Peatcosm: Experimental Insights into Climate Change Effects on Peatland Carbon Cycling and Trace Gas Flux.

Acid peatlands develop thick, organic, base-poor peat soils that are perennially saturated to near the surface, and are typically dominated by Sphagnum mosses, Ericaceae, and sedges. Climate change is expected to alter the water balance and plant functional groups of these ecosystems, with expected impacts on carbon balance and trace gas flux. However, there is debate about whether these changes will predictably drive increasing peat carbon mineralization and lower methane flux. In the PEATcosm experiment we used a large mesocosm approach that minimized microtopographic effects to examine the role of simulated summer drought and change in plant functional groups (removal of sedges or Ericaceae) in carbon cycling and trace gas flux. Lower water table and higher sedge abundance both increased porewater redox potential. Net CO₂ uptake and methane efflux were both lower under low water tables and when Ericaceae were removed.  Although potential phenol oxidase activity was not increased by treatments, phenolic content per unit dissolved organic carbon did decline with lower water tables, and cellulose decomposition was strongest just above the water table, suggesting that moisture and oxygen limitations to decomposition were important constraints on overall carbon balance. For methane flux, the fact that removal of Ericaceae reduced CH₄ flux under low water tables suggests that Ericaceae roots stimulate methanogens, suppress methanotrophs, or both. Illumina rDNA sequencing does not support the latter, as when only Ericaceae are present shallow peat methanotrophs represent a larger fraction of the bacterial community. Combined with lower fluxes under sedges, this information suggests that methanogenesis may have been upregulated under Ericaceae alone. Overall, our results indicate that plant functional groups and water table interact to regulate trace gas flux in peatland ecosystems, so models of climate change impacts on peatland carbon cycling should include plant community dynamics.