Fire-Induced Multiple Changes in Electron Transfer Properties of Peat Soil Organic Matter: The Role of Functional Groups, Graphitic Carbon, and Iron.

Peatland fires induced changes in electron transfer properties and relevant electroactive structures of peat soil organic matter (PSOM) remain ambiguous, impeding comprehension of postfire biogeochemical processes. Here, we revealed temperature-dependent electron exchange capacity (EEC) of PSOM dynamics through simulated peat soil burning (150-500 °C), which extremely changed postfire microbial Fe-nanoparticles reduction and methanogenesis. EEC diminished significantly (60-75% loss) due to phenolic-quinone moieties depletion with increasing temperature, regardless of oxygen availability. The final EEC in oxic burning surpassed that of anoxic burning by 1.5 times, attributed to additional quinones from oxygen incorporation. Notably, EEC exhibited heat resistance up to 200 °C and stabilized above 350 °C. Additionally, fire reshaped the EEC-relevant redox-active moieties. Heterocyclic-N generated from burning predominantly contributed to the electron-accepting capacity (EAC) alongside quinones, while phenolic moieties and bonded Fe(II) enhanced the electron-donating capacity (EDC). However, the preferential binding of heterocyclic-N to Fe(II) restricted the EDC of Fe(II). Interestingly, the decrease in EAC declined its electron-shuttling effects in microbial Fe nanoparticle reduction, but fire-induced graphitic carbon formation increased the electrical conductivity (EC) of PSOM, promoting electron transfer. Further, enhanced EC may facilitate methanogenesis in postfire peatlands. These findings advance our understanding of elemental biogeochemical cycles and greenhouse emission mechanisms in postfire peatlands.