| Nowadays global warming is one of the major environmental problems, which will bring a huge challenge to the survival and development of human society, human survival, sustainable economic and social development. Climate scientists said that we must stop increasing greenhouse gas emissions and reduce emissions in2015to2020, global greenhouse gases emissions required to achieve80%of1990levels by2050if we want to prevent average global temperatures rise2℃.The carbon is one of the important factors of global climate change. Methane, as the most important greenhouse gas, of which contribution to the global warming is up to15%. According to statistics, the content of methane in the atmosphere has been increased by about2.5times from the Industrial Revolution (0.72ppm) to2005(1.77ppm). In recent years, many studies showed that hydropower, which has been widely accepted as a clean energy, may be an important source of atmospheric CO2and CH4. Especially, in the tropics, where large number of submerged vegetation with high temperature cause large emission of greenhouse gases. Three Gorges Reservoir is the world’s largest artificial reservoir for hydroelectric power and beacause the Three Gorges project was constructed, a special ecosystem of the water level fluctuating zone which have the height of30m meters was produced between normal store water level175m and flood prevention control water level145m. Studies have shown that the water level fluctuating zone is the core zone of the methane emissions. So far, however, report about methane emission from Three Gorges reservoir was rare, and carrying out the research about methane emission from water level fluctuating zone and water-air interface is significance.In this paper, a typical water level fluctuating zone was selected as the research area, which located in Zhenxi town (107°27’E,29°54’N), Fuling city, Chongqing. It is the first time, the diurnal and seasonal variation of methane emission about three elevations (155,165and175m) in water level fluctuating zone and permanent flooding point140m and180m elevation was observed using static opaque chamber-GC techniques for two year. Through long-time field sampling and observations, we get several important conclusions as follows:The result of the diurnal variation shows that that diurnal variation of methane emission presented a feature of peak in sunny and clear day of water level fluctuating zone. The maximum rate of methane flux generally occurred about11:00or13:00. In sunny daytime, the diurnal variation of ecosystem respiration were exponentially depedent on air temperature (R2=0.55-0.42, P<0.01). The average emission fluxes of different season were0.019mg-m-·h-1(Spring),0.121mg·m-2·h-1(Summer),0.121mg-m-2h-1(Autumn) and0.027mg·m-2h-1(Winter).In entire exposed period, the methane emission flux showed different characteristics on ecosystems of different elevation ecosystems. The methane emission flux on180m elevation is relatively stable, but the fluctuations in the spring and summer is higher than that of in autumn and winter, and there is no obvious peak of the methane flux in observation period. The methane emission flux on175m elevation does not have a significant variation and there are two obvious peaks on February17and May3,2012. The emission flux on165m and155m elevation is small in entire exposed period. During entire exposed period, the order of average methane emission flux for different elevations was155m (4.11mg-m-2·d-1)>180m (1.82mg·m-2d-1)>175m (1.77mg-m-2·d-1)>165m (1.24mg·m-2·d-1). The difference of175m,165m and155m elevation was not significant (P>0.05), and significantly less than that of155m elevation (P<0.05).In flooding period, methane emission fluxes increased significantly and the peak appear at this period. During the flooding period, the order of average methane emission flux for different elevations was155m (25.44mg-m-2·d-1)>140m (25.11mg·m-2·d-1)>165m (23.75mg·m-2·d-1)>175m (6.27mg·m-2·d-1). The difference of165m,155m and140m elevation was not significant (P>0.05), and significantly higher than175m elevation (P<0.05).The result of the monthly variation showed that the monthly average methane emission fluxes are positive on140m elevation, and the trend is increased gradually at first and then gradually decreases,the emission flux maximum appeared in June and the minimum appeared in November. The monthly average methane emission fluxes are positive on155m elevation, the emission flux decrease gradually from January to June. The monthly average methane emission fluxes are positive on165m elevation. The monthly average methane emission fluxes are positive on175m elevation except in October. Totally, the maximum and minimum of monthly average methane emission fluxes appeared in June on140m elevation and in March on180m elevation. In the Hydro-Fluctuation, the maximum and minimum of monthly average methane emission fluxes appeared in February on165m elevation and in October on175m elevation.The result of the annual emissions showed that the order of the annual methane emission from August2011to August2012for different elevations was140m95.40kg-ha-1)>155m (74.82kg·ha-1)>165m (38.29kg·ha-1)>175m (14.70kg·ha-1)>180m (8.66kg·ha-1), the difference reached a significant level (P<0.05). The annual methane emission from August2011to August2012on175m elevation was96.3%higher than the emission from August2010to August2011and the annual methane emission from August2011to August2012on180m elevation was78.5%higher than the emission from August2010to August2011.Correlation analysis showed that the moisture content of the soil is the main influencing factors of methane emission. The methane flux and soil water content showed a significant positive correlation (P<0.01) throughout the observation period on180m,175m and165m elevation. At the same time, the temperature, the carbon content can also affect the landing methane emissions. A significantly positive correlation was observed between the fluxes of methane across the water-air interface and the DOC concentrations and the water temperature. Also, methane emission fluxes and water temperature is a significant positive correlation on155m elevation (P<0.05), methane emission fluxes and the DOC in the water is a significant positive correlation on140m elevation (P<0.05). |
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