Methane Emissions And Carbon Absorbing Effects Of The Chongxi Wetland

Wetlands are large sources of atmospheric gaseous methane (CH4). With global warming, CH4 emissions from wetlands gradually become important research problem. CH4 emission is a complex process and depends on many factors, with mixed results. Function of carbon source/sink of estuaries is different from other inland wetlands because of the special hydro logical period and vegetation status. Estuaries play an important role in global change as the interface of the sea and the land. However, studies about CH4 emission from estuary, especially freshwater estuary, are not very much.Investigations in this study were carried out at the Chongxi wetland at the west end of Chongming Island, Changjiang River estuary during the 2008 and 2010. The spatial and temporal variability of CH4 emission rates from different tidal elevations and vegetative distribution areas were measured using a modified static closed chamber technique. The effects of tidal submergence frequencies and vegetation at different elevation on CO2 fixation and CH4 emission were analyzed. Such knowledge would contribute to management strategies in ecological engineering to reduce the greenhouse effect. And the carbon absorbing effect of the Chongxi wetland was measured by analyzing carbon fixation and carbon emission. The main results of this study were summarized as follows:(1) CH4 emissions from the different elevation sites were measured, and CH4 emissions from vegetated areas, nonvegetated areas, and clipping vegetation areas were analyzed to explore the vegetation effects. CH4 emissions from the freshwater estuarine wetlands had remarkable seasonal variations, with the highest values occurring in summer and late-summer and the lowest emission occurring in the early spring and winter. CH4 fluxes at different elevation sites showed an evident increase from the higher elevation areas to the lower ones, the annual CH4 emissions from the higher elevation areas and the lower elevation areas were 15 g m-2yr-1,100.87 g m-2yr-1, respectively; However, the minimum values occurred in the mudflats (0.438 g m-2yr-1). High submergence frequency and duration of tide increased CH4 emissions; however, the difference of CH4 emissions from vegetated areas and nonvegetated areas was larger than that from different tidal submergency. This suggests that the change of CH4 emissions along elevation were mainly affected by the vegetation. Vegetation supplied C-substrates for CH4 production. Clipping vegetation significantly reduced CH4 emission,6.65 g m-2yr-1 and 42.58 g m-2yr-1, from clipping sites at the higher elevation areas and the lower elevation areas. This suggests that vegetation can transport CH4 emission especially during the growing season.Higher net photosynthetic rate of plants at the lower elevation areas not only led to more CH4 emission but also higher carbon fixation. However, the ratio of CH4 emission to CO2 fixation was smaller in the higher elevation areas than it was in the lower elevation areas,0.3% and 1.6%, respectively.(2) Self-designed experimental mesocosm was used to measure CH4 emission and the effect of vegetation transport on CH4 emission in the different tidal stage. During a cycle of flood current and ebb current, from pre-submergence, during the period of submergence to post-submergence, CH4 emissions from the site of long-term submergence and the site of short-term submergence reached the minimum (1.21 mgCH4 m-2h-1) and maximum (1.18 mgCH4 m-2h-1) when sites were submerged. Clipping vegetation remarkablely reduced CH4 emission, and the difference of CH4 emissions between the site of long-term submergence and the site of short-term submergence was not much. It was obvious that Phragmites australis transported the CH4 emission from soil and contributed 41.5%-69% of the total CH4 emissions.(3) The study mainly focused on aerobic CH4 emissions from aboveground tissues of Phragmites australis. The stems and leaves were cut into small sections and sealed for measuring CH4 emissions from these detached tissues. The results showed that CH4 emissions from detached stems were 0.2-0.7μL L-1. CH4 emission was higher in the lower parts of stems than that in the higher parts of stems and leaves, which confirmed the transport effect of Phragmites australis. The results may imply Phragmites australis do not produce CH4 in aerobic environment, but transport microbially produced CH4 from wetland soil to the atmosphere, and transport effect mainly exists in the lower parts of stems but not in leaves.(4) CH4 emissions were different in the different vegetation areas. A wood plant Alnus trabeculosa led to lower CH4 emission from soil than that from Phragmites australis. The Alnus trabeculosa not only reduced CH4 emission, but also increased the environmental heterogeneity, which played an active role in the wetland ecosystem.(5) Net ecosystem exchange of carbon and CH4 emission were studied by open-path eddy covariance and modified static closed chamber technique, respectively. The results showed that the annual carbon fixation was -0.225 kgCO2-C m-2yr-1, and CH4 emission was 0.075 kgCH4-C m-2yr-1. In summary, Annual carbon exchange of the Chongxi wetland was -0.15 kgC m-2 yr-1. The Chongxi wetland reduce the carbon in the atmosphere.Althought the Chongxi wetland is the source of CH4 emission, it is also the sink of carbon dioxide. In summary, the Chongxi wetland has the effect of carbon fixation and carbon absorbing.