Initial Responses of Methane Cycling to Deep Peat Heating in a Bog Ecosystem.

Northern peatlands are among the most important ecosystems in the global carbon cycle.  These wetlands store roughly one-third of terrestrial soil carbon and are responsible for a significant fraction of global methane emissions.  A pressing question in global biogeochemistry remains whether these large soil carbon stores will be released to the atmosphere as methane in response to warming.  A mechanistic answer to this question is necessary for Earth system models to predict ecosystem-climate feedbacks in response to ongoing global change.  The Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) project will ultimately manipulate temperature (+0, +2.25, +4.5, +6.75 and +9 °C) and atmospheric carbon dioxide concentrations (Ambient, +850 ppmv) within a bog ecosystem in northern Minnesota.  Deep peatland heating through the entire ~2 m peat profile was initiated in June of 2014, allowing for the unique exploration of the impact of deep warming on methane dynamics in this peatland.  Differential warming was greatest at depth, but surface depths warmed progressively following continued deep peat heating.  In the months following the initiation of deep peatland heating, net methane flux showed a positive correlation with deep peat temperature, with the exception of winter months where an extensive ice layer likely limited methane emissions. Measurements of net methane production through the depth profile in September 2014 and June 2015 suggest that surface peat was more responsive to increases in soil temperature and that observed increases in net methane flux were likely driven by the response of these surface soils.  In contrast, methane production in deep peat was relatively insensitive to warming and there were no impacts of warming on porewater methane concentrations or isotopic composition through the peat profile.  Across all treatments, methane emitted from the bog surface reflected recently fixed photosynthate (~5-15 years old), further suggesting that warming treatments have not increased methane production from old, deep peat.