Pyrogenic carbon accelerated electron transfer process of dissolved organic matter in paddy soil

As a ubiquitous and stable phase of soil organic matter, pyrogenic carbon (PyC)-produced by the incomplete combustion of biomass during wildfire burning, fossil fuel consumption, and engineered manufacturing-exhibits exceptional electron-mediating capacity. Dissolved organic matter (DOM), a highly redox-active component in the environment, participates in many biogeochemical cycles and transformations through its electron transfer process. PyC can actively interact with DOM through adsorption and aggregation. Although both function as electron shuttles, their cooperative mechanisms remain poorly understood, particularly how PyC's dual electron transfer pathways (direct electron transfer and redox cycling) modulate DOM's electron shuttling. Here, we conducted paddy microcosm incubations in combination with electrochemical analyses to investigate the effect of PyC on DOM redox cycles. Results demonstrated that the electron transfer rate mediated jointly by DOM and PyC (1000 °C) was 14 times that of PyC (400 °C), showing positive correlations with both the conductivity and electron exchange capacity (EEC) of PyC. Further mechanistic study using anthraquinone-2,6-disulfonate (AQDS), a widely used analogue of the redox centers of DOM, revealed that the strong adsorption of AQDS on PyC (1000 °C) formed a redox continuum, in which the rapid direct electron transfer through the conductive PyC matrices accelerated the redox cycles of AQDS molecules and extended their availability for electron transfer. In contrast, PyC produced at lower temperature range (400 °C) did not exhibit similar kinetic promotion effect on AQDS electron transfer, which aligned with the low conductivity and EEC. This study has highlighted the combined electron transfer effect of PyC and DOM, emphasizing the importance of their biogeochemical redox interactions as well as their effects on nutrients and contaminants transformations in soil and aquatic ecosystems.