Methane Emissions And Influencing Factors Of Methane Production From Thermokarst Lakes In The Central Qinghai-Tibet Plateau

In the context of global warming,permafrost is undergoing a process of degradation,and the degradation of permafrost will lead to the formation of thermokarst landforms,of which thermokarst lake is a relatively common thermokarst landforms.After the formation of the thermokarst lake,the organic carbon originally stored in the permafrost is released,which accelerates the emission of methane and has a significant impact on the carbon cycle in the region.Thermokarst lakes are widely distributed in permafrost regions of the Qinghai-Tibet Plateau.However,it is not clear about the process and influence factors of methane emission from the thermokarst lake in the central Qinghai-Tibet Plateau.In this study,42 thermokarst lakes along the Qinghai-Tibet Highway in the central part of the Qinghai-Tibet Plateau were taken as the research objects.Based on the monitoring and indoor experiments,the physical and chemical properties and CH₄ production rate variation characteristics of the sediment from different vegetation types of thermokarst lakes were emphatically studied,and the main factors affecting the methane production rate of the sediment from thermokarst lakes were analyzed.The main results of this study are as follows:（1）There are significant differences in methane concentration and flux in the thermokarst lake in the central Qinghai-Tibet Plateau during the ice cover period,ice melting period,non-ice period and icing period.In terms of methane concentration,the ice cover period（42.67±57.77μmol/L）>ice melting period（6.50±13.65μmol/L）>freezing period（3.41±7.47μmol/L）>non-ice period（2.03±2.83μmol/L）and the methane concentration in the ice cover period is significantly higher than that in the other three periods.In terms of methane flux,the ice melting period(32.03±69.11mmol·m^-2·d^-1)>icing period(10.65±20.49 mmol·m^-2·d^-1)>non-icing period(3.48±5.09 mmol·m^-2·d^-1)and the ice melting period and icing period are the key periods of methane emission from thermokarst lake.（2）There are differences in the methane production rate of the sediment from different vegetation types in the central Qinghai-Tibet Plateau,and the organic carbon content is the main factor.Accumulative methane production pattern of thermokarst lake under different vegetation types at 5℃:alpine swamp meadow type lakes(17.42±30.86μg·g^-1dw)>alpine meadow type lakes(0.15±0.12μg·g^-1dw)>alpine steppe type lakes(0.08±0.05μg·g^-1dw)>alpine desert type lakes(0.010±0.001μg·g^-1dw).With the increase of vegetation coverage,the rate of methane production also increases.The increase in temperature also significantly promotes the methane production process,and follows the same pattern as at 5℃.Among them,thermokarst lakes of alpine meadow type are the most sensitive to temperature rise.Finally,through correlation analysis,stepwise regression and other methods,it is found that organic carbon and total nitrogen in the sediment are the main factors in the methane production process of the sediment.（3）Based on the cultivation and control experiments of lake sediment,it has been shown that the input of exogenous carbon can promote its methane production process,and the direct promoting effect is greater than the indirect promoting effect.The average content of iron bound carbon in the sediment of the thermokarst lake is 13%,with the iron bound carbon content range of 7.33%-16.85%for the alpine swamp meadow type thermokarst lake,and the average iron bound carbon content is 10.41%.The iron bound carbon content range of 4.51%-38.93%for the alpine meadow type thermokarst lake,and the average iron bound carbon content is 15.56%.Both sodium acetate,which is a carbon source directly utilized by sediment microorganisms,and oxalic acid,which is a carbon source that disrupts iron bound carbon structures,promote the generation of methane in the sediment of thermokarst lake.The directly promoting carbon source,sodium acetate,increases the final cumulative methane production at 5℃and 15℃by approximately14 and 29 times,respectively.The indirectly promoting oxalic acid carbon source increases the final cumulative methane production at 5℃and 15℃by approximately 2 and 5 times,respectively.The direct promoting effect of sodium acetate is stronger than the indirect promoting effect of oxalic acid.The study clarified the characteristics of methane concentration and flux in the thermal melting lake pond during the ice cover period,the ice melting period,the non-ice period and the icing period,explored the methane production rate of the sediment of the thermokarst lake and its influencing factors,and also explored the impact of external carbon input on the methane production of the sediment of the thermokarst lake.This will enhance the understanding of the carbon cycle process of the Qinghai-Tibet Plateau thermokarst and provide important data for future climate change prediction.