Seasonal dynamics of methane emissions from a subarctic fen in the Hudson Bay Lowlands

Abstract. Ecosystem-scale methane (CH₄) flux (F_CH4) over a subarctic fen at Churchill, Manitoba, Canada was measured to understand the magnitude of emissions during spring and fall shoulder seasons, and the growing season in relation to physical and biological conditions. F_CH4 was measured using eddy covariance with a closed-path analyser in four years (2008–2011). Cumulative measured annual F_CH4 (shoulder plus growing seasons) ranged from 3.0 to 9.6 g CH₄ m⁻² yr⁻¹ among the four study years, with a mean of 6.5 to 7.1 g CH₄ m⁻² yr⁻¹ depending upon gap-filling method. Soil temperatures to depths of 50 cm and air temperature were highly correlated with F_CH4, with near-surface soil temperature at 5 cm most correlated across spring, fall, and the shoulder and growing seasons. The response of F_CH4 to soil temperature at the 5 cm depth and air temperature was more than double in spring to that of fall. Emission episodes were generally not observed during spring thaw. Growing season emissions also depended upon soil and air temperatures but the water table also exerted influence, with F_CH4 highest when water was 2–13 cm below and lowest when it was at or above the mean peat surface.