Study On Characteristics Of Methane Flux In Wetland Of Fengdu Longhe Fluctuation Zone Under Water Level Change

Wetland methane emissions are one of the major sources of global greenhouse gases and are becoming major global environmental and climate issues.Wetland is an ecosystem between ocean and land that plays an irreplaceable role in controlling greenhouse gas emissions such as methane.The littoral zone is a special type of wetland ecosystem that periodically floods or exposes the water to specific areas of the water surface due to seasonal fluctuations in water levels.Since the Three Gorges Reservoir's experimental water storage operation in 2003,the formation of the water-level fluctuation zone in the Three Gorges Reservoir area has been closely related to the operation and dispatching mode of the reservoir.The reservoir area adopts“anti-seasonal”operation mode.The water level in winter is 175m at the highest and the water level in summer is at 145m,which makes the reservoir fall zone with a vertical drop of 30m and an area of about 348.9 km².The water-falling zone has frequent fluctuations in water level,but it has certain regularity,and the water level is an important environmental factor that affects methane production,oxidation and transmission.There are few reports on the characteristics of methane fluxes and their influencing factors in the Canyon-type Rivers in the Canyon-type Rivers affected by the Three Gorges reservoir area.In order to fully understand and understand the knowledge system related to the interaction between the periodic water level fluctuation of the wetland of the reservoir and the methane flux characteristics,it provides a reference for the qualitative and quantitative study of greenhouse gas emissions such as reservoir methane.In the two operation periods from August 2016 to July 2018,the three water level changes in the flooding period,transition period and dry season were taken as the study area in the Fengdu section of the Longdu River.Under the condition of cyclical fluctuation of water level,static box/floating-gas chromatography was used to carry out methane flux in the wetland of the upper reaches(S1),midstream(S2)and downstream(S3)of the Fengdu section of the Longhe River Basin.Systematic research on feature analysis.The indoor simulated culture experiment was carried out by using the typical water-fluctuating zone soil affected by the reservoir water level,and the relationship between soil methane emission potential and water level conditions was analyzed to support the monitoring of in-situ flux in the early stage.The main results are as follows:(1)During the whole observation period,the overall change trend of the daily variation of methane flux in the wetland of the Fengdu Long River is“convex”,that is,the methane flux at noon is higher than the methane flux in the morning and afternoon.(2)Methane emissions have spatial and temporal differences.Because the S1sampling point in the Fengdu section of the Longhe River Basin is not affected by the backwater of the Three Gorges Reservoir,the methane emission flux fluctuates around zero at different water level cycles and the emissions are relatively stable,which may be related to the relatively stable soil properties or state.The methane flux during the different water levels of S2 is,flooding period(0.178 mg·m^-2·h^-1)>transition period(0.149 mg·m^-2·h^-1)>falling period(0.141 mg·m^-2·h^-1).The methane flux during the S3sampling point at different water levels is,flooding period(0.220 mg·m^-2·h^-1)>transition period(0.107 mg·m^-2·h^-1)>Falling period(0.028 mg·m^-2·h^-1).(3)Using Principal Component Analysis and Pearson correlation method,the methane flux at the S1 sampling point was significantly correlated with soil organic carbon(SOC)in the environmental factors examined(R=0.963,P<0.05),and the methane flux at the S2 sampling point.The correlation between the amount and the temperature inside the box was extremely significant(R=0.991,P<0.01),and the methane flux at the S3 sampling point was significantly correlated with soil temperature(R=0.951,P<0.05).However,there is a correlation between the methane flux at the S1,S2 and S3 sampling points and the environmental factors in the pore water in the environmental factors of pore water,but the correlation is not significant.(4)Changes in water level conditions play an important role in the production of methane.Through analysis of variance,it was found that there was a significant difference between the cumulative emissions of methane during the flooding,transition and dry periods(P<0.01).From the average analysis of methane emission potential,the methane emission potential during the flooding period(mean value is about 2.07ng·g^-1·d^-1)is relatively higher than the transition period(mean value is about 1.24ng·g^-1·d^-1)and the dry period(mean value of about 1.48 ng·g^-1·d^-1).(5)Under laboratory conditions,no matter which water level the simulated culture was in,compared with the blank control group,different concentrations of SO₄^2-,NO₃^-,Fe³⁺,and Mn⁴⁺electron acceptors were added to the methane emission potential during the 7th day of the culture time.It has a certain inhibitory effect,but as the culture time increases,its inhibitory effect gradually decreases.