Methane (CH4) is an important greenhouse gas that also affects the chemistry and oxidation capacity of the atmosphere (Cicerone and Oremland 1988). The current burden of methane in the atmosphere is about 4700Tg (1 Tg =1 million tons) (Wahlen et al. 1989), and the global annual emission is ca. 505 Tg CH4 year−1 (Crutzen 1991). Ca. 80% of the total methane emission is of modern biogenic origin, whereas only 20% is due to fossil carbon sources (Wahlen et al. 1989). The increase in the tropospheric methane concentration from 0.7 ppm in preindustrial times to the present value of 1.7 ppm can be attributed mainly to expanding agricultural activities, such as rice cultivation and animal husbandry (Bouwman 1989). Natural wetlands are the largest source of atmospheric methane; the estimated global source strength is ca. 110 Tg CH4year−1 of which ca. 60% is attributed to tropical wetlands (Bartlett and Harriss 1993). However, the strengths of individual sources of atmospheric methane can only be estimated with broad ranges of uncertainty.