MICROMETEOROLOGICAL MEASUREMENTS OF METHANE FLUX IN A MINNESOTA PEATLAND DURING 2 GROWING SEASONS

Methane flux was measured, employing the micrometeorological eddy corr elation technique, during two growing seasons (1991 and 1992) in a pea tland in Minnesota. As compared to 1991, the 1992 season was wetter an d cooler Here we examine the seasonal distributions of CH4 flux and th e relationship between concurrently measured CH4 and CO(2)fluxes. Midd ay CH4 flux was low (1.5 mg m(-2) h(-1)) during late May in both seaso ns. Subsequently, the flux ranged from 2.5 to 5.5 mg m(-2) h(-1) durin g early June to early July in both years. Methane flux peaked at 6.5 m g 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 du ring late July to late August in 1991 may have reduced the methane emi ssion. During mid August-mid October in 1992, the water table was cons istently high and the flux ranged from 2.0 to 3.0 mg m(-2) h(-1). As c ompared 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 t able. Integration over the growing season (late May to mid October), i ndicated that this ecosystem released approximately 10.4 and 11.5 g C m(-2) of CH4 in 1991 and 1992, respectively. We examined our concurren t measurements on methane flux and canopy photosynthesis under a varie ty of environmental conditions from different parts of the growing sea son. On a time scale of the entire season, the overall patterns of met hane 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 me thane flux during these selected periods were relatively small. The sl opes and correlation coefficients of linear regressions between methan e flux and photosynthesis data varied widely. Accordingly, a close cou pling between short-term (day to day) variations in methane flux and c anopy photosynthesis was not evident.