Effects Of Terrrain And Vegetation Cover On Ecosystem Respiration And Methane Uptake Of Alpine Meadow

Rising concentration of carbon dioxide(CO₂)and methane(CH₄)are important contributors to global warming.Terrestrial ecosystem is an important source or sink of CO₂ and CH₄and plays an important role in regulating the concentration of both in the atmosphere.Permafrost regions are important carbon pool in terrestrial ecosystems.Rapid climate change may lead to the decomposition of large quantities of organic carbon stored in permafrost,forming a positive feedback process with climate change.Alpine meadow is a widely distributed grassland type in the permafrost area of Qinghai-Tibet Plateau,accounting for 52.77%of its area.At present,there are many studies on ecosystem respiration(Re)and CH₄ flux of alpine meadows,and significant progress has been made in the estimation of carbon fluxes(Re and CH₄ flux),seasonal dynamics,and analysis of impact mechanisms at the point scale.However,there are few studies on alpine meadow carbon fluxes under different topography and vegetation cover conditions,and the lack of understanding of the spatial pattern of carbon fluxes and its controlling factors at small watershed scale limits the accurate assessment of regional scale carbon exchange on the Qinghai-Tibet Plateau.In this paper,the Mountain Fenghuo located in the Yangtze River source of the Qinghai-Tibet Plateau was selected as the study region,and the alpine meadow ecosystem was taken as the research object.In the study region,the permafrost is widely distributed and the terrain fluctuates greatly.Under the influence of climate change and overgrazing,the alpine meadow in the region has been degraded to different degrees.Relying on the small watershed comprehensive observation field in the Mountain Fenghuo,the carbon flues of alpine meadow was observed under different terrain conditions(different slope aspects and slope positions),and vegetation coverage(high vegetation covered patches and patch)during 2017 and 2018 for two consecutive growing seasons,combined with soil physical and chemical properties and mathematical statistic methods to clarify the spatial pattern of alpine meadow soil properties and further analyze the influence of the terrain and vegetation on carbon fluxes and the driving factors of carbon fluxes under the small watershed scale.1.The soil temperature of the sunny slope was higher than that of the shady slope,and there was no significant difference in soil moisture(soil water content(g%)and soil volume water content(v%)).There were no significant differences in soil properties,including soil organic carbon,dissolved organic carbon,microbial biomass carbon and nitrogen content,soil saccharase and cellulase activities and plant biomass between shady and sunny slopes.The soil temperature and water content at the bottom position of slopes were higher than other positions.The better hydrothermal conditions at the bottom resulted in higher contents of soil organic carbon,dissolved organic carbon and microbial biomass carbon(nitrogen)as well as higher saccharase and cellulase activities.The bare patches significantly reduced the soil volume water content and plant biomass in the alpine meadow,so the concentrations of soil organic carbon,soil total nitrogen,dissolved organic carbon,dissolved total nitrogen,microbial biomass carbon and nitrogen,mineral nitrogen concentration and the activities of saccharase and cellulase in the bare patches were less than grassland.However,the soil p H of bare patches were significantly greater than high vegetation covered patches.2.The Re and CH₄ uptake of alpine meadow had the similar seasonal dynamics.The Re and CH₄ flux of alpine meadow were larger in July and August,while lower in other months of the growing seasons,which was closely related to the seasonal changes of soil temperature and water.At the small watershed scale,the growing seasons average Re of alpine meadow was 4.8 to 7.32 g CO₂ m^-2 d^-1 under different terrain conditions.Under the influence of soil hydrothermal condition,microbial activity and substrate concentration,the maximum of Re was 7.32±1.06 g CO₂ m^-2 d^-1at the bottom of shady slope,while the lowest Re was 4.8±0.71 g CO₂ m^-2 d^-1at the upper of the shady slope with the lowest soil temperature and barren soil.In the study region,the alpine meadow was a CH₄ sink during the growing seasons under different terrain conditions.The average CH₄ uptake range was during the growing seasons ranged from1.6 to 10.48?g CH₄m^-2 h^-1.Under the influence of soil volumetric water content and water filled pore space,the CH₄ uptake rate at the bottom position of the slopes were significantly less than the middle and upper of the slopes.3.The bare patches significantly reduced the Re of alpine meadow,and the average Re rates of the bare patches and the high vegetation covered patches during the growing seasons were 2.26 g CO₂ m^-2 d^-1 and 6.17 g CO₂ m^-2 d^-1under different terrain conditions,respectively,which was mainly because the bare patches reduced the microbial activity of the alpine meadow.The average rates of CH₄ uptake in the bare patches and high vegetation covered patches during the growing seasons was 25.4?g CH₄ m^-2 h^-1 and 6.61?g CH₄ m^-2 h^-1,respectively.The CH₄ uptake rate of bare patches was significantly greater than high vegetation covered patches at the middle and upper of the slopes,but their CH₄ uptake rate were similar at the bottom of the slopes.Soil moisture(Soil volume water content)was the main factor regulating the spatial variation of CH₄ uptake in alpine meadow.As the basic geomorphic unit of the Qinghai-Tibet Plateau,the small watershed is a key link connecting the point scale and the regional scale.It is of great significance to clarify the spatial pattern and regulatory factors of the carbon fluxes at the small watershed scale for the accurate evaluation of the carbon exchange at the regional scale.The results show that the soil properties and carbon fluxes of alpine meadow show obvious spatial differences under different terrain conditions at the small watershed scale,and the altitude has more obvious effect.Meanwhile,the change of vegetation cover also significantly affected the soil properties and carbon exchange of alpine meadow.The spatial variation of soil properties under different terrains and vegetation cover,especially the difference of microorganisms and soil moisture,are the main reason for the spatial variation of carbon flux in alpine meadow at the small watershed scale.At present,when the land surface process models quantify the carbon exchange amount at the regional scale,each pixel unit represents a large space range,and the effect terrain and vegetation cover on carbon fluxes is often be ignored at small watershed scale,which affects the evaluation accuracy of the model.The mountainous and hilly areas of the Qinghai-Tibet Plateau account for about 60.58%of the total area,and about half of the alpine meadows are experiencing varying degrees of degradation.Therefore,the impact of terrain and vegetation cover on carbon exchange should be taken into full consideration at the regional scale carbon exchange assessment and land surface process model to improve the accuracy of the assessment.