Peatlands are among Earth’s most important terrestrial ecosystems due to their massive soil carbon (C) stores and significant release of methane (CH4) into the atmosphere. Methane has a sustained-flux global warming potential 45-times greater than carbon dioxide (CO2), and the accuracy of Earth system model projections relies on our mechanistic understanding of peatland CH4 cycling in the context of environmental change. The objective of this study was to determine, under in situ conditions, how heating of the peat profile affects ecosystem-level anaerobic C cycling. We assessed the response of CO2 and CH4 production, as well as the anaerobic oxidation of CH4 (AOM) , in a boreal peatland following thirteen months of deep peat heating and fourteen months of subsequent whole-ecosystem warming (surface and deep heating) as part of the Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) project in northern Minnesota, USA. The study utilizes a regression-based experimental design including five temperature treatments that warmed the entire 2 m peat profile from 0 to +9 °C above ambient temperature beginning in June of 2014. Soil cores were collected at 25, 50, 75, 100, 150 and 200 cm depths from each experimental chamber at the SPRUCE site and anaerobically incubated at in situ temperatures over the course of 1-2 weeks.
Results/Conclusions
Methane and CO2 production in surface peat were positively correlated with seasonal and experimentally elevated temperature, but no consistent temperature response was observed at depth (75-200 cm) following both deep peat and whole-ecosystem warming. Surface peat had greater CH4 production rates than deeper peat, implying that the increased CH4 emissions observed in the field were largely driven by surface peat warming. Additionally, the CO2:CH4 ratio was inversely correlated with temperature in the surface, indicating that surficial anaerobic respiration becomes more methanogenic with warming. We also observed AOM occurring throughout the entire peat profile, with the highest rates observed at the surface and preliminary data suggesting a positive correlation with increasing temperature. While SPRUCE will continue for many years, our initial results suggest that the vast C stores at depth in peatlands are relatively unresponsive to warming and any response will be driven largely by surface peat.