东北典型城郊菜地土壤冻融过程中镉的迁移转化行为研究

Seasonal freeze-thaw action is a typical climate characteristic of the mid- and high-latitude regions, which can have important influences on the environmental chemical behaviors of heavy metals in soils. However, the related researches are mainly based on the indoor simulation with extreme freeze-thaw conditions, and the conclusions are different from the results under actual field situation. For the status quo of Cd pollution in the typical peri-urban vegetable soil of northeast China, based on the in-situ soil freeze-thaw experiments, this project intends to: illustrate the influent mechanisms of freeze-thaw action on the aging behavior of exogenous Cd in soil, by means of monitoring the dynamic changes of Cd content and chemical forms in soil/solution and soil microbial activity, etc.; discuss the release kinetics and its key influencing factors of Cd from contaminated soil under the action of freezing and thawing, by means of analyzing the physico-chemical properties and Cd chemical forms of different sizes of soil aggregates; prove up the influences of freeze-thaw action on the migration behaviors of Cd in soil, by means of studying the colloid-facilitated transport process of Cd and its flux; reveal the effects of freeze-thaw action on the bioavailability of soil Cd, by means of studying the dynamics of Cd uptake and accumulation in vegetable and its physiological response mechanisms. The implementation of this project will help to enrich the research contents of migration and transformation processes of soil heavy metals, and to provide theoretical foundations for predicting the heavy metal bioavailability in the peri-urban vegetable soil of northeast China during the freeze-thaw season and ensuring the safety of regional agricultural products.

季节性冻融作为中高纬度地区的典型气候特征，显著影响着土壤中重金属的环境化学行为。然而，相关研究多基于极端冻融条件的室内模拟，其结论与田间实际情况存在差异。针对我国东北典型城郊菜地存在Cd污染的现状，本项目拟基于田间原位冻融试验，通过对土壤及溶液中Cd含量、化学形态和土壤微生物活性等的动态监测，阐明冻融作用对土壤中外源Cd老化行为的影响机制；通过对不同粒级土壤团聚体理化性质和Cd化学形态的分析，探讨冻融过程中污染土壤Cd的释放动力学及其主要影响因素；通过对土壤胶体携带Cd在土壤中运移过程及其通量的研究，探明冻融作用对污染土壤中Cd迁移行为的影响；通过对蔬菜体内Cd吸收、积累动力学过程其生理响应机制的研究，揭示冻融作用对土壤中Cd生物有效性的影响机制。本项目的开展将有助于丰富土壤中重金属迁移转化过程的研究内容，并为预测冻融季东北城郊菜地土壤重金属的生物有效性和保证区域农产品安全提供理论依据。

中高纬度地区的季节性冻融过程影响着土壤中重金属的迁移转化行为和生物有效性，但相关研究多基于极端冻融条件的室内模拟实验，其结论与田间实际情况存在差异。本项目以吉林省长春市城郊菜地土壤为研究对象，通过室内模拟实验和田间原位实验，探讨了冻融作用对土壤中镉迁移转化行为的影响。结果表明：（1）田间实际冻融条件下土壤对镉的吸附和解吸能力与极端冻融条件下的研究结果存在显著差异，冻结时长、冻融速率和次数的增加能够增加土壤对镉的最大吸附量，冻融幅度的增加减小了土壤对镉的最大吸附量，土壤对镉的专性吸附比例随着冻融速率、冻结时长、冻融次数和冻融幅度的增加而增加。（2）相较于室内老化条件，田间条件下外源镉在土壤中的老化速率慢、土壤中镉的向下迁移量大、土壤中镉的动力学释放速率快且释放量大、土壤中镉的生物有效性高。（3）冻融作用导致土壤大团聚体和微团聚体镉负载量的降低，冻融作用促进全土和各粒径团聚体中可交换态镉向可还原态和残渣态转化且转化量随冻融幅度和冻结时长的增加而增大。（4）相较于秋季冻融，春季冻融增加了土壤胶体的淋出量和胶体结合态镉的比例、降低了土壤团聚体稳定性、降低了土壤大团聚体中镉负载量并增加了<0.25 mm团聚体中镉负载量。（5）冻融作用显著增加镉在菠菜根部的累积并降低菠菜叶部镉累积量，冻融作用增加了菠菜叶内移动活性较强的乙醇提取态镉含量并降低了移动活性弱的乙酸和盐酸提取态镉含量。本项目研究成果不仅可以丰富和完善土壤中重金属迁移转化行为的研究内容，还可以指导季节性冻融区城郊菜地的蔬菜生产实践。