Brackish Water Rewetting Enables Resilient Methane Suppression across Coastal Peatland Land Uses.

Peatland rewetting with freshwater reduces carbon dioxide (CO2) emissions but often increases methane (CH4), undermining its climate benefits. Brackish water, rich in sulfate (SO42-), offers a potential alternative for CH4 mitigation. However, its effectiveness and dominant mechanisms across different coastal land uses remain unclear, hindering broader application. Here, we conducted a four-month incubation experiment using intact soil cores from eight drained coastal peatlands representing four land-use types: grass-cut, grass-graze, arable, and unmanaged. We assessed CH4 emissions from soils rewetted with either natural brackish water (SO42- = 392 ± 2 mg L-1) or synthetic water containing 0% (simulating freshwater), 50%, or 100% of the natural SO42- concentration. Results showed that natural brackish water significantly suppressed CH4 emissions by ∼90% compared to freshwater, with stronger suppression in grass-cut, arable, and unmanaged soils, but limited effects in grass-graze sites. Such differences were attributed to land-use-induced soil variations, where higher pH and lower methanogen relative abundance enhanced suppression. Mechanistically, natural brackish water primarily suppressed CH4 via physiological inhibition of methanogens (likely through salt stress and hydrogen sulfide toxicity). This effect persisted after SO42- reduction inhibition (day 90) and lasted until the end of incubation, reflecting resilience to short-term SO42- depletion. Our study advances understanding of CH4 dynamics and informs targeted use of brackish water rewetting for CH4 mitigation in coastal peatlands.