Methane emissions from the Amazon floodplain

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 CH4year(-1) (Crutzen 199 1). Ca. 80 per cent of the total methane emission is of modern biogenic origin, whereas only 20 per cent is due to fossil carbon sources (Wahlen et al. 1989). The increase in the tropospheric methane concentration from 0.7ppm 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. 110TgCH4year(-1) of which ca. 60 per cent is attributed to tropical wetlands (Bartlett and Harriss 1993). However, the strengths of individual sources of atmo spheric methane can only be estimated with broad ranges of uncertainty. Methane is generated in the last step of anaerobic fermentation of organic matter. The inundation of wetlands cuts off the oxygen supply to the soil resulting in anaerobic conditions and, thus, favoring methane production. Methane can be released from submerged soils to the atmosphere by diffusion, ebullition, and through the aerenchyma of plants (Wassmann et al. 1992).