Methane Emission from Natural Wetlands in China:Summary of Years 1995-2004 Studies

From studies undertaken during 1995-2004,annual budgets of CHemissions from natural wetlands and its temporal and spatial variations were examined throughout China,and various factors influencing CHemissions were also evaluated. The seasonal variation in CHemissions that increased with increasing plant growth reached its peak in August;decrease in the emissions was found in freshwater marshes but not in peatlands.Emissions were mainly controlled by temperature and depth of standing water.Low CHemissions at the early plant growing stages were not because of deficiency of organic C for CHproduction but because of low temperatures.Low temperatures not only reduced CHproduction but also stimulated CHoxidation by lowering the activity of other aerobic microbes which left more Oin the rhizosphere for methanotrophs.Low summer temperatures (below 20℃) in the Qinghai-Tibetan Plateau lowered CHproduction and CHemission resulting in little or no seasonal variation of emissions.Diel and spatial variation in CHemissions depended on plant species.For plants that transport CH4 using the pressure-driven convective through-flow mechanism, diel variation in CHemissions was governed by diel variation of solar energy load (that produces temperature and vapor pressure differences within various plant tissues) and stomatal conductance.For plants that transport gases using the molecular diffusion mechanism only,the diel variation of CH4 emissions was because of differences in the magnitude of Oproduced through photosynthesis and then delivered into the rhizomes and/or rhizosphere for CHoxidation.Emergent plants could transport more CHthan submerged plants because the former transport CHdirectly into the atmosphere rather than into water as do submerged plants where CHcan be further be oxidized during its diffusion from water to the atmosphere.Emergent plants with high gas transport capacity could not only transport more CH4 into the atmosphere but also live in deeper water,which in turn would inundate more plant litter,resulting in increased availability of C for CH4 production.Annual CHemission from natural wetlands in China was estimated to be 1.76 Tg, up to 1.17 Tg of which was emitted from freshwater marshes. CH4 emission from freshwater marshes mainly occurred during the growing season and less than 8% was released during the freeze-thawing period despite the fact that thawing efficiently released CHfixed in ice column into the atmosphere.