Variations And Influencing Factors Of CH₄ And CO₂ Emissions From Thermokarst Lakes On The Qinghai-Tibet Plateau

Permafrost in the northern hemisphere accounts for 24%of the land area and stores a large amount of organic carbon.In case of global warming,permafrost is undergoing an extensive degradation,and its stored"old carbon"is also in an unstable state.Thermokarst is a special landform formed by the degradation of ice-rich permafrost or the melting of thick underground ice.Different from the slow degradation of permafrost,the thermokarst process will cause rapid exposure of deep organic matter,which has a significant impact on the regional carbon cycle.Thermokarst lakes are one of the main types of thermokarst landforms.However,the carbon release rate of thermokarst lakes on the Qinghai-Tibet Plateau and their response to climate change are still unclear.This research aims to answer the following two scientific questions:what is the rate of CH₄and CO₂ emission from thermokarst lakes on the Qinghai-Tibet Plateau and its main controlling factors?In this study,42 thermokarst lakes along the highway in the central Qinghai-Tibet Plateau were selected,and continuous monitoring was carried out during the growing season from June 2019,September 2019 and from June to September 2020.By combining field monitoring and laboratory experiment analysis,it focuses on the physical and chemical properties of different vegetation types in thermokarst lakes and the change characteristics of greenhouse gas release rates,and analyzes the main influencing factors of greenhouse gas release from thermokarst lakes.The main conclusions of this study are as follows:(1)The release of greenhouse gases from thermokarst lakes on the Qinghai-Tibet Plateau has significant monthly and day-night differences.During the growing season,the average diffusion flux of methane in thermokarst lakes was 3.48±5.09 mmol·m^-2·d^-1,and the average emission flux of carbon dioxide was 20.67±12.81 mmol·m^-2·d^-1.Th e flux of methane emission from thermokarst lakes shows a downward trend from Jun e to September,June(4.90±6.74 mmol·m^-2·d^-1)>July(3.65±5.09 mmol·m^-2·d^-1)>August(2.64±3.22 mmol·m^-2·d^-1)>September(1.88±2.76 mmol·m^-2·d^-1).The emission flux of carbon dioxide presents a different pattern,August(27.61±7.09 mmol·m^-2·d^-1)>July(22.10±15.70 mmol·m^-2·d^-1)>June(20.61±12.50 mmol·m^-2·d^-1)>September(10.47±8.43mmol·m^-2·d^-1).In addition,the release of methane from thermokarst lakes on the Qing hai-Tibet Plateau has obvious diurnal characteristics,while the amount of carbon dioxi de released does not fluctuate significantly during the day and night.(2)There are significant differences in the physical and chemical parameters and greenhouse gas emission fluxes of different vegetation types on the Qinghai-Tibet Plat eau.The conductivity,SO₄^2-concentration and other physical and chemical indicators increase with the decrease of surrounding vegetation,and there are significant differen ces between thermokarst lakes of different vegetation types.The methane flux of ther mokarst lakes decreases with the decrease of the surrounding vegetation coverage:alp ine swamp meadow(8.06±6.97 mmol·m^-2·d^-1)>alpine meadow(3.14±4.17 mmol·m^-2·d^-1)>alpine steppe(2.28±3.73 mmol·m^-2·d^-1)>alpine desert(0.08±0.06 mmol·m^-2·d^-1).T he carbon dioxide emission flux of thermokarst lakes on the Qinghai-Tibet Plateau als o changes with vegetation:alpine swamp meadow(28.01±10.68 mmol·m^-2·d^-1)>alpine meadow(20.55±13.22 mmol·m^-2·d^-1)>alpine desert(17.18±10.12 mmol·m^-2·d^-1)>alpi ne steppe(17.12±13.02 mmol·m^-2·d^-1).(3)The release of greenhouse gases from thermokarst lakes on the Qinghai-Tibet Plateau is mainly affected by water quality indicators and environmental factors.Using the stepwise regression analysis method to analyze the physical and chemical indicators of lake water and sediments,it is found that the most important factor affecting the methane flux of thermokarst lakes on the Qinghai-Tibet Plateau is the concentration of SO₄^2-in the water body.It indicates that the anaerobic oxidation of methane is an important process that affects the methane flux in thermokarst lakes of the Qinghai-Tibet Plateau.The most important factors affecting the carbon dioxide flux of thermokarst lakes are water temperature and the concentration of SO₄^2-in the water body,indicating that the decomposition of DOC and the oxidation of methane are the main processes that affect the carbon dioxide flux of thermokarst lakes on the Qinghai-Tibet Plateau.This study has clarified the characteristics of the release rate of greenhouse gases in thermokarst lakes,and explored the main controlling factors that affect the release of greenhouse gases in thermokarst lakes.The cognition of the feedback relationship between the carbon cycle of ecosystems in permafrost regions and climate warming provides important data for future climate change projections.