The Effect Of Freeze-thaw Cycle On The Stability Of Black Soil And Its Mechanism

The northeast black soil area is the largest food producing area in China,which is located in the high latitude area.It is short in summer,hot and rainy,long in winter,cold and dry.The soil layer of 0?20 cm shows the characteristics of"day thawing and night freezing",and the phenomenon of freezing and thawing cycle is strong.Under the influence of freeze-thaw cycle,the physical and chemical properties of soil such as bulk density,water stable aggregate and soil shear strength have changed,and further affect the soil erodibility,which is one of the main driving factors of soil erosion and soil erosion in this area.In this study,the typical black soil of Keshan farm,Keshan County,Heilongjiang Province was selected as the research object.According to the actual situation of the field measurement,the initial moisture content of the soil was divided into four water gradients:saturated moisture content?43.39%?,field moisture content?32.25%?,field moisture content?70%?and air dried soil moisture content?3.34%?The number of times was set to 0,1,3,5,7,10,15,30.According to the local meteorological data,the soil samples with different water gradients and different times of freezing and thawing after the indoor simulated freezing and thawing treatment were analyzed by wet sieving,LB method and SEM scanning electron microscope,in order to reveal the influence of freezing and thawing cycle on the stability of northeast black soil and its mechanism.The main results were as follows:?1?At the same time,the initial water content and the number of freeze-thaw cycles were the key factors affecting the stability of soil aggregates in the freeze-thaw cycle.After 30 freeze-thaw cycles,the proportion of>1 mm water stable aggregates and the average weight diameter of soil aggregates decreased significantly?P<0.05?,with the decrease range of 90.81%?97.43%and 69.26%?75.06%,respectively.The damage rate(PAD_0.25,PAD_1.0)and fractal dimension increased significantly?P<0.05?,and the increase of PAD_1.0 was large,reaching 91.78%?98.82%.It could be seen that the freeze-thaw cycle cutting The water stability of soil aggregate was reduced by the ability of splash erosion resistance of weak soil.The initial moisture content was the key factor of the freeze-thaw cycle.When the initial moisture content was lower than that of the air dried soil,after the freeze-thaw cycle,the freeze-thaw cycle had no significant influence on the average weight diameter and fractal dimension of the water stable aggregate.With the increase of the initial water content,the composition of soil particle aggregates changed significantly.The proportion of water stable aggregates with a diameter of>1 mm decreased significantly,the average weight diameter decreased by 69.26%,and the fractal dimension increased by6.03%.The freeze-thaw cycle played a role in breaking the water stable aggregates.When the initial water content was the field water capacity,with the further increase of the initial water content,the crushing effect of freeze-thaw cycle on soil water stable aggregates tended to be stable.The number of freeze-thaw cycles was also a key factor affecting the stability of soil aggregates in the freeze-thaw cycle.In the early stage of freeze-thaw cycle?0,1,3,5,7 times?,with the increase of freeze-thaw cycle times,PAD_0.25,PAD_1.0 significantly increased,soil aggregate particles were broken,the average weight diameter gradually decreased,fractal dimension gradually increased,after 10 times of freeze-thaw,PAD_1.0 reached87.04%?96.43%,nearly 100%,the average weight diameter decreased 62.35%?71.18%,with the further freeze-thaw cycle times The average weight diameter and fractal dimension of 15 and 30 times of freeze-thaw were not significantly different from that of 10 times of freeze-thaw,indicating that the stability of soil aggregate would gradually decrease and tended to be stable with the increase of freeze-thaw cycles.?2?In order to explore the changes of the internal structure of soil aggregates during the freeze-thaw cycle,SEM images were obtained by scanning the aggregates with SEM.In order to further observe the changes of the internal structure of soil aggregates,IPP?image pro?was used Plus)was used to process the SEM images of soil,and the indexes such as area porosity,porosity roundness,porosity abundance and fractal dimension of pores were extracted.It was found that freeze-thaw cycle had a significant impact on the change of soil aggregate pore structure.At the same time,the initial water content and the number of freeze-thaw cycles were the key factors affecting the soil aggregate pore structure in the freeze-thaw cycle.When the initial water content is lower than that of the air dried soil,after the freeze-thaw cycle,the freeze-thaw cycle had no significant effect on the soil area porosity.With the increase of initial water content,the porosity of soil area changed significantly,and the porosity of soil area decreased significantly.With the increase of freeze-thaw cycles,the porosity of soil area increased to 6.97%?23.15%after 30freeze-thaw cycles.During the whole freeze-thaw cycle?1,3,5,7,10,15,30?,the pore abundance and fractal dimension increased compared with the zero freeze-thaw data.In the early stage of freeze-thaw cycle?0,1,3,5,7 times?,the pore rounding rate increased with the increase of freeze-thaw times,but after 7 freeze-thaw cycles,the pore rounding rate fluctuated under 10,15 and 30 freeze-thaw cycles.The evolution law of pore structure of soil sample microstructure showed that freezing and thawing destroy the connection between soil particles,which increased the number of pores and made the pore structure more rough and irregular.And that made the structure of black soil more easily broken and deformed.The weakening effect of freezing and thawing cycle on the structure of black soil may be the cause of the stability decline of black soil aggregate.?3?In the black soil area of Northeast China,after freeze-thaw cycle,fast wetting?FW?treatment had the most destructive effect on the stability of black soil aggregates,and it was significantly different from slow wetting?WS?and mechanical disturbance?SW?treatment.For the three different initial particle size aggregates of 1-2mm,2-3mm and 3-5mm,the stability performance under three different crushing mechanisms was SW>WS>FW,which indicated that after freeze-thaw treatment,the stability of black soil aggregates was SW>WS>FW The main damage mechanism to the black soil aggregates after circulation was dissipation under the condition of rainstorm.In this study,the freeze-thaw cycle had obvious destructive effect on the aggregate under three kinds of crushing mechanisms,which showedthat the process of soil moisture condensation and expansion will have a significant impact on the internal structure of the aggregate,and the aggregate fragility would be improved.It was found that the initial moisture content and the number of freeze-thaw cycles were the key factors that affect the stability of different crushing mechanisms after freeze-thaw cycles.The initial moisture content mainly affected the stability of aggregates by affecting the proportion of air and water in aggregates and the volume of water condensation expansion.In the process of freeze-thaw cycle,the more times of freeze-thaw cycle,the more times of soil water volume change,more serious damage to aggregate structure.Therefore,in the area of frequent freeze-thaw cycle,if there iweas sudden rainstorm,it was easy to produce large area of soil erosion.