Effect Of Soil Physical Crust On Wind Erosion Of Sandy Loess In Northern Loess Plateau Under Freezing And Thawing

The northern part of the Loess Plateau is a wind erosion and water erosion crisscross area,which is also an area with strong freeze-thaw action.The interaction between freeze-thaw action and wind erosion and water erosion further intensifies soil erosion.At the same time,the soil physical crust formed by rainfall splash and runoff on the surface is widely distributed in this area.The existence of soil physical crust can inhibit wind erosion,while the anti wind erosion ability of freeze-thaw crust is rarely reported,Therefore,this study takes sand loess as the research object,combined with indoor simulated rainfall,indoor simulated freeze-thaw and indoor wind tunnel test,systematically studies the effect of freeze-thaw on the strength of crust and the physical and chemical properties of soil crust,discusses the wind erosion resistance of physical crust with different strength before and after freeze-thaw,and reveals the erosion reduction effect and mechanism of physical crust on soil wind erosion under freeze-thaw.The results can provide a theoretical basis for the prediction and control of soil erosion in this area.The main conclusions are as follows:（1）The changes of crust strength and physical and chemical properties of soil under freeze-thaw conditions were clarified.Before freezing and thawing,the influence of rainfall intensity on crust development was greater than that of rainfall duration.With the increase of rainfall intensity,the shear strength,hardness and thickness of crust increased.After freeze-thaw,the shear strength and hardness of crust decreased exponentially with the increase of freeze-thaw times.The crust thickness under different crust strength has no significant change with freeze-thaw,but the crust thickness of high-strength crust before and after freeze-thaw is greater than that of low and medium strength crust.After freeze-thaw,the unit weight of crust with different strength decreased significantly,and the porosity of crust increased significantly.The greater the strength of crust,the smaller the change range of unit weight and porosity of crust.With the increase of freeze-thaw times,the skin roughness of the skin with different strength increases first and then decreases,but has no significant change trend with the initial moisture content.After freeze-thaw,the aggregate content>0.25mm,average weight diameter and geometric average diameter of the crust decreased significantly under different crust strength,and the aggregate content>0.25mm decreased with the increase of crust strength,while the average weight diameter and geometric average diameter increased first and then decreased.When the initial water content is higher,with the increase of freeze-thaw times,the content of sand and erodible particles in the crust with different strength increases.The soil organic matter of crust with different strength decreased after freeze-thaw,while the content of calcium carbonate did not change significantly with the times of freeze-thaw and the initial water content.After freeze-thaw,the correlation between physical and chemical properties of soil crust is high,and freeze-thaw is more sensitive to crust intensity factor and aggregate factor,while other factors are weak sensitive.（2）The changes of wind erosion characteristics of soil with physical crust before and after freeze-thaw were clarified.Before and after freeze-thaw,the amount and intensity of wind erosion of soil with or without crust increased significantly with the increase of wind speed,and the height and rate of sediment transport also increased significantly.The sediment transport rate decreases exponentially with the height of sediment transport.With the increase of crust strength,the amount and intensity of wind erosion decrease significantly,and the height and rate of sediment transport also decrease.Compared with the soil without crust,the low-strength crust before freezing and thawing can reduce the wind erosion by more than 90%.After freeze-thaw,the amount of wind erosion of soil with different strength crust decreased by 12.64,16.09 and 17.70 times than that of soil without crust;2.87,3.74,10.75 times;0.57,0.42,2.18times.The higher the crust strength,the stronger the soil resistance to freeze-thaw.Under the wind speed of 13m·s^-1,the wind erosion intensity of soil without crust after freeze-thaw is reduced by more than 80%compared with that before freeze-thaw,and the increase rate of freeze-thaw on skinned soil is more than 500%.After freeze-thaw,the sediment transport height of soil without crust is 16-20cm lower than that before freeze-thaw,and the sediment transport rate is more than 79%lower than that before freeze-thaw.The sediment transport height of soil with different skinning strength is 7-16cm higher than that before freeze-thaw,and the sediment transport rate is at least 3 times higher.（3）The effect of physical crust with different strength before and after freeze-thaw was calculated,the contribution rate of soil wind erosion affected by physical crust was clarified,and the mechanism of soil wind erosion caused by physical crust under freeze-thaw was revealed.Before freeze-thaw,the corrosion reduction effect of physical crusts with different strength is more than 99%.With the increase of crust strength,the corrosion reduction effect of physical crust is greater.The corrosion reduction effect of physical crusts with different strength after freeze-thaw varies with the initial water content.The higher the initial water content,the smaller the corrosion reduction effect of crusts.When the initial water content is 4%and 8%,the corrosion reduction effect of different strength crusts is 90%and 70%respectively.When the initial water content is 12%,the corrosion reduction effect of low and medium strength crusts is negative,but the corrosion reduction effect of high strength crusts is 61.54%.The contribution of wind speed to wind erosion of soil crust（37.01%）>crust strength（24.53%）>times of freezing and thawing（19.57%）>initial moisture content（13.26%）.Freeze-thaw mainly affects the wind erosion intensity by affecting the crust unit weight,crust hardness,crust shear strength and aggregate content greater than 0.25mm of the crusted soil.Among them,the soil wind erosion intensity has a significant negative correlation with the above crust factors,and the correlation is in the order of crust unit weight>crust hardness>crust shear strength>aggregate content greater than 0.25mm>geometric average diameter,and is closely related to the crust unit weight,crust hardness,crust shear strength The aggregate content greater than0.25mm has a good nonlinear relationship.