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 (SO₄^2-), 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 (SO₄^2- = 392 ± 2 mg L^-1) or synthetic water containing 0% (simulating freshwater), 50%, or 100% of the natural SO₄^2- 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 SO₄^2- reduction inhibition (day 90) and lasted until the end of incubation, reflecting resilience to short-term SO₄^2- depletion. Our study advances understanding of CH₄ dynamics and informs targeted use of brackish water rewetting for CH₄ mitigation in coastal peatlands.