Study On The Effects Of Different Freezethaw Cycle Conditions On Soil Structure And Water Characteristics Of Farmland

The black soil area in the middle and high latitudes of Northeast China is not only the largest grain production base in my country,but also a typical seasonal frozen soil area in my country.Due to the particularity of the natural environment in seasonally frozen soil regions,black soil will experience repeated freeze-thaw cycles.Under the action of freeze-thaw cycles,soil structure characteristics and water characteristics will change,which will deteriorate the crop planting environment and affect the soil in the black soil region of Northeast China.Characteristics and crop planting environment of farmland soil.This experiment takes cultivated black soil in a typical area of Northeast China as the research object.Through indoor simulated freeze-thaw experiments,three soil initial mass moisture contents of 20%,24%,28%（W1,W2,W3）are set,0mm,30mm,60mm,90mm（L0,L1,L2,L3）4 snow depths,a total of 12 treatments.The overall test adopts a gradient freeze-thaw cycle temperature of-16°C-4°C,-12°C-8°C,-8°C-12°C,-4°C-16°C,and each freeze-thaw temperature cycle is 2 times,a total of 8 freeze-thaw cycles melting process.By collecting soil samples to measure snow characteristics,soil structure and water characteristics related indicators,analyze the change law of snow characteristics under differ ent freeze-thaw cycle temperatures,and then explore the changes in soil structure characteristics and water characteristics under different treatments,and further identify freeze-thaw The main influencing factors of soil structure changes under the actio n of cycling,and explore the relationship between soil structure and soil moisture characteristics under different treatments.The main research content and results are as follows:（1）By analyzing the influence of different freeze-thaw cycle temperatures on the characteristics of snow cover,it is found that within the same freeze-thaw cycle temperature,the snowmelt rate decreases with the increase of snow depth.Moreover,the increase of freeze-thaw cycle temperature and the increase of snow depth will increase the snowmelt rate of snow.During the melting process,the snow density first increased and then stabilized,and its value ranged from 0.013 to 0.638 g/cm³.The increase in the temperature of the freeze-thaw cycle promotes the heat exchange betwee n the snow cover and the atmosphere,which increases the rate of change in snow density.（2）By analyzing the effects of freeze-thaw cycles on soil structure under the treatment of different snow cover depths and initial soil moisture content,it is found that the freeze-thaw effect significantly increased the total porosity of the soil and significantly decreased the bulk density of the soil,but the increase of snow cover depth and the initial moisture content of the soil reduced the variation of the tot al porosity and soil bulk density.The distribution of soil pore size changed under the action of freeze-thaw,and the increase of the initial moisture content of the soil inhibited the increase of the macropore content and the decrease of the micropore content in the soil after freeze-thaw,while the micropore size of the soil after freeze-thaw decreased with the increase of snow depth,and the macropore size increased with the increase of snow depth,and the rest of the pore size did not fluctuate much.Freeze-thaw also reduced soil particle size,but the increase of initial soil moisture content would inhibit the transition of microaggregates to clay particles under freeze-thaw,while the continuous increase of initial moisture content would transform soi l particle size to microaggregates.In addition,freeze-thaw also changed soil stability,among which Mean weight diameter（MWD）,Geometric mean diameter（GMD）,>0.25 mm hydrostable aggregates(WR_>0.25)and soil erodability K decreased and the aggregate failure rate（PAD）and fractal dimension D increased.Soil stability is mainly affected by the content of large aggregates（>2 mm）,small aggregates（0.25-2 mm）and clay particles（<0.106 mm）.Multivariate ANOVA showed that the number of freeze-thaw times was the main driving factor for soil structure change,the initial soil moisture content played a secondary role in soil structure,and the change of snow depth had little effect on soil structure.（3）By analyzing the effects of freeze-thaw cycle on soil moisture characteristics under treatment with different snow depth and initial soil moisture content,it was found that the increase of initial moisture content before freeze-thaw test enhanced the water-holding capacity of soil.After freeze-thaw test,the water-holding capacity of soil under high suction is weakened,and the water-holding capacity under low suction is enhanced,and the increase of snow depth and initial soil moisture content has an inhibitory effect on the c hange of soil moisture characteristics after freeze-thaw test.In addition,after freeze-thaw testing,the soil field water holding capacity（FC）,permanent wilt point（PWP）,plant effective water content（PAWC）and S_inf decreased,and FC and PWP increased significantly with the increase of snow depth.By analyzing the effects of different treatments on soil hydrothermal conditions,it was found that the lower the freeze-thaw temperature and the smaller the snow depth,the faster the corresponding soil free zing rate.For every 4°C increase in freeze-thaw cycle temperature,the average soil thawing time of 0-10 cm soil layer decreased by 8 h,while the soil temperature of 0-10 cm soil layer increased by 0.1°C for every 10mm increase in snow depth.The freeze-thaw cycle will cause soil moisture to migrate to the upper soil,and the higher the freeze-thaw cycle temperature,the greater the soil water migration.（4）Through the study of the correlation between soil structure and water characteristics,it is found that the soil water characteristics change due to the change of soil structure.When the initial moisture content of the 0-10 cm soil layer is small（20%,24%）,the distribution of soil pore size is the dominant factor affecting the characteristics of s oil moisture;when the initial moisture content of the soil is high（28%）,the distribution of soil particle size Situation is the dominant factor affecting soil water characteristics,and soil structure has a high degree of influence on water characterist ics.The increase of initial soil moisture content in the 10-20 cm soil layer will reduce the correlation between soil structure and moisture characteristic indicators.In summary,the freeze-thaw cycle will significantly change the soil properties such as soil structure and moisture characteristics.Different snow depths and initial soil moisture content will significantly affect the change trend of freeze-thaw cycles on soil properties,thereby changing the soil environment and crop habitat.This study ca n provide a theoretical basis for predicting the sowing and growth environment of soil crops in spring under different environmental conditions,and provide theoretical support for maintaining the health of farmland soil habitats in seasonally frozen regio ns.Provide scientific and technological support to ensure the health of agricultural water and soil environment and the sustainable development of agriculture.