Micrometeorological measurements of methane flux in a Minnesota peatland during two growing seasons

Methane flux was measured, employing the micrometeorological eddy correlation technique, during two growing seasons (1991 and 1992) in a peatland in Minnesota. As compared to 1991, the 1992 season was wetter and cooler Here we examine the seasonal distributions of CH4 flux and the relationship between concurrently measured CH4 and CO(2)fluxes. Midday CH4 flux was low (1.5 mg m(-2) h(-1)) during late May in both seasons. Subsequently, the flux ranged from 2.5 to 5.5 mg m(-2) h(-1) during early June to early July in both years. Methane flux peaked at 6.5 mg m(-2) h(-1) during mid July in 1991. The peak flux (8.0 mg m(-2) h(-1)) in 1992 occurred 3 weeks later. A sustained drop in water table during late July to late August in 1991 may have reduced the methane emission. During mid August-mid October in 1992, the water table was consistently high and the flux ranged from 2.0 to 3.0 mg m(-2) h(-1). As compared to 1991, CH4 flux during this time in 1992 was higher by about 1.0 mg m(-2) h(-1) because of the overriding influence of the water table. Integration over the growing season (late May to mid October), indicated that this ecosystem released approximately 10.4 and 11.5 g C m(-2) of CH4 in 1991 and 1992, respectively. We examined our concurrent measurements on methane flux and canopy photosynthesis under a variety of environmental conditions from different parts of the growing season. On a time scale of the entire season, the overall patterns of methane flux and canopy photosynthesis were similar in both years. Canopy photosynthesis, however, showed large day-to-day changes in response to variations in temperature and moisture. Corresponding changes in methane flux during these selected periods were relatively small. The slopes and correlation coefficients of linear regressions between methane flux and photosynthesis data varied widely. Accordingly, a close coupling between short-term (day to day) variations in methane flux and canopy photosynthesis was not evident.