Comparison of heavy metal speciation, transfer and their key influential factors in vegetable soils contaminated from industrial operation and organic fertilization

Purpose: In this study it is investigated whether typical pollution sources cause differences of heavy metal (Cd, Cu, Pb, and Zn) pollution in the soil-leafy vegetable systems. Materials and methods: Soil and leafy vegetable samples were collected in the fields contaminated by industrial operation sources (IOS) and organic fertilizer sources (OFS), respectively, in the Yangtze River Delta (YRD) areas. The differences of heavy metal pollution were compared in soil-vegetable systems contaminated by IOS and OFS (IOS- and OFS-systems). Those mainly included (1) Cd, Cu, Pb, and Zn contents in soils and vegetables; (2) heavy metal fractions distribution in soils and their connection with the heavy metal in leafy vegetables; (3) the key influential factor of heavy metal accumulation of leafy vegetable; and (4) the ecological risk of heavy metal pollution in soil-vegetable systems. Results and discussion: The mean pollution index (PI) of Cd, Pb, Cu, and Zn was 5.05, 1.89, 0.72, and 1.21 in IOS-soils (soils contaminated by IOS), and 2.51, 0.25, 0.64, and 0.71 in OFS-soils (soils contaminated by OFS), respectively. For PIS > 1, the percentages of Cd were 100% in both sources. The IOS caused more significant Cd and Pb pollution in vegetables, but OFS induced higher Cu and Zn accumulation. The Cd in soils had a relatively higher bioavailable fraction proportion (the sum of water-soluble, exchangeable and carbonate-bound fraction), mobility, and transferability in IOS-soils. Nevertheless, such indicators for Cu and Zn were higher in OFS-soils. Compared with IOS-soils, OFS-soils were characterized by a higher proportion of metals bound to humic acid and organic matter. Redundancy analysis (RDA) results showed that the accumulation of Cd, Pb, Cu, and Zn in IOS-vegetables (vegetables contaminated by IOS) was mainly controlled by F3-Cd, F5-Pb, F5-Cu, and F3-Zn in soils, respectively. Moreover, F1-Cd, F4-Pb, F2-Cu, and F2-Zn could be the dominant influential factors of Cd, Pb, Cu, and Zn absorption by OFS-vegetables (vegetables contaminated by OFS), respectively. Risk assessment results revealed that the IOS-systems had higher heavy metal pollution level than OFS-systems. Conclusions: The source of heavy metals is one of the main factors for the mobility and transfer of heavy metals in the soil–plant systems. There are obvious differences in pollution characteristics between IOS- and OFS-soils. Compared with OFS, IOS caused the more serious ecological risk of heavy metal pollution to the soil-vegetable systems.