| Climate warming is a global environment issue today, the main reason for which is the increasing concentration of greenhouse gases in atmosphere. Carbon dioxide, as the most important greenhouse gas, whose contribution to the global warming is up to56%.In recent years, many studies showed that hydropower, which has been widely accepted as a clean energy, may be an import 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 as a large-scale hydropower play an important role in flood control, power generation, shipping and ecological scheduling. 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. The water level fluctuating zone has a special and complex environment, and is an important part of the ecosystem of Three Gorges region. The migration and transformation process of substances impacted by water level change in water level flucturting zone, and the carbon cycles is different from natural ecosystems, so the emission of CO2can not be ignored. So far, however, report about CO2emission from Three Gorges reservoir was rare, and carrying out the research about CO2emission 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 ecosystem respiration about three elevations (155,165and175m) in water level fluctuating zone and180m elevation was observed using static opaque chamber-GC techniques for one year. The flux of CO2across the water-air interface was measured with floating static opaque chamber-GC techniques. In addition, to quantify ecosystem respiration about water level fluctuating zone and CO2flux across the water-air interface, more systematic analysis was carried out on the influencing factors like as environmental, physical and chemical factors. Through long-time field sampling and observations, we get several important conclusions as follows:(1) The diurnal variation of ecosystem respiration presented a feature of peak in sunny and clear day of water level fluctuating zone.The maximum rate of ecosystem respiration values generally occurred about13:00or15:00. Besides, ecosystem respiration showed an obvious difference with season change, on average, which order in daytime was flowering autumn>summer>spring>winter. Moreover, the range of ecosystem respiration was larger in summer and autumn in daytime compared winter and spring. During daytime in different seasons, the ecosystem respiration also varied significantly in each observation time. In sunny daytime, the diurnal variation of ecosystem respiration were exponentially depedent on soil temperature of5cm depth, soil surface temperature, air temperature and temperature in chamber under sunny days.(2) The ecosystem respiration of each elevation (155m,165m,175m and180m) reached to a maximum in summer or autumn, and which was the lowest in winter or at the beginning of water level decline. During entire exposed period, the order of ecosystem respiration for different elevations was165m (1055.10mg·m-2·h-1)>175m (985.49mg·m-2·h-1)>155m (793.60mg·m-2·h-1)>180m (776.91mg·m-2·h-1). Form May to August of2011, the cosystem respiration of four elevations in the order:175m (1871.25mg·m-2·h-1)>165m (1408.96mg·m-2·h-1)>180m (1154.53mg·m-2·h-1)>155m (763.04mg·m-2·h-1). From august2010to august2011, annual average of ecosystem respiration of175m elevation (715.84mg·m-2·h-1) was lower compared with which of180m elevation (776.43mg·m-2·h-1). The order of annual total respiration was180m (68.05tCO2·hm-2)>175m (62.68tCO2·hm-2)>165m (46.86tCO2·hm-2)>155m (21.90tCO2·hm-2).(3) The ecosystem respiration was significantly influenced by temperature, an exponential relationship between ecosystem respiration and soil temperature of5cm depth could be received; also, temperature in the chamber would directly affect the ecosystem respiration. Taking into account temperature and WFPS for the effects of ecosystem respiration, the correlation coefficients was from0.325to0.735.In addition, the influence of surface soil (0～10cm) pH on the ecosystem respiration of175m,180m elevations were pronounced. The SOC, TN contents in surface soil (0-10cm) were the important factors that affect the ecosystem respiration about175m elevation, which showed significantly positive, while a significant negative correlation was observed between ecosystem respiration and SMBC contents. Correlation analysis showed that a significantly positive, negative correlation was observed between ecosystem respiration about155m,165m elevations and the DOC contents (0～10cm), respectively.(4) The results showed that Three Gorges reservoir is a CO2source during observation period (from March to September2011), the flux value was13.08～215.88,30.23～186.mg·m-2·h-1for the water-air observation station of140m,145m elevation, and the mean flux value was85.05,89.31mg·m-2·h-1, respectively. Seasonal variation characteristics in the CO2emission were significant, and the fluxes of CO2across water-air interfrace reached a maximum in June and July, presenting a feature of peak.(5) Correlation analysis showed that a significantly positive correlation was observed between the fluxes of CO2across the water-air interface and the DOC concentrations. Also, the influence of water temperature to the water-air observation station of145m elevation on CO2across was pronounced. |
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