Effects of experimental warming and elevated CO2 on surface methane and CO­2 fluxes from a boreal black spruce peatland

Abstract:

High latitude peatlands represent a major terrestrial carbon store sensitive to climate change, as well as a globally significant methane source. While elevated atmospheric carbon dioxide concentrations and warming temperatures may increase peat respiration and C losses to the atmosphere, reductions in peatland water tables associated with increased growing season evapotranspiration may alter the nature of trace gas emission and increase peat C losses as CO₂ relative to methane (CH₄). As CH₄ is a greenhouse gas with twenty times the warming potential of CO₂, it is critical to understand how surface fluxes of CO₂ and CH₄ will be influenced by factors associated with global climate change. We used automated soil respiration chambers to assess the influence of elevated atmospheric CO₂ and whole ecosystem warming on peatland CH₄ and CO₂ fluxes at the SPRUCE (Spruce and Peatland Responses Under Climatic and Environmental Change) Experiment in northern Minnesota. Belowground warming treatments were initiated in July 2014 and whole ecosystem warming and elevated CO₂ treatments began in August 2015. Here we report soil iCO₂ and iCH₄ flux responses to the first year of belowground warming and the first two months of whole ecosystem manipulation. We also leverage the spatial and temporal density of measurements across the twenty autochambers to assess how physical (i.e., plant species composition, microtopography) and environmental (i.e., peat temperature, water table position, oxygen availability) factors influence observed rates of CH₄ and CO₂ loss. We find that methane fluxes increased significantly across warming treatments following the first year of belowground warming, while belowground warming alone had little influence on soil CO₂ fluxes. Peat microtopography strongly influenced trace gas emission rates, with higher CH₄ fluxes in hollow locations and higher CO₂ fluxes in hummock locations. While there was no difference in the isotopic composition of the methane fluxes between hollow and hummock locations, δ¹³CH₄ was more depleted in the early and late growing season, indicating a transition from hydrogenotrophic to acetoclastic methanogenesis during periods of high photosynthetic input.