Impact of rainfall regime on methane flux from a cool temperate fen depends on vegetation cover

Climate change projections forecast an intensification of the precipitation regime for many regions of the globe, including central North America, with fewer, larger events interspersed between longer periods devoid of rain or snow. This shift has the potential to affect the carbon cycling of peatland ecosystems, including the flux of methane from the peat. We conducted a field manipulation experiment where irrigation treatments were used to simulate different seasonal rainfall regimes. The treatments were designed such that total seasonal rainfall was held constant but discrete event frequency and magnitude were altered between treatments in a poor fen in southern Ontario, Canada. The rainfall regime was controlled over three vegetation types: Sphagnum capillifolium (moss); Carex oligoperma (sedge); and Chamaedaphne calyculata (shrub). Decreasing rainfall frequency from thrice-weekly to bi-monthly [coupled with 6X increase in event intensity] led to significantly greater CH4 flux from the moss and sedge communities in the latter third of the growing season. The shrub communities were unaffected by the changing rainfall regime. A companion lab mesocosm experiment revealed the control the fluctuating water table had on the CH4 fluxes from the vegetation community, particularly from the moss communities. Overall, there were significantly greater CH4 fluxes from all communities with increasing days since the previous rainfall event. As precipitation frequency decreases results of this study demonstrate the potential for increased CH4 flux to the atmosphere from peatland areas dominated by Sphagnum and herbaceous species. Wetland restoration and creation projects should consider these effects on peatland carbon cycling and function.