| The soil at the active layer of permafrost is the most active soil layer,which is located in the middle circle of the complex interaction between atmosphere,permafrost and ecosystem,and is the main layer reflecting the degradation of permafrost underclimate change.The active layer mainly participates in the process of yield and confluence in the headwater region,and influences the amount of water resources and the temporal and spatial distribution of water resources in the basin,which is of great significance to the climate development,water resources change and ecosystem under the synergistic freeze-thaw cycle in the future.However,the complex change of soil environment in active layer is a continuous and long process with strong spatial heterogeneity.And the research on the mechanism of soil material composition,water flow and its accompanying hydrological process changes in permafrost is not well studied.Therefore,under the vegetation types of alpine meadow in permafrost regions,we choose observation sites of different vegetation coverage,through the measurement of soil properties and hydraulic properties and continuous monitoring of hydrothermal process,and combined with the simulation analysis of relevant models,to study the mechanism of the change of hydrothermal process in the active layer of frozen soil under the conditions of freeze-thaw cycles and vegetation degradation,which can provides a theoretical basis for the mechanism study of ecohydrological process and the change of water resources in the high altitude cold region.And part of the conclusions are as follows:(1)Vegetation degradation and freeze-thaw action leads to the change of soil properties and soil hydraulic properties in the active layer of permafrost and the degradation of alpine vegetation ecosystem.Soil basic properties and water physical properties of high coverage meadow are completely different from those of low coverage meadow.For example,the soil of the high-coverage meadow has high sand content,low clay content,good porosity,low bulk density(BD),good water permeability,good water efficient utilization,poor water holding capacity,and soil-water characteristic curves(SWCC)steepness under high matrix potential gradually flattens with the decrease of matrix potential.However,the low coverage meadow had high clay particle content,fine particle content reached 95%,high BD,poor porosity,good water holding capacity of 20.9% on average,and poor water permeability and soil water use ability,which were only 0.002 m/d and 9.6%,respectively.The SWCC of the low coverage meadow was higher than that of the high coverage meadow,the slope of the curve is small and changes weakly with decreasing matrix potential.Because the root system slows down the degradation of surface soil,the saturated water content(?S),the field water capacity(?FC),the permanent wilting water content(?PWP),the saturated water conductivity(KS),the available soil water content(?AWC),and the unavailable soil water content(?UWC)of the 10~20 cm depth meadow soil with medium coverage are close to those of the high coverage meadow with only 0.5%,6.0%,6.8%,0.29m/d,0.8% and 5.5% difference,respectively.Vegetation degradation is closely related to soil properties and interacts with each other.(2)Soil hydraulic properties are closely related to soil basic physical and chemical properties.The correlation analysis and stepwise linear regression analysis showed that the main influencing factors of soil moisture under high matrix potential(?>-330 hpa)were porosity and BD,followed by soil organic matter,which had nothing to do with soil structure.However,at lower matrix potential(?<-330 hpa),soil structure is the main influencing factor,and with the decrease of matrix potential,the effect of soil structure becomes more intense,and the effect of porosity is no longer affected.?FC and ?PWP are functions of sand particles.?AWC and KS have basically the same influencing factors,both of which indicate the movement of soil water.(3)Soil temperature in the active layer changed in a unimodal form and reached two extreme values in August and January respectively.The soil moisture changes dramatically with the critical temperature(0 ?).Soil properties and vegetation affect the change of soil temperature.The range of soil temperatures in the 0~50 cm depth soil profile with low coverage are 38.2,29.7,25.9,23.1 and 21.3 ?, respectively,and that in the high coverage is 32.6,27.2,23.6,20.6 and 18.5 ?,respectively.The soil root layer has the function of water conduction and water retention in the precipitation period and no precipitation period,respectively,and regulates the distribution characteristics of soil water on the profile.The root layer of the soil in the low coverage meadow shows the stratified accumulation phenomenon of actual soil water.In permafrost soil,the rate of change was 0.37?/month and 0.27 ?/month respectively.The water shortage of vegetation ecosystem mainly depends on the difference between actual soil water content and ?PWP,rather than the absolute value of actual soil water content.(4)There is a significant correlation between soil moisture and soil temperature,which shows a stronger correlation during the soil freezing period in the active layer,with the correlation coefficient r all greater than 0.8,and a poor correlation during the thawing period,with r all less than 0.8.The freeze-thaw curves of the active layer in the study area are similar to those of SWCC,showing a lag phenomenon.Under the change of soil temperature,soil profile water redistributes,and soil water content in the middle and low coverage root layers with water shortage increases while other layers decrease.Therefore,the interaction between vegetation change and freeze-thaw cycle affects the change of water heat relationship. |
PDF Full Text Request