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.