Interannual, seasonal, and retrospective analysis of the methane and carbon dioxide budgets of a temperate peatland

Three years (2009–2011) of near‐continuous methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes were measured with the eddy covariance (EC) technique at a temperate peatland located within the Marcell Experimental Forest, in northern Minnesota, USA. The peatland was a net source of CH 4 and a net sink of CO 2 in each year with annual carbon budgets of −26.8 (±18.7), −15.5 (±14.8), and −14.6 (±21.5) g C m −2  yr −1 for 2009–2011, respectively. Differences in the seasonal hydrometeorological conditions among the three study years were most pronounced during 2011, which was considerably warmer (+1.3°C) and wetter (+40 mm) than the 30 year average. The annual CH 4 budget was +11.8 (±3.1), +12.2 (±3.0), and +24.9 (±5.6) g C m −2  yr −1 for the respective years and accounted for 23%–39% of the annual carbon budget. The larger CH 4 emission in 2011 is attributed to significant warming of the peat column coupled with a high water table position throughout the entire growing season. Historical (1991–2011) CH 4 emissions were estimated based on long‐term hydrometeorological records and functional relationships derived from our 3 year field study. The predicted historical annual CH 4 budget ranged from +7.8 to +15.2 (±2.7) g CH 4 ‐C m −2  yr −1 . Recent (2007–2011) increases in temperature, precipitation, and rising water table at this site suggest that CH 4 emissions have been increasing, but were generally greater from 1991 to 1999 when average soil temperature and precipitation were higher than in recent years. The global warming potential (considering CO 2 and CH 4 ) for this peatland was calculated based on a 100 year time horizon. In all three study years, the peatland had a net positive radiative forcing on climate and was greatest (+187 g C m −2 ) in 2011. The interannual variability in CH 4 exchange at this site suggests high sensitivity to variations in hydrometeorological conditions.