Study On The Relationship Between Loess Microstructure And Soil-water Characteristic Curve Under Different Vegetation Cover

With the strategies of “The Belt and Road” and “Ecological Protection and High-quality Development of the Yellow River Basin” put forward in succession,the ecological environment of the Loess Plateau region has been improved significantly,and notable results have been achieved in water and sand management,flood control and disaster mitigation.However,due to the arid climate and concentrated precipitation,soil and water conservation projects such as terraces-silt dams and urbanisation development have directly changed the eco-hydrological cycle process in the area,while agricultural production and engineering activities have caused damages to surface vegetation,altered the soil structure and soil-water characteristic curve parameters to varying degrees and affected the water-holding capacity and infiltration properties of the soil.Therefore,researching on the relationship between the microstructure of loess and soil-water characteristic curves for different vegetation cover type can help grasp the dynamic changes of soil and water resources in the process of vegetation restoration on the Loess Plateau,and to guide the design of ecological water conservancy projects in the Loess Plateau region more scientifically and effectively.This paper was funded by the National Natural Science Foundation Project of“Micro-meso-macroscale study on the mechanism of loess slope disaster under the influence of environment” and the State Laboratory of Loess and Quaternary Geology key Project of“Simulation and simulation of void microstructure and preferential flow characteristics in key zones of loess”.The study was carried out on the loess soils under three types of vegetation cover: grass,shrub and tree in the Bailu loess tableland area of Anchun Town,Lantian County,Xi’an City.Using the contact filter paper method to measure the soil-water characteristic curves,and the changes in the microstructure of loess particles and pores were quantitatively analyzed by Bettersize 2000 laser particle size meter and scanning electron microscope(SEM).Based on these methods,the relationship among the microstructure of loess soils and the soil-water characteristic curves and the changing characteristics of the water-holding capacity of loess under different vegetation cover were investigated.The main research results of this paper are as follows:(1)The texture of loess under different vegetation cover in the Bailu loess tableland area is fine,but there are some variation in particle size composition.The particle size composition of loess in grass,shrub and tree are dominated by coarse chalk particles(particle size10～50μm)and clay particles(particle size <5μm),with coarse chalk accounting for 43.91%,45.24% and 41.34% of the total composition,and clay accounting for 36.31%,34.18% and40.33% of the total composition,respectively.The distribution curves of the percentage content of the particle intervals all showed a triple-peaked pattern,with peak particle sizes ranging from 0.21 to 0.34 μm,0.82 to 0.91 μm and 21.28 to 27.12 μm,respectively.By calculating the particle volume fractal dimension for different particle size compositions,it was found that the particle volume fractal dimension reflects the percentage of coarse and fine particles in the soil.The higher the percentage of fine particles,the higher the particle volume fractal dimension and,conversely,the higher the coarse particle content,the lower the particle volume fractal dimension.(2)Based on the Scanning Electron Microscope images,the loess particle types are classified into six forms,such as point,flake,elongated,multi-angled,prismatic particles and agglomerates,according to the spatial geometry,particle size,and the particle contact methods are classified into four categories,such as direct point contact,indirect point contact,direct surface contact and indirect surface contact.SEM images at different magnifications show that: grass loess is mainly in direct point contact and direct surface contact,the soil structure is relatively unstable;shrub loess is mainly in direct point contact,indirect point contact and direct surface contact,the soil structure is relatively stable;tree loess is mainly in direct point contact,indirect point contact and indirect surface contact,the soil structure is the most stable.(3)Using PCAS(Praticles,Pores and Cracks Analysis System)and IPP(Image-Pro Plus)image processing software to quantitatively analyze the porosity,pore size and pore fractal dimension of loess.The results show that there are some differences in the pore distribution characteristics of loess under different vegetation cover,and the average surface porosity of grass,shrub and tree loess is 34.48%,31.69% and 35.99% respectively,showing that tree loess > grass loess > shrub loess.The pores of the loess are all predominantly microporous(pore size <2μm),but due to their small pore size,they have relatively little influence on the degree of face porosity.The number of medium pores(pore size of 8～32μm)is less than 1%,but due to their relatively large pore size,they have the greatest influence on the degree of face porosity.Grass,shrub and tree loess pores have fractal characteristics,with pore fractal dimensions of 1.7936,1.7788 and 1.7032 respectively,all among 1 and 2.Grass loess pore fractal dimension is the largest,the higher the complexity of the pore structure,tree loess pore fractal dimension is the smallest,the lower the complexity of the pore structure.(4)The results of the soil-water characteristic curves show that under the same substrate suction conditions,the volumetric water content of the soil always shows that tree loess >grass loess > shrub loess,with tree loess having the strongest water-holding capacity,followed by grass loess and shrub loess the weakest.Vegetation cover conditions have an influence on soil grain and pore structure,which in turn has an influence on the water holding capacity of the soil.The influence of the distribution characteristics of the loess grain and pore structure on the soil-water characteristic curve is mainly analysed through the grain and pore quantification data and the characteristic values of the soil saturated water content and residual water content obtained from model fitting.Saturated water content and porosity are equal in size,the greater the porosity,the greater the saturated water content of the soil.The residual water content is related to the adsorption capacity of the particles and the number of micro-pores,the finer the soil particles,the greater the content of tiny pores and the greater the residual water content.