Experimental Study On Effects Of Freeze-thaw Conditions On Structure Characteristics And Detachment Capacity Of Black Soil

The northeast black soil region is located in the seasonal frozen soil area,and the surface soil structure changes obviously under seasonal freeze-thaw cycles,which affects the resistance of soil erosion.During the spring thawing period,the phenomenon of seepage is common.In addition,the longitudinal ridge-tillage system used in some cultivated land promotes the snowmelt runoff to flows in the furrows,which leads to the detachment of the surface soil and intensifies soil erosion.This has seriously threatened the sustainable development of sociely and agriculture.In this study,the typical black soil in Northeast China was selected as the studied soils,and the change processes of characteristics of soil pore structure and aggregate stability under freeze-thaw effects were quantitatively studied using the indoor simulated freeze-thaw cycle test.According to the changes of soil pore structure characteristics under freeze-thaw action,the effects of soil pore structure on soil water holding capacity were evaluated.Meanwhile,the effects of freeze-thaw action on soil shear strength and detachment rate were studied through simulated snowmelt runoff and seepage tests,and the quantitative relationships between soil shear strength and detachment rate were analyzed with the seepage and no-seepage conditions.Based on soil soil microstructure characteristics,the mechanise of simulated snowmelt runoff and seepage on the soil detachment rate was explored.The main results were as followed:(1)Under the effects of freeze-thaw cycles,soil pore structure became looser and more porous,and the complexity and connectivity of pore structure obviously increased.The results indicated that with the increasing number of freeze-thaw cycles,the total imaged porosity,porosity with diameter >500 μm,porosity of elongated pores,branch density,fractal dimension,and connectivity density increased significantly,and the porosity with diameter<100 μm and100-500 μm and regular porosity increased with a fluctuating trend,while the irregular porosity fluctuated.However,the length density,fractal dimension,and connectivity density decreased under the 10 freeze-thaw cycles,and then increased with the increase in freeze-thaw cycle.With the increase in soil initial water content,the effects of freeze-thaw on pore structure were strengthened,which significantly increased the total imaged porosity and porosity with diameter >500 μm,porosity of elongated pores,but significantly decreased branch density,fractal dimension,and connectivity density.(2)Freeze-thaw cycles affected the distribution of soil aggregates and water-stable aggregates,and the dynamic change processes depended on the initial soil water content and the number of freeze-thaw cycles.Freeze-thaw effects decreased the mean weight diameter and geometric mean diameter of soil aggregates and water-stable aggregates,but increased the fractal dimension.Freeze-thaw effects reduced soil aggregate stability,and the decrease was the largest when the soil initial moisture content was 30%.Meantimes,the low-frequency of freeze-thaw cycles strengened aggregate water stability with an initial water content of 20%,but aggregate water stability gradually decreased with the increase in the number of freeze-thaw cycles.(3)The effects of freeze-thaw cycles significantly influenced soil water retention capacity.During freeze-thaw cycles,soil saturated water content significantly increased.Field water capacity and available water content first increased under low-frequency of freeze-thaw cycle,but gradually decreased with the increase in freeze-thaw cycles.However,freeze-thaw effects had no significant effects on the permanent wilting point.Freeze-thaw effects can increase soil saturated water content and reduce field water capacity and available water content by changing soil pore structure(increases in macroporosity and elongated porosity,pore structure with high complexity and good connectivity).(4)Freeze-thaw effects significantly decreased soil shear strength,but significantly increased soil detachment rate under simulated snowmelt runoff.Compared with the no-seepage condition,the formation of seepage significantly reduced the soil shear strength,thus increased soil detachment rate.Under no-seepage and seepage conditions,soil detachment rate by simulated snowmelt runoff increased with the decrease in soil shear strength with a power function.(5)Freeze-thaw effects affected soil shear strength by changing soil microstructure characteristics(soil pore structure and aggregate stability).The results of partial least squares regression showed that mean weight diameter and fractal dimension of water-stable aggregates,mean weight diameter of aggregates,<100 μm porosity and irregular porosity were the main structure characteristics that control soil shear strength.Thus,the decrease in soil shear strength mainly depended on the decrease in the structure stability of soil aggregates and water‐stable aggregates and the decrease in small porosity.