Study On Characteristics And Impact Factors Of Greenhouse Gas Emissions From Different Interfaces Of Long River Fluctuation Zone

For a long time,hydropower has been regarded as clean energy.However,the construction of the reservoirs may greatly change the ecological functions of the original habitat,and then have a great impact on the greenhouse gas emissions of the entire reservoir area.The Three Gorges Reservoir is currently the largest hydropower project in the world,and its"anti-seasonal"operation mode has formed a fluctuation zone with a water level drop of nearly 30 meters.As the fluctuation zone is periodically in a cycle of submergence/drying,its material migration and energy exchange are more complicated,and the carbon cycle is quite different from the natural habitats.Its contribution to greenhouse gas emissions comes from both shoreside swamps and water body.Therefore,the research on greenhouse gas emissions in the entire fluctuation zone of the Three Gorges Reservoir is of great significance for assessing the contribution of large-scale reservoir construction to greenhouse gas emissions.In order to understand the variation and differences of greenhouse gas emissions between two interfaces,and to comprehensively estimate the global warming potential and source/sink function of the entire fluctuation zone,the main tributary of the Three Gorges Reservoir-the Long River in the Fengdu section was taken as the study area.Static box-gas chromatography was used to carry out methane and carbon dioxide flux at the soil-air interface and water-air interface at various areas(upstream 1#,midstream2#,and downstream 3#)in the fluctuation zone of the Long River in the Fengdu section for 17 months.Meanwhile,the sediment index(including pore water index),surface water index and environmental parameters were measured.The main results obtained are as follows:(1)Diurnal variation and differences of greenhouse gas emissionsThe average Diurnal methane flux at the water-air interface is greater than that at the soil-air interface,and the difference is significant(P<0.05).In particular,the largest difference of methane flux at two interfaces occurs during morning-noon period(258.2?g·m^-2·h^-1).On the contrary,the carbon dioxide flux at the water-air interface is smaller than that at the soil-air interface,and the difference is significant(P<0.05).The largest difference of carbon dioxide flux between two interfaces occurs during the afternoon-evening period,reaching 89.2 mg·m^-2·h^-1.(2)Monthly variation and difference of greenhouse gas emissionsThe monthly average methane emissions from the water-air interface are higher throughout the year,and the difference is significant(P<0.05).The largest difference between two interfaces occurs in the spring or winter(February or April).On the contrary,the carbon dioxide emissions from the soil-air interface are higher,and the difference is significant(P<0.05).The largest difference between two interfaces occurs in summer or autumn(July or September).(3)Global warming potential and source/sink function of the fluctuation zoneIn general,the methane emissions from the water-air interface are higher than that from the soil-air interface.Two interfaces of 1#behave as methane sinks throughout the year.Two interfaces of 2#behave as methane sources throughout the year.3#is relatively special,whose soil-air interface behaves as methane sinks,while the water-air interface behaves as methane sources.On the contrary,the carbon dioxide emission from the soil-air interface is higher than that from the water-air interface,and carbon dioxide emissions from 1#,2#,and 3#all behave as carbon dioxide sources throughout the year.On a century scale,the global warming potential of wetland of the Long River in the Fengdu section is represented as:soil-air interface>water-air interface,2#,3#>1#.(4)Interannual variation of global warming potential and the source/sink functionIn 2020,the cumulative emissions of methane from the soil-air interface of the entire wetland system increased by 7.1%and the cumulative emissions of carbon dioxide decreased by 10.8%.In 2020,the cumulative emissions of methane from the water-air interface of the entire wetland system dropped by 18.38%,while the cumulative emissions of carbon dioxide dropped by 25.4%.As far as each wetland,the global warming potential is consistent with the magnitude affected by the water level,which is represented as 3#>2#>1#.In terms of interannual variation,the total warming potential of the three wetlands in 2020 decreased by 7.9%,14.1%,and 20.4%,respectively.Taking the average of three,the total warming potential of wetland of the Long River in 2020 dropped by 14.1%.(5)The impact of environmental factors on the greenhouse gas emissionsThe methane emission flux from soil-air interfaces of 1#,2#,3#are extremely significantly positively correlated with soil organic carbon,sediment temperature,and the temperature in the static box respectively(P<0.01).The carbon dioxide emission flux from soil-air interface of 1#is significantly positively correlated with pore water dissolved organic carbon(P<0.05).The carbon dioxide emission flux from soil-air interface of 2#is extremely significantly positively correlated with pore water dissolved organic carbon(P<0.01)and significantly positively correlated with pore water Mn(?)(P<0.05).The carbon dioxide emission flux from soil-air interface of 3#is significantly positively correlated with pore water Mn(?)(P<0.05).The methane emission flux from water-air interface of 1#has significant negative correlation with pore water Fe(?)and surface water NO₃^-(P<0.05),while it has extremely significant negative correlation with surface water Fe(?)(P<0.01).The methane emission flux from water-air interface of 2#has significant negative correlation with surface water NO₃^-and surface water dissolved inorganic carbon(P<0.05).The methane emission flux from water-air interface of 3#has a significant positive correlation with the temperature in the static box(P<0.05);The carbon dioxide emission flux from water-air interfaces of 1#,2#,3#is extremely significantly positively related to the surface water temperature,soil organic carbon,and the temperature in the static box respectively(P<0.01).In addition,the carbon dioxide emission flux from water-air interfaces of 3#is significantly negatively related to surface water dissolved inorganic carbon(P<0.05).The methane emission flux from soil-air interface has relatively few impact factors,and the relationship between temperature,soil organic carbon and methane flux is extremely significant(P<0.01).The correlation between environmental factors and methane flux at water-air interface is not as significant as that at soil-air interface.The impact factors of carbon dioxide flux at water-air interface are mainly temperature index and carbon index.Due to the influence of water level fluctuation,the correlation between environmental factors and carbon dioxide flux at water-air interface is more significant than that at soil-air interface.