Accumulation Pathway of Cd, Pb, and Zn in Chinese Cabbage under the Condition of Exogenous Pollution Superposition in High Geological Background Area

Abstract

Northwest Guizhou is a karst area with a high geological background. Affected by historical soil zinc smelting, the heavy metal content of atmospheric dust in the region is high, and soil pollution is severe. In order to explore the accumulation pathway of heavy metals in leafy vegetables, Chinese cabbage was used as the test crop, and the geological high background soil and zinc smelting-contaminated soil with the same contents of Cd, Pb, and Zn were selected. A pot experiment was carried out in the polluted area of zinc smelting and the non-polluting control area. The heavy metal content, enrichment coefficient (BCF), and transport coefficient (TF) of Chinese cabbage were studied under open-air, plastic mulching film, and greenhouse cultivation conditions. The results showed that the contents of Cd, Pb, and Zn in Chinese cabbage in the polluted area and the control area were 0.10-1.01 and 0.10-0.91 mg·kg-1, 0.31-0.62 and 0.23-0.37 mg·kg-1, and 7.50-32.74 and 4.88-21.79 mg·kg-1, respectively. Overall, the contents of heavy metals in the polluted area were relatively high. The contents of Cd and Pb in Chinese cabbage planted in soil with a high geological background met the requirements of the national food safety standard limits. Affected by atmospheric deposition, the contents of Pb and Zn in Chinese cabbage in the polluted area were significantly higher than that in the control area, and the difference in Cd was insignificant. The proportions of weak acid-soluble Cd, Pb, and Zn in the contaminated soil were 48%, 3.0%, and 16%, respectively, which were 3.15, 1.01, and 1.57 times higher than those in the control soil with a high geological background. Affected by the activity of heavy metals, the contents of Cd and Zn in Chinese cabbage planted in the contaminated soil exceeded the national standard and were significantly higher than those in the control soil. The root-soil BCF of Cd, Pb, and Zn in polluted soil was significantly higher than that in the control soil, and the BCF of Cd and Zn was higher than that of Pb. The TF aboveground root Cd and Zn in Chinese cabbage was significantly higher than in the control soil, whereas the TF aboveground root Pb in the polluted area was significantly higher than that in the control area. The Pb content of Chinese cabbage in the two study areas showed open field>plastic mulching film>greenhouse cultivation. In conclusion, the content of Cd and Zn in Chinese cabbage was greatly affected by the activity of heavy metals in soil, and the main accumulation pathway was root absorption and transportation. In addition to root absorption, atmospheric deposition was an important accumulation pathway of Pb. Therefore, in areas with high geological backgrounds, attention should be paid to controlling the exposure risk of Cd and Zn in leafy vegetables planted on exogenously polluted soils. Additionally, greenhouse cultivation could effectively reduce the accumulation of Pb.