Summertime greenhouse gas fluxes from an urban bog undergoing restoration through rewetting

Rewetting can promote the ecological recovery of disturbed peatland ecosystems and may help to revert these ecosystems to carbon dioxide (CO2) sinks. However, rewetting of disturbed peatlands can also cause substantial emissions of methane (CH4) and possibly nitrous oxide (N2O). This study quantified summertime emissions of the three major long-lived greenhouse gases (GHGs) CO2, CH4 and N2O; from undisturbed, disturbed and rewetted soils in the Burns Bog Ecological Conservancy Area (BBECA), a 20 km2 urban bog located in Delta, British Columbia, Canada. Four sites were chosen that represent different stages before or after ecological recovery in the BBECA: (i) a relatively undisturbed scrub pine / Sphagnum / low shrub ecosystem; (ii) a Rhynchospora alba / Sphagnum ecosystem that was disturbed by peat mining more than 65 years ago; (iii) a R. alba / Dulichium arundinaceum ecosystem that was disturbed by peat mining 50 years ago and rewetted five years ago; and (iv) a disturbed and rewetted surface with little vegetation cover that was cleared of vegetation 16 years ago and rewetted two years ago. The GHG fluxes from soils and ground vegetation were measured at all sites during June–August 2014, using a portable non-steady-state chamber system for CO2 and syringe sampling and laboratory analysis for CH4 and N2O fluxes. All four sites exhibited net GHG emissions into the atmosphere, dominated by CH4, which contributed 81–98 % of net CO2 equivalent (CO2e) emissions. Overall, the median CH4 flux for all measurements and sites was ~74 mg m-2 day-1 (~30–410 mg m-2 day-1, 25th–75th percentiles). Fluxes in the rewetted (water-saturated) sedge ecosystem were highest, with a quarter of the values higher than 3,000 mg m-2 day-1 (median 78 mg m-2 day-1). Exchange of CO2 due to photosynthesis and respiration was of secondary importance compared to soil CH4 emissions. Continuous CO2 flux measurements using the eddy covariance approach in the disturbed and rewetted R. alba / Sphagnum site showed that the entire ecosystem, which included tall vegetation, was a weak CO2 sink during the summer (average summertime CO2 uptake of 3.59 g CO2 m 2 day 1). No significant emission or uptake of N2O was observed. The results showed that CH4 emissions dominated the net GHG emissions in this disturbed bog at different stages of recovery.