基于高密度采样的土壤重金属分布特征及迁移速率

Distribution and Migration of Heavy Metals in Soil Profiles by High-Resolution Sampling

重金属在土壤剖面中的垂直分布特征是土壤自身理化性质和外界条件影响下重金属迁移和积累的综合反映，也是了解土壤重金属污染程度和修复治理的基础,传统的用于农业土壤研究的层次采样技术和方法，只能粗略了解重金属在土壤层次中的分布状况，不足以确定污染物的垂直迁移距离和速度．本研究通过对钢铁厂周边3种不同利用类型土壤进行垂直方向的高密度采样分析，即在0～40cm深度内以2cm间隔，在40～80cm以5cm间隔，80cm以下以10cm间隔采样，对重金属元素Cu、Zn、Pb、Cr、Cd的剖面垂直分布进行了研究．结果表明，在几乎未受人为干扰的林地土壤中，可见重金属在土壤表层明显富集，证实其来源是大气沉降，其中Zn、Pb向下迁移的距离很小，Cu、Cd迁移稍快，Cr向下淋溶迁移的深度没有明显的界限；在受人为影响的旱地和水稻田中，重金属在耕层土壤中被人为扰动混合，但在耕层界面以下仍然有明显的向下迁移．研究表明，垂直方向高密度采样可以比较精确地理解重金属在土壤中的分布模式，通过重金属元素含量与稳定元素Ti含量的比值及其变化，还可以估计在特定时间内污染物的迁移距离和迁移速度，各种重金属具有不同的迁移能力，其相对顺序为Cd〉Cu〉Zn〉Pb．

The vertical distribution of heavy metals in soils profiles is a result of heavy metals accumulation and migration under combining influence of edaphic factors and environmental conditions. It＇s an important basis for evaluation of heavy metals pollution and remediation of contaminated soils. By traditional sampling methods, i.e. , soils were sampled according to pedogenetic horizons, only very general information about element migration can be learned. In the current study, three sites near a steel factory were selected to represent three types of land use, i.e. forest, dry land for vegetable cultivation and rice paddy field. Soils were sampled horizontally by high-resolution sampling method. In the top of 40 cm soils were sectioned in 2 cm intervals, then 5 em intervals in next 40 em, and 10 cm intervals in the last 20 em of profile. Total content of Cu, Zn, Pb, Cr and Cd were determined, and the vertical distribution of Cu, Zn, Pb, Cr, Cd in every profile was analyzed. The results indicated that enrichment of heavy metals appeared in the upper most layer of the natural forest soil that without any anthropic disturbance, and this phenomenon proved that heavy metals were coming from atmospheric deposition. We found that Cu, Zn and Pb moved downward in a short distance, Cd migrated relatively faster than Cu, Zn and Pb, while Cr had no recognizable location of migration front. In the soil profiles of dry land and paddy field, there were influences of agricultural practice, the distribution and movement of metals were thus different form those of the forest soil. In cultivated layer heavy metals were evenly distributed because soils in the upper layer were mixed by cultivation, however, bellow the cultivated layer obvious migration took place again. It is concluded that different heavy metals have different mobility and there is such a relative order： Cd〉Cu〉Zn〉Pb. The study shows that the distribution pattern can be obtained with the currently adopted hlgh-resolution sampling method, than traditional horizon sampling, and more precise migration distance and rates can be estimated.