Pyrogenic carbon drives abiotic methane formation: Hydroxyl radicals and reductive capacity mediate demethylation

Methane (CH₄) oversaturation in oxygenated waters has challenged the long-held view that biogenic CH₄ production is strictly anoxic. Abiotic CH₄ formation has been observed in oxidative, photochemical, and radical-driven systems. However, the potential role of pyrogenic carbon (PyC) remains largely unknown. PyC is a fire‑derived material that is widely transported into and suspended within lakes, rivers, and coastal waters. Here, we investigated the abiotic methanogenic activity of PyC derived from rice straw, pine twig, and peanut shell at pyrolysis temperatures of 300, 500, and 700 °C under environmentally relevant aqueous conditions. In water-phase batch experiments, PyC promoted CH₄ production from exogenous methyl donors and, to a lesser extent, from its own surface-bound methyl groups. Stable carbon isotope tracing with dimethyl sulfoxide (DMSO-¹³C₂) confirmed that methyl groups of DMSO were incorporated into CH₄, yielding substantial ¹³C enrichment. Under oxic water conditions, electron paramagnetic resonance (EPR) spectroscopy, benzoic acid probing, and correlation analyses supported a reactive oxygen species (ROS) mechanism in which hydroxyl radicals (·OH) derived from PyC drove demethylation. Under anoxic conditions, CH₄ formation occurred only in the presence of an exogenous methyl donor, and the yield correlated positively with the electron-donating capacity of PyC. Notably, the production was enhanced by factors typical of aquatic environments, including acidic pH, dissolved Fe²⁺, and light exposure, whereas environmental aging suppressed it. These findings identify PyC as a previously unrecognized driver of abiotic CH₄ production and highlight its potential role in carbon cycling and methylated organic compound transformation in fire-affected environments.