Quantifying the effects of water table and soil temperature on the emission of methane from peat wetland at the field scale

Methane emissions from peal wetlands have been shown to be very sensitive to soil temperature and the position of the water table in controlled environment studies and in the field. However, these studies have been made using small chambers which average fluxes over areas ranging from 0.05 to 0.3 m(2) and spatial variability in methane emission from these ecosystems commonly spans several orders of magnitude over linear scales of a few metres. In this study, fluxes of methane from peat wetlands in Caithness, Scotland were measured over a 14 d period in May and June 1994 using eddy covariance methods, which provided 30 min mean methane fluxes averaged over areas of typically 10(4) to 10(5) m(2), overcoming the small-scale spatial variability in emission. The mean methane emissions varied with wind direction over the areas of bog with different water tables. Within each sector of the fetch, the mean methane emission increased with temperature in the range 7-11 degrees C by approximately a factor of two (5 mu mol m(-2) h(-1) degrees C-1), close to the response obtained in controlled conditions. The rates of methane emission From the bog averaged 39 mu mol m(-2) h(-1) with peak values approaching 170 mu mol m(-2) h(-1). The responses of the methane emission to temperature and water table at this site have been used to calculate the seasonal variability in emission rates and annual emissions From continuous measurements of water table and meteorological variables over the Caithness peatlands. Annual emission from these peatlands was estimated to be 0.43 mol m(-2) yr(-1), and From U.K. peatlands a range of 40-141 kT yr(-1) was predicted. The responses were used to simulate the effects of increasing mean temperature by up to 4 degrees C and varying the water table by up to +/- 2 cm. (C) 1998 Elsevier Science Ltd. All rights reserved.