Annual greenhouse gas budget for a bog ecosystem undergoing restoration by rewetting

Abstract. Many peatlands have been drained and harvested for peat mining, agriculture, and other purposes, which has turned them from carbon (C) sinks into C emitters. Rewetting of disturbed peatlands facilitates their ecological recovery and may help them revert to carbon dioxide (CO₂) sinks. However, rewetting may also cause substantial emissions of the more potent greenhouse gas (GHG) methane (CH₄). Our knowledge of the exchange of CO₂ and CH₄ following rewetting during restoration of disturbed peatlands is currently limited. This study quantifies annual fluxes of CO₂ and CH₄ in a disturbed and rewetted area located in the Burns Bog Ecological Conservancy Area in Delta, BC, Canada. Burns Bog is recognized as the largest raised bog ecosystem on North America's west coast. Burns Bog was substantially reduced in size and degraded by peat mining and agriculture. Since 2005, the bog has been declared a conservancy area, with restoration efforts focusing on rewetting disturbed ecosystems to recover Sphagnum and suppress fires. Using the eddy covariance (EC) technique, we measured year-round (16 June 2015 to 15 June 2016) turbulent fluxes of CO₂ and CH₄ from a tower platform in an area rewetted for the last 8 years. The study area, dominated by sedges and Sphagnum, experienced a varying water table position that ranged between 7.7 (inundation) and −26.5 cm from the surface during the study year. The annual CO₂ budget of the rewetted area was −179 ± 26.2 g CO₂–C m⁻² yr⁻¹ (CO₂ sink) and the annual CH₄ budget was 17 ± 1.0 g CH₄–C m⁻² yr⁻¹ (CH₄ source). Gross ecosystem productivity (GEP) exceeded ecosystem respiration (R_e) during summer months (June–August), causing a net CO₂ uptake. In summer, high CH₄ emissions (121 mg CH₄–C m⁻² day⁻¹) were measured. In winter (December–February), while roughly equal magnitudes of GEP and R_e made the study area CO₂ neutral, very low CH₄ emissions (9 mg CH₄–C m⁻² day⁻¹) were observed. The key environmental factors controlling the seasonality of these exchanges were downwelling photosynthetically active radiation and 5 cm soil temperature. It appears that the high water table caused by ditch blocking suppressed R_e. With low temperatures in winter, CH₄ emissions were more suppressed than R_e. Annual net GHG flux from CO₂ and CH₄ expressed in terms of CO₂ equivalents (CO₂ eq.) during the study period totalled −22 ± 103.1 g CO₂ eq. m⁻² yr⁻¹ (net CO₂ eq. sink) and 1248 ± 147.6 g CO₂ eq. m⁻² yr⁻¹ (net CO₂ eq. source) by using 100- and 20-year global warming potential values, respectively. Consequently, the ecosystem was almost CO₂ eq. neutral during the study period expressed on a 100-year time horizon but was a significant CO₂ eq. source on a 20-year time horizon.