Study On The Effect Mechanisms Of Soil Aggregate Turnover On Cadmium Retention Behaviors Under Freeze-thaw Actions

The quality of soil is closely related to the health of residents.Because of the rapid developments of urbanization and industrialization,the gradually accumulation of cadmium in the agricultural soil of China has widely occurred.Freeze-thaw cycles are widespread in mid-high latitude and high-altitude regions of China,generally occurring in seasonal permafrost layers in autumn and spring.During the soil freeze-thaw process,phase changes of soil moisture and accompanying phenomena lead to the variations in soil properties,which in turn affect the migration and transformation behavior of heavy metals in soil.Soil is heterogeneous and composed of soil aggregates with different sizes.Aggregates serve not only as the structural units that accumulate and hide pollutants in soil but also as the smallest units that protect soil from erosion.Freeze-thaw cycles primarily affect the migration and transformation behavior of pollutants in soil by influencing soil aggregate turnover.This study reveals the impact mechanism of freeze-thaw cycles on the retention capacity of soil cadmium from the perspective of soil aggregate turnover.To reveal the relationship between soil aggregate turnover and heavy metal retention capacity under freeze-thaw cycles,three researchs were conducted in this thesis:（1）Firstly,we tracked the soil aggregate turnover processes under different freeze-thaw conditions（including soil moisture content,freezing temperature using rare earth oxide tracers to and freeze-thaw cycle number）.Then,we performed cadmium adsorption-desorption isotherm experiments using soil aggregate after the freeze-thaw treatments.Finally,we combined the soil results from soil aggregates turnover and adsorption-desorption isotherm experiments to reveal the turnover pathways of soil aggregates under different freeze-thaw conditions and to explore the impact of freeze-thaw cycles on the retention capacity of soil cadmium;（2）Based on the differences between natural spring freeze-thaw conditions and autumn freeze-thaw conditions,we conducted the freeze-thaw-leaching simulation experiments using the cadmium-contaminated soil columns.The aim of this part was to reveal the impact of seasonal freeze-thaw cycles on the migration capacity of soil cadmium;and（3）Based on field in-situ freeze-thaw experiments,we investigated the fate mechanisms of exogenous cadmium in soil.Finally,the relationships among freeze-thaw conditions,soil aggregate turnover,and soil cadmium retention characteristics were revealed.The main research conclusions are as follows:（1）The stability of soil aggregates was affected by freeze-thaw conditions.Under various freeze-thaw condition simulation studies,when the field water holding capacity was 20%,soil aggregate stability decreased with increasing freeze-thaw cycles,and the effect of freeze-thaw on turnover of soil aggregates was not obvious;when the field water holding capacity was 60%,the mean weight diameters of soil aggregates increased by 54.37%（freeze-thaw amplitude varied from-4°C to 5°C）and 48.10%（freeze-thaw amplitude varied from-18°C to 5°C）after one freeze-thaw cycle,and the direction of turnover of soil aggregates can be described as“breakdown progressively and aggregate directly”;when the field water holding capacity was 100%,the mean weight diameter of soil aggregates increases by 15.67%（freeze-thaw amplitude varied from-4°C to 5°C）and 5.41%（freeze-thaw amplitude varied from-18°C to 5°C）after one freeze-thaw cycle,and the direction of turnover of soil aggregate can be described as“breakdown progressively and aggregate directly”.In seasonal freeze-thaw experiments,freeze-thaw action increased the mean weight diameter of soil aggregates by 50.00%（autumn freeze-thaw）and 45.83%（spring freeze-thaw）for the 5-10 cm depth,and by 50.00%（autumn freeze-thaw）and 33.33%（spring freeze-thaw）for the10-15 cm depth,respectively.In the field freeze-thaw experiments,the mean weight diameter of soil aggregates increased after field autumn freeze-thaw,with non-frozen soil columns having a mean weight diameter of 246.20μm and frozen soil columns having a mean weight diameter of 286.20μm,respectively.（2）Freeze-thaw action changed the retention capacity of soil aggregates for cadmium.The increase in retention capacity under different freezing temperatures was:-4°C>-18°C;the turnover process caused by freeze-thaw was an important factor affecting the retention capacity of soil aggregates for cadmium,with larger turnover on soil aggregates leading to weaker retention capacity for cadmium.During the desorption process,freeze-thaw action reduced the specific adsorption ratio of soil aggregates,and the cadmium desorption ratio of<106μm soil aggregates was significantly higher than that of≥106μm soil aggregates.Field freeze-thaw actions reduced the aging degree of retained cadmium in soil;after 43 days of aging,the acid-soluble cadmium in the field freeze-thaw group was higher than that in the non-freeze-thaw group（17.49%>8.15%）.Freeze-thaw action can also change the mass loading of cadmium among different size soil aggregates.The distribution of cadmium in<106μm soil aggregates increased under the seasonal freeze-thaw action,with the increase in magnitude being:spring freeze-thaw>autumn freeze-thaw.（3）Freeze-thaw action affected the migration behavior of cadmium in soil.During the leaching process of field freeze-thaw soil,soil cadmium migration was dominated by the acid-soluble cadmium,accounting for 68.97%of total cadmium.Freeze-thaw action increased the migration of soil cadmium,with increases of 10.6%and 20.1%（infiltration at 5 cm and 10 cm）;freeze-thaw action reduced the content of acid-soluble cadmium in the soil cadmium migration process but promoted the release of soil colloids,thereby promoting the migration of reducible cadmium.The degree of colloid release from soil aggregates under spring freeze-thaw（1139.16 mg）was greater than that under the autumn freeze-thaw（989.76 mg）;autumn freeze-thaw increased the migration capacity of soil cadmium,but spring freeze-thaw reduced the migration capacity of soil cadmium:the total cadmium concentration in the filtrate of control group was 29.00 mg L^-1,the total cadmium concentration in the autumn freeze-thaw group filtrate was 36.11 mg L^-1,and the total cadmium concentration in the spring freeze-thaw group filtrate was 23.92 mg L^-1,respectively.By tracing soil aggregates with rare earth elements,we found that the soil mobile colloids were mainly from the<106μm soil aggregates,indicating that soil micro-aggregates were less stable during the leaching process and more prone to release colloids.This thesis reveals the mechanisms of cadmium retention in soil under freeze-thaw action from multiple perspectives.It not only expands and deepens the research ideas and contents of heavy metal migration and transformation behavior in soil of the seasonal freeze-thaw area,but also has important scientific significance and practical value for protecting black soil resources and ensuring the safety of agricultural products.