more on phenolic types than abundance, with monophenol-like molecules (C < 12) being more significant than polyphenol-like (C ≥ 12). Quinone moieties in pyPDOM are associated with high-oxygen condensed aromatics, and their depletion reduces the electron-accepting capacity, weakening its electron shuttle effect in microbial iron reduction. Our findings enhance the understanding of the changes in PDOM redox properties during fires.