P.C. Alvala and L. Marani
Methane (CH4) is a trace gas with an important role in both troposphere due to the reactions with the hydroxyl radical and the formation of organic radicals, and the stratosphere, because of its participation in the chlorine and water vapor chemistry. Methane is also an important greenhouse gas, with a relative contribution of about 20% for the global greenhouse effect (Wuebbles and Hayhoe 2002). Many works have evidenced that an increase of the methane concentration began around 1800 with the industrial era, and the consequent increase of human activities, including the utilization of fossil fuels, cattle raising and rice plantations (Blake and Rowland 1988; Stern and Kaufmann 1996; Dlugokencky et al. 1998; Etheridge et al. 1998). Presently, the CH4 concentration is about 1,770 ppbv (part per billion by volume), but it presents a large inter-annual variation caused by eruptions, such as one of the Mt. Pinatubo, or by alterations of the wetlands emissions, mainly due to alterations in the precipitation and temperature regimes in the range between 30c and 90c N (Dlugokencky et al. 1996; Walter et al. 2001).
Global budget evaluations indicate that natural wetlands have a substantial contribution to tropospheric methane, estimated as 100 Tg CH4 y-1, or about 20% of the global emission (Wuebbles and Hayhoe 2002). Although most of the wetlands are located in the temperate region of the Northern Hemisphere (Lehner and Doll 2004), many experiments have pointed that the wetlands in the tropical region emit 66 Tg CH4 y-1, or 60% of the emissions from the natural wetlands (Bartlett and Harriss 1993). The emission of methane from the wetlands is influenced by many environmental factors, such as temperature, dissolved organic carbon (DOC), pH, oxyreduction potential (pO), water depth, and floating vegetation. Qualitative relations between the methane fluxes and the factors have been determined, but direct relations are more difficult to establish due to the complex mechanisms involved in
Divisao de Geofísica Espacial, Instituto Nacional de Pesquisas Espaciáis, DGE-INPE, Av. dos Astronautas, 1.758, CEP 12227-010, Sao Jose dos Campos - SP, Brazil e-mail: [email protected]
S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.7, © Springer-Verlag Berlin Heidelberg 2009
the process, starting with the production in the sediment, which is followed by the transport though the water column and the subsequent liberation to the atmosphere. Many experiments have been performed in the tropical region, especially in the Amazonian wetland, showing that it is an important source of the global budget of atmospheric methane, with an emission range estimated from 1.73 to 21.0 Tg CH4 y-1 for the whole Amazon basin (Devol et al. 1990; Bartlett et al. 1990; Melack et al. 2004).
Previous work in wetlands indicated that the transport of methane from the sediments to the water surface might occur by three main pathways, which are dependent on each ecosystem: (i) diffusion through the water column, (ii) ebullition from the sediments and (iii) transport through the plant stems (Bartlett et al. 1990; Devol et al. 1990; Keller and Stallard 1994; Bastviken et al. 2004). In Amazonia, the ebullition transport is very important, sometimes accounting for more than 60-70% of the total emission to the atmosphere, while the transport by plants is dependent on the plant type and how it covers the surface.
Present estimates of methane emission from tropical wetlands have an excessive dependence on data from Amazonia, with insufficient information of seasonal and spatial variation in the emission from other tropical wetlands, as evidenced by Smith et al. (2002). There are a few studies covering some important tropical and subtropical wetlands, which are not yet characterized.
One of the most important wetlands in Brazil is the Pantanal, which is covered by savanna-like vegetation and is flooded seasonally. Its total area is estimated about 138,183 km2, with most of this area within Brazil, in the upper Paraguay River basin. Flooding in the region is clearly seasonal, with its maximum flooded area occurring at the end of March. Marani and Alvala (2007) showed flux measurements for one year, and they identified the importance of the Pantanal floodplains as a source of methane to the atmosphere, with bubble fluxes constituting the major contribution to the general flux. This work presents a study of flux data obtained in five Pantanal sites during 2004 and 2005, and evaluates emissions from floodplains and lakes and their possible relationship with some environmental variables.
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