Deep peat warming increases surface methane and carbon dioxide emissions in a black spruce-dominated ombrotrophic bog

Boreal peatlands contain approximately 500 Pg carbon (C) in the soil, emit globally significant quantities of methane (CH₄ ), and are highly sensitive to climate change. Warming associated with global climate change is likely to increase the rate of the temperature-sensitive processes that decompose stored organic carbon and release carbon dioxide (CO₂ ) and CH₄ . Variation in the temperature sensitivity of CO₂ and CH₄ production and increased peat aerobicity due to enhanced growing-season evapotranspiration may alter the nature of peatland trace gas emission. As CH₄ is a powerful greenhouse gas with 34 times the warming potential of CO₂ , it is critical to understand how factors associated with global change will influence surface CO₂ and CH₄ fluxes. Here, we leverage the Spruce and Peatland Responses Under Changing Environments (SPRUCE) climate change manipulation experiment to understand the impact of a 0-9°C gradient in deep belowground warming ("Deep Peat Heat", DPH) on peat surface CO₂ and CH₄ fluxes. We find that DPH treatments increased both CO₂ and CH₄ emission. Methane production was more sensitive to warming than CO₂ production, decreasing the C-CO₂ :C-CH₄ of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ¹³ C of CH₄ suggesting an increasing contribution of acetoclastic methanogenesis to total CH₄ production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH₄ is <2%, CH₄ represents >50% of seasonal C emissions in the highest-warming treatments when adjusted for CO₂ equivalents on a 100-year timescale. These results suggest that warming in boreal regions may increase CH₄ emissions from peatlands and result in a positive feedback to ongoing warming.