Pore Size Distribution Of Freezing-Thawing Soil And Its Influencing Factors

Affected by global warming,the freezing state of near surface soil in cold regions has changed in the past few decades.The influence of climate change on the soil water content before freezing,number of freeze-thaw cycles,snowmelt water and other factors can not be ignored.Freezing and thawing can significantly change the soil structure.In addition to the soil particles themselves,the changes of soil pore system existing inside and between particles can directly determine the changes of soil structure.The change of soil structure is the basis and essence of soil water and heat transfer capacity,mechanical properties and even microbial activities.The volume change caused by ice water phase change during soil water freezing and thawing leads to the change of soil pore system.The initial water content before freezing and the number of freeze-thaw cycles determine the amplitude and frequency of ice water phase change volume,which is the key factor affecting the change of soil pores.In order to better understand the complex hydrothermal movement and reveal the change law of soil pores in the freezing and thawing process.Taking black soil,meadow soil and chernozem,which account for the largest cultivated land area in Heilongjiang Province,as the research object,aiming at the influence of freeze-thaw cycle on soil pore distribution characteristics,the unsaturated hydraulic conductivity and macropore distribution characteristics of remolded soil columns in different freeze-thaw stages were measured by tension infiltrometer and ethylene glycol aqueous solution.The pore size distribution of undisturbed soil under different freeze-thaw cycles and initial water content was quantified by NMR.The effects of freezing and thawing on soil water movement parameters were compared and analyzed.The main results are as follows:（1）The feasibility of ethylene glycol aqueous solution as infiltration solution was verified.The law of water infiltration during soil freezing under negative pressure is clarified,and the differences of soil infiltration parameters are analyzed.The hydraulic conductivity of frozen soil under low temperature is revealed.Adding ethylene glycol into water will not change the wetting ability of the solution to soil particles,and the contact angle between soil and ethylene glycol aqueous solution and deionized water is the same.Based on the principle of tension infiltrometer,it is basically feasible to measure the infiltration characteristics and pore distribution of frozen soil with ethylene glycol aqueous solution.The ice formed by soil freezing will fill the soil pore space and block the water infiltration channel.When the frozen soil temperature decreases to-10℃,the unsaturated hydraulic conductivity of the three soils under tension is between 0.0036-0.4951cm/h.The unsaturated hydraulic conductivity corresponding to the actual matrix potential energy after tension conversion is<10^-10m/s,which can be ignored.On the premise of not changing the freezing state of soil,when reached a certain negative temperature,the unsalinized frozen soil can be regarded as impermeable.（2）The change law of macropores in remolded soil at different freeze-thaw stages is revealed,and the effects of freeze-thaw cycle numbers and initial water content on macropore flow and macropore distribution characteristics of remolded soil are clarified.For remolded soil before freezing,the number of soil macropores per unit area is inversely proportional to the soil pore size.The number of macropores is the least,but it contributes the most to the saturated flow.There was no significant difference in soil steady infiltration rate and unsaturated hydraulic conductivity under different initial water content;However,the infiltration rate and cumulative infiltration amount of soil with high initial water content are relatively small.After freezing,the greater the initial water content of soil,the more ice crystals will be produced,and the more the infiltration parameters will decrease.The number of pores with the smallest pore diameter becomes more and the number of pores with the largest pore diameter decreases,so that the infiltration water flow is more evenly distributed in pores of all sizes.After soil melting,the number and porosity of pores are slightly larger than those before freezing.The greater the tension value,the greater the difference of infiltration parameters after soil freezing and thawing with different initial water content.The more freeze-thaw cycle numbers,the more macropores and effective porosity,but the proportion of macropore flow did not increase significantly.Whether the soil is frozen or not,the large and medium pores with a pore diameter of more than 0.05mm conduct almost all the infiltration water.（3）The pore size distribution of undisturbed soil before and after freeze-thaw is revealed,and the effects of freeze-thaw cycle numbers and initial water content on the pore characteristics of undisturbed soil are clarified.For undisturbed soil,the initial water content before freezing has little effect on the pore distribution.The soil pore diameter spans multiple orders of magnitude and is generally distributed at 0.001μm-100μm.The first freeze-thaw cycle has the greatest impact on the pore structure.After the first freeze-thaw cycle,the decline of the proportion of small pores is significantly greater than that of large pores,but the total porosity of the soil becomes larger,and the growth range of soil porosity will accumulate with the increase of the number of cycles.For black soil and chernozem with high initial water content,when the freeze-thaw cycle numbers are added to 10 or 20 times,there is melting settlement under the influence of soil self weight,so that the total porosity difference between soils with different initial water content after freezing and thawing is small,but the porosity components of pores with different pore diameters are significantly different.The residual plant roots make the meadow soil less affected by the difference of water content before freezing,but also limit the development of macropores in the later stage of freeze-thaw.（4）The advantages,disadvantages and applicability of tension infiltrometer and nuclear magnetic resonance method in the determination of pore characteristics of freeze-thaw soil are compared,and the characteristic threshold of the variation law of loam porosity accumulation curve is obtained.The root of the difference of pore structure between undisturbed soil and remolded soil under freeze-thaw cycle is analyzed.NMR method can improve the measurement accuracy of pore size to micron and nano scale,but due to its basic principle,it can not measure the pore size distribution of frozen soil.Tension infiltrometer can be used in frozen soil,but the accuracy range is more suitable to describe the variation law of large and medium pores in soil.The value of surface relaxation intensity only affects the absolute value of pore size,and will not affect the relative proportion of pore size distribution.The pore size distribution of the three soils used in the experiment is relatively concentrated,and a single surface relaxation strength can be used.The inflection point of three soil porosity accumulation curves is near 0.1μm,it is recommended that 0.1μm is one of the critical thresholds of equivalent pore size of loam.The original structure of undisturbed soil is more stable,and its macropores can accommodate more soil water volume expansion.The pore structure of remolded soil is completely destroyed in the manufacturing process,which makes it more sensitive to the volume change under the influence of freeze-thaw.The undisturbed soil that has experienced many freeze-thaw cycles is more vulnerable to the influence of soil frost heave and thawing settlement,which weakens the variation range of soil pores.（5）The effect of freezing and thawing on soil water movement parameters was revealed.Based on the influence of initial water content and freeze-thaw cycle numbers on soil pore structure,appropriate suggestions are put forward for the control of soil water state before freezing.The increase of large porosity after freezing and thawing is the fundamental reason for the enhancement of soil infiltration capacity.Freeze-thaw cycle can improve soil water retention,but its effect decreases with the increase of soil profile depth.The higher the initial water content,the less likely the liquid water is to freeze and the lower the supercooling temperature.The influence of freezing and thawing on water movement parameters varies with soil type,bulk density and initial water content.More freeze-thaw cycle numbers and higher initial water content can promote the development of soil structure.From the perspective of agriculture,freeze-thaw cycle is a favorable factor,which can make the topsoil soil more loose,increase the large porosity of the melted soil,and make the soil have a more uniform pore system after thawed.This can improve the hydraulic characteristics of the cultivated layer soil and is conducive to water movement in the melting period.Under the trend of global warming,if the temperature is high in winter,there is less snow and the soil moisture content is low before freezing,the impact of freezing and thawing on soil structure will be weakened and the input cost of agricultural measures during spring ploughing will be increased.When the soil is relatively dry before freezing,proper irrigation can be considered to keep the initial water content above 20%.