Wetlands

Wetland emissions dominate the natural sources of CH4. The amounts of CH4 produced vary greatly from area to area, with changes in temperature, water level and organic carbon content all being important controlling factors. Current estimates of global CH4 emissions from wetlands range between 100 and 250 Tg CH4/year.

The process of CH4 production in wetland soils (methanogenesis) involves the microbial mineralization of organic carbon under anaerobic conditions in the waterlogged soil. In the absence of oxygen, the organic carbon (usually simple carbon compounds such as acetate) is used as an alternative terminal electron acceptor and so provides a source of energy for the metha-nogens in these anoxic soils. When water levels fall, as they often do in the summer months, CH4 emissions from wetlands can be greatly reduced or even cease completely as oxygen concentrations rise in the soil.

Through widespread land drainage and changes in land use, humans have had a huge impact on wetland CH4 emissions in many areas of the world. Additionally, increased nitrogen and sulphate deposition from the atmosphere, again resulting from human activity, may also greatly affect net CH4 emis sions from wetland areas. Some studies have shown an increase in CH4 consumption (CH4 oxidation) as a result of increased nitrogen deposition, while others have reported inhibition. Sulphate deposition, which can occur due to emissions of sulphur dioxide (SO2) from fossil fuel burning, can favour sulphate-reducing bacteria over methanogenic bacteria in waterlogged soils, thus actually reducing CH4 production.

The potential for control of CH4 emission from wetlands lies largely in land use policy. Under certain circumstances draining of wetlands could greatly reduce emissions. However, certain wetland environments such as peat bogs have been shown to vastly increase their emissions of CO2 in response to such draining - so offsetting any net reduction in CH4 emissions. In addition there is the problem of habitat destruction, with many wetland animal, plant and insect species already being endangered. Addition of sulphate-based fertilizers to wetlands may also limit methanogenesis by favouring sulphate-reducing bacteria, but again this may have negative impacts on ecosystem function and biodiversity through acidification of the soils, not to mention the significant financial costs that would be involved.

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