Study On Interaction Mechanism Of Water And Carbon In Alpine Meadow Of Qilian Mountains Under Grazing Disturbance

Alpine meadow ecosystem is widely distributed in the west of China,and is the main site of grazing activities,which has an important influence on regional economic and social development.However,due to the complexity of the ecosystem water-carbon interaction and the uncertainty of climate change,the current understanding of the process and mechanism of the alpine meadow ecosystem water-carbon interaction needs to be further strengthened.Therefore,to explore the interaction mechanism of water and carbon in alpine meadow under the disturbance of grazing,especially to identify the environmental factors and key processes leading the carbon sequestration and water production functions of alpine meadow ecosystem,is conducive to scientific guidance of animal husbandry activities and has great significance for regional ecological governance and environmental protection.In this study,the improved IBISi model was used to simulate plant carbon sequestration and water yield in the alpine meadow ecosystem of the Qilian Mountains under the realistic grazing scenario,driven by high quality meteorological data after downscaling by machine learning,and verified by remote sensing net primary productivity(NPP)data and hydrological station observation data.The validated model was used to simulate the NPP and water yield depth from 2001 to 2020 under no-grazing,0.5 times grazing,realistic grazing,1.5 times grazing and double grazing,and output ten key process quantities,including canopy evaporation,canopy transpiration and leaf area index.Then principal component analysis method was used to identify the dominant processes that had important effects on the carbon sequestration and water production of alpine meadow,the changes of each dominant process were analyzed.On this basis,the interaction relationship between water and carbon in alpine meadow of Qilian Mountains was explored by redundancy analysis method combined with four environmental factors including temperature,precipitation,relative humidity and wind speed.Finally,driven by future climate model data,the changes of NPP and water yield depth of alpine meadow in Qilian Mountain were predicted under climate change and different grazing scenarios,and the conclusions were as follows:(1)Under different grazing conditions,the NPP and water yield of alpine meadow in Qilian Mountain ranged from 210 to 262.53 g C/m~2 and 114.2 to 116.2 mm,respectively,and the rate of change ranged from 2.53 to 3.13 g C/(m~2·a)and 1.85 to1.89 mm/·a.All of them decreased with the increase of grazing intensity.NPP and water yield depth were positively correlated in the north,middle and south of the study area,and negatively correlated in the west and east of the study area.(2)In the alpine meadow ecosystem of Qilian Mountains,soil infiltration,surface evaporation,leaf area index(LAI),canopy transpiration,canopy evaporation and photosynthetic active radiation were the dominant processes that had important effects on the carbon sequestration capacity of vegetation.The dominant processes that have important influence on the carbon sequestration capacity of vegetation include soil infiltration,surface evaporation,canopy transpiration,soil moisture change and canopy evaporation.Among the environmental variables,precipitation has the highest total explanation for all the key processes affecting carbon sequestration and water production capacity,which are 23.28%and 55.86%,respectively.Grazing activities resulted in a significant decrease in leaf area index(LAI),canopy transpiration and canopy evaporation,but a significant increase in surface evaporation and soil flow,resulting in a decrease in NPP and water yield.(3)In the realistic grazing scenario,about 18%of the precipitation water is trapped and evaporated by the canopy,about 15%of the water flows directly through the slope,and the remaining 67%of the water penetrates into the soil.After entering the soil,about 24%of the precipitation water is reabsorbed by plants for transpiration,about 24%of the water directly evaporates from the surface into the atmosphere,and about 7%of the water becomes a part of the ecosystem water production in the form of soil flow.The final water production accounts for about 22%of the total precipitation.Among them,canopy interception,canopy transpiration and soil water content changes are closely related to plant carbon sequestration,which form the link of water-carbon interaction in alpine meadow ecosystem.(4)Under the future climate change scenario,NPP and water yield depth of alpine meadow in Qilian Mountains showed a significant upward trend,and the change rate of NPP and water yield depth under SSP126 scenario was lower than that under SSP585scenario.Under the SSP126 and SSP585 scenarios,grazing activities resulted in a decrease of 17.9 g C/m~2 and 16.8 g C/m~2 in the average multi-year NPP,respectively.In conclusion,the carbon sequestration and water production capacity of alpine meadow in Qilian Mountains have been improved under climate change and grazing disturbance.Precipitation plays a leading role in water production and controls the change of carbon sequestration capacity together with temperature.Under grazing disturbance,the leaf area of vegetation decreases,which affects transpiration,evaporation and canopy interception.Under certain climatic conditions,the relationship between carbon sequestration and water production in Qilian Mountain is mainly affected by canopy evaporation,canopy transpiration and soil surface evaporation.In the future,appropriate engineering measures can be taken to reduce surface water evaporation and improve water yield in the alpine meadow pasture area of Qilian Mountains.