典型区域大气沉降重金属的土壤环境行为及其对作物的富集效应

Atmospheric depositions of heavy metals are the important sources of heavy metals in the soil environment. Therefore, it is necessary to understand the heavy metal behaviors in the soil environment, bioaccumulation effects on crops and the relationship of role transformation with the original heavy metals in the soils. In this project, the around areas of a large non-ferrous metal smelter are selected as typical areas to study the dynamics of heavy metals in the environment. Soil exchanges from gradient areas of atmospheric depositions (high deposition area, low deposition area and background area) and the effect of crop bioaccumulation (“soil-crop system”) are designed. The transportation process of heavy metals on the interface of atmosphere-soil-biology is taken as the research object. Through the comparative study of the fractions of the newly-deposited and the original heavy metals in the soil environment, the variation law of heavy metals in the soils and the transportation and accumulation of the heavy metals in soil-crop system are studied. Additionally, the main-control factors and transformation rates of the newly-deposited heavy metals are determined in the soils. The current study is to reveal the contribution of atmospheric heavy metals accumulating in the “soil-crop system” and to explain the mechanism of heavy metals from atmospheric depositions influence on the interface of atmosphere-soil-biology. The project is a crossover study of atmospheric science and soil science. The results will clear the soil environmental effect of the heavy metal from atmospheric deposition, and provide scientific basis for the risk control of heavy metals and crop production for food security in the large areas of farmland in southern China, where the concentrations of heavy metals in both soils and crops exceed the environmental quality standard.

大气重金属沉降是土壤环境中重金属的重要来源，弄清这部分重金属在土壤中的环境行为、对作物富集效应以及与土壤原有重金属的角色转化关系十分必要。本项目选择大型有色金属冶炼厂周边作为典型区域，通过在大气重金属沉降通量梯度区域（高沉降区、低沉降区、背景区）设置土壤置换试验及生物富集效应（“土壤-作物系统”）试验，以重金属在大气-土壤-生物界面迁移过程为对象，对比研究土壤中大气沉降重金属和土壤原有重金属形态在土壤环境中的变化规律以及在“土壤-作物系统”中的迁移富集效应。明确大气沉降重金属在土壤中形态转化的主控因子及转化速率，揭示大气沉降重金属对“土壤-作物系统”重金属累积的贡献，解释大气沉降重金属在大气-土壤-生物界面迁移的影响机制。该项目是大气环境科学与土壤科学的交叉研究，研究结果将明确大气沉降重金属的土壤环境效应，为南方大面积土壤和作物重金属“双超标”农田风险管控和粮食安全生产提供理论依据。

我国是大气重金属排放大国，大气沉降已成为农田生态系统镉的重要来源，弄清这部分重金属在土壤中的环境行为、对典型作物的富集效应以及与土壤原有重金属的角色转化关系十分必要。本项目选取其中3个大气沉降梯度区（高沉降区，中沉降区、背景区）进行土壤置换实验，种植典型蔬菜（小白菜）和水稻，连续累积设置3年，分析大气“新”沉降重金属形态对土壤重金属形态的影响及在土壤中的老化模型，阐明大气“新”沉降重金属对典型农作物的富集贡献及效应。并取得了以下重要结果：（1）风频、距冶炼厂距离以及其比值是影响冶炼厂周边重金属沉降通量在空间分布特征的关键驱动因素，降雨是影响重金属在冶炼厂周围沉降通量在沉降时间上分布的重要影响因素；大气湿沉降溶解态重金属占80%以上，干沉降中酸溶态占总量的28-80%，说明大气沉降重金属输入到地表生态系统后具有较高的生物有效性。（2）大气重金属沉降显著增加土壤表层（0-2 cm）重金属总量、有效态含量以及有效态比例，但高活性的大气沉降重金属进入土壤后会被迅速老化，且成一阶指数衰减，老化速率与土壤阳离子交换容量和粘土、土壤有机质呈显著正相关。（3）在高沉降区，通过0.5年和1.5年沉降，Cu、Cd、Pb沉降总量只占土壤库存量的0.3% -16%和0.8%-32%，但在小白菜可食部分中贡献可达到36%-87%和36%-91%，且两年之间并没有显著差异，说明大气沉降对小白菜重金属含量的累积主要来自于植株生长期间的“新”沉降重金属，茎叶直接吸收大气重金属沉降是小白菜地上部分的重金属累积的主要途径，占总金属累积量的38-99%。（4）沉降区水稻重金属的富集不仅有来自土壤原有重金属通过根部迁移方式的贡献，也有来自大气沉降的贡献，置换到中、高沉降区的无污染土壤种植的水稻籽粒中Cu和Cd积累量来自大气沉降的贡献分别达到54.7-61.9%、41.3-59.1%和50.7-83.3%、60.7-83.3%。通过本项目的研究，揭示了大气沉降重金属在土壤中老化过程，弄清了大气沉降重金属对农作物的富集机制及贡献份额。