Abstract: Background, aim, and scope The reservoir has always been a source of water and electricity providing energy and convenience for human beings. However, due to the huge water storage capacity, complex dam operation and maintenance process, the reservoir has become the main carrier of carbon cycle in the ecological environment, which emits loads of greenhouse gas to atmosphere. As one of the world's major dams, the Three Gorges Dam transports many carbon compounds to nature. And this phenomenon has arose the worldwide scholars’ attention. However, there is no uniform method to monitor the absorption and emission of greenhouse gas from reservoir till now. As the typical tributary of the Three Gorges Reservoir, Pengxi river plays a significant role in carbon cycle and chemical efflorescence weathering in reservoir region Consequently, this paper seeks to establish a method for quantifying gas emissions and to analyze how greenhouse gases change over the course of a year. Moreover, it is also requested what is the major attributer for emission of greenhouse gas.. This paper supervises and analysis the fluxes of CO2 and methane in water - gas interface in Pengxi River from May 2016 to February 2017. Materials and methods In reference of previous literature, this study adopted the combination of Headspace balance method and model estimation to obtain the partial pressures of carbon dioxide and methane in surface water and exchange fluxes of CO2 and methane. Additionally, this paper also used field measuring instruments to measure the physical and chemical variables so as to analyze the effects of these factors on greenhouse gas release. The correlation between the partial pressures and fluxes of CO2 and methane and environment variables were analysed by means of the spearman correlation index based on SPSS software. Results Through researching, it was achieved that Pengxi River was the source of greenhouse gas emission. The average value of p(CO2) in surface of waters was analysed to be (1807.635±315.605) μatm in Pengxi River, while that of p(CH4) was (218.7725±127.9425) μatm; and the average value for CO2 flux in water - gas interface was (32.53±3.86) mmol·m-2·d-1, CH4 flux average value was（0.208±0.143）mmol/(m2·h）, the trend of the partial pressure and flux almost remained the same. Moreover, in terms of physical and chemical factors, water temperature, Dissolved oxygen (DO) and pH are proved to be generally higher in summer. Discussion In comparison with other similar study in literature, it can be concluded that the diffusive of CO2 was higher in mainstream than that in tributary. The CO2 flux in the Pengxi River Basin was moderate, higher than the flux in some small reservoir but smaller than that in large lakes and rivers. And the data for CO2 in this paper are much similar with previous study in Pengxi River. With respect to methane, the amount of methane released from tropical waters is greater. The amount of methane released from Pengxi River located in subtropical zone is not only much smaller than that in tropical waters, but also less than that from other rivers in subtropical zone as well as rivers in temperate zone. Moreover, there was a significant positive correlation between CH4 flux, p(CH4), water temperature, and pH, while the CO2 diffusion flux at the water-air interface was significantly positively correlated with p(CO2) and negatively correlated with DO, pH, and Chl-a. Other environmental factors showed vague effects on the fluxes. Conclusions Compared with the diffusive fluxes of greenhouse gases in typical rivers in the world, the study found that the release of CO2 flux in the Pengxi River Basin was moderate, while the diffusion flux of CH4 was small. DO, pH, Chl-a and water temperature have apparent impact on the release of greenhouse gas. Furthermore, the emission of greenhouse gas is greater when water carry larger capacity of that gas. Recommendations and perspectives This research methods can be applied to other reservoirs and extensive analysis of the effects of other influencing factors on greenhouse gas emissions such as nitrogen compounds. In addition, the conclusions which have been achieved give evidence for controlling greenhouse gas emissions in future study.
Keywords: Pengxi River; CO2; CH4; diffusion flux; environmental factors