| As one of the main vegetable supply bases in the city,the soil quality of peri-urban vegetable area is closely related to the health of residents.Due to the special location,heavy metal pollution exists in the peri-urban vegetable soil.At middle and high latitude areas,due to the periodic change of environmental conditions such as temperature,soil has a seasonal freezing and thawing cycle（FTC）process.When exogenous heavy metals enter the soil,they are mainly accumulated on the surface where the freezing-thawing process also happens.Freeze-thaw cycle changes the physical and chemical properties of soil through changing soil water and heat,and has a further effect on the content,chemical form and migration of heavy metals in soil.This thesis aims to provide a better understanding of the effect mechanisms of how freeze-thaw process on the Cd migration and transformation behavior in peri-urban vegetable fields in northeast China.The peri-urban vegetable area was used as the research subject in this thesis.Our research studied the Cd adsorption and desorption characteristics of FTC treated soils and the distribution of Cd in contaminated soil aggregates by conducting indoor simulation experiments of different FTC treatments（FTC amplitude,FTC rate,FTC length and FTC frequency）.Besides,our research investigated on the dynamics of soil Cd content and chemical form as well as microbial processes in different freezing-thawing periods.The effects of FTC on the migration and transformation of Cd in peri-urban vegetable fields and related mechanisms were investigated.The main research results are as follows:（1）Soil adsorption capacity of Cd was higher than that of the unfrozen control after freezing-thawing treatment at different rates,length and cycles.In the study of FTC amplitude,the adsorption amount of Cd was the largest in the soil treated by freezing temperature of-2℃,followed by the soil treated by-15℃and-5℃.The adsorption amount of soil Cd decreased with the increase of FTC rate,FTC length and FTC frequency.In the Langmuir model,the adsorption capacity of Cd increased with the increase of FTC frequency,FTC length and FTC rate,while in the Freundlich model,the adsorption capacity of soil Cd decreased with the increase of FTC amplitude.The proportion of soil specific adsorption increased with the increase of FTC rate,FTC length and FTC frequency.In the FTC amplitude experiment,the sequence of soil specific adsorption proportion was:-2℃>-15℃>-5℃.（2）The Cd mass loading of>2 000μm aggregates decreased after freeze-thaw treatment,which decreased with the increase of FTC frequency,FTC amplitude and the decrease of FTC length.In the study of FTC rate,the largest reduction was caused by the treatment of 20℃h-1,followed by the treatment of 1℃h-1 and 3℃h-1.The Cd mass loading of<250μm aggregates decreased after freeze-thaw treatment,which decreased with the increase of FTC amplitude,FTC length and FTC frequency.In bulk soil,the acid-extractable Cd content decreased after freezing-thawing treatment,which decreased with the increase of FTC rate and FTC frequency.However,in the FTC amplitude and FTC length experiment,the reduction of-10℃and 12 h treatments were significant.The content of reducible Cd increased with the increase of FTC amplitude,FTC length and FTC rate,but decreased with the increase of FTC frequency.The content of residual Cd increased with the increase of FTC amplitude and FTC length.In the aggregates of various particle sizes,the content of acid-extractable Cd in>2 000μm aggregates decreased with the increase of FTC length.The acid-extractable Cd in size of 2 000-1 000μm particle decreased with the increase of FTC amplitude,FTC rate and FTC length,and the residual Cd increased with the increase of FTC amplitude.The acid-extractable Cd in 1 000-500μm particle decreased with the increase of FTC length.In<250μm particle,the acid-extractable Cd content decreased with the increase of FTC rate and FTC length,while the content of reducible Cd increased with the increase of FTC rate,and the residual Cd increased with the increase of FTC amplitude.The correlation between Cd mass loading and organic carbon mass loading in different freezing-thawing treatments were very significant（r=0.927-0.985,p<0.01）,and the correlation coefficient decreased with the decrease of particle size.（3）The Cd content in the 5-10 cm soil layer increased by 0.31 mg kg-1,0.18 mg kg-1 and 0.08 mg kg-1 in the Autumn FTC period of 2018,Spring FTC period of 2019and Autumn FTC period of 2019,respectively.The Cd contents in 10-15 cm layer increased by 0.05 mg kg-1 and 0.13 mg kg-1 in Spring and Autumn FTC periods of2019,respectively.The Cd in lab soil gradually migrated from the 0-5 cm layer to the lower layer with the increase of aging time,and the Cd content in the 5-10 cm layer and the 10-15 cm layer increased by 0.20 mg kg-1 and 0.18 mg kg-1,respectively.The aging rates of acid-extractable Cd in the 0-5 cm layer of soil were different:the acid-extractable Cd in the field decreased from the initial value to 0.91 mg kg-1 in the0-17 days of rapid aging,and slowly decreased to 0.85 mg kg-1 after 375 days of aging.The acid-extractable Cd in lab soil decreased from initial value to 0.75 mg kg-1in 0-60 days of rapid aging,and slowly decreased to 0.73 mg kg-1 after 375 days of aging.Different chemical form of Cd content in 5-10 cm layer increased after 400days aging.The acid-extractable Cd content were increased by 0.14 mg kg-1（field）and 0.01 mg kg-1（lab）,and the reducible Cd content increased by 0.47 mg kg-1（field）and 0.19 mg kg-1（lab）,respectively.The oxidized Cd content increased 0.12 mg kg-1（field）and 0.16 mg kg-1（lab）,respectively.Soil urease activity and acid-extractable Cd content gradually changed from positive correlation to negative correlation with aging time（p<0.05）,while soil acid phosphatase activity and reducible Cd content had positive correlation（p<0.05）.There was a positive correlation between the two enzyme activities in soil（p<0.05）,and the correlation increased with the increase of soil depth and aging time. |
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