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

Abstract

Methane flux was measured, employing themicrometeorological eddy correlation technique,during two growing seasons (1991 and 1992) in apeatland in Minnesota. As compared to 1991, the1992 season was wetter and cooler. Here we examinethe seasonal distributions of CH4 flux and therelationship between concurrently measured CH4and CO2fluxes. Midday CH4 flux was low(1.5 mg m-2 h-1) during late May in bothseasons. Subsequently, the flux ranged from 2.5 to5.5 mg m-2 h-1 during early June to earlyJuly in both years. Methane flux peaked at6.5 mg m-2 h-1 during mid July in 1991. The peak flux (8.0 mg m-2 h-1) in 1992occurred 3 weeks later. A sustained drop in watertable during late July to late August in 1991 mayhave reduced the methane emission. During midAugust–mid October in 1992, the water table wasconsistently high and the flux ranged from 2.0 to3.0 mg m-2 h-1. As compared to 1991,CH4 flux during this time in 1992 was higher byabout 1.0 mg m-2 h-1 because of theoverriding influence of the water table. Integration over the growing season (late May to midOctober), indicated that this ecosystem releasedapproximately 10.4 and 11.5 g C m-2 of CH4in 1991 and 1992, respectively. We examined our concurrent measurements on methaneflux and canopy photosynthesis under a variety ofenvironmental conditions from different parts of thegrowing season. On a time scale of the entireseason, the overall patterns of methane flux andcanopy photosynthesis were similar in both years. Canopy photosynthesis, however, showed large day-to-daychanges in response to variations in temperatureand moisture. Corresponding changes in methane fluxduring these selected periods were relatively small. The slopes and correlation coefficients of linearregressions between methane flux and photosynthesisdata varied widely. Accordingly, a close coupling between short-term (day to day) variations inmethane flux and canopy photosynthesis was not evident.