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 (CH ), 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 (CO ) and CH . Variation in the temperature sensitivity of CO and CH production and increased peat aerobicity due to enhanced growing-season evapotranspiration may alter the nature of peatland trace gas emission. As CH is a powerful greenhouse gas with 34 times the warming potential of CO , it is critical to understand how factors associated with global change will influence surface CO and CH 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 CO and CH fluxes. We find that DPH treatments increased both CO and CH emission. Methane production was more sensitive to warming than CO production, decreasing the C-CO :C-CH of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ C of CH suggesting an increasing contribution of acetoclastic methanogenesis to total CH production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH is <2%, CH represents >50% of seasonal C emissions in the highest-warming treatments when adjusted for CO equivalents on a 100-year timescale. These results suggest that warming in boreal regions may increase CH emissions from peatlands and result in a positive feedback to ongoing warming.