Effects of human enclosure and farming activities on heavy metals in sediments/soils of the coastal reclamation areas in the Yangtze Estuary

Purpose The enclosure and reclamation of wetland sediments altered soil properties and heavy metal levels in coastal areas. This study focuses on the effects of anthropogenic enclosure and reclamation activities on the physicochemical properties and the biotoxicity of heavy metals in sediments/soils in the Yangtze Estuary. And, the purpose was to explore the factors affecting heavy metals in different reclamation stages and to provide reasonable suggestions for stabilizing heavy metals in farmland transformed from enclosed land. Materials and methods According to the reclamation situation, sampling areas were divided into natural wetland (NW), the enclosure area (EA) separated from NW by a 2-m-high embankment, the reclamation area with short-term (3 years) rice cultivation history (RAS), and the reclamation area with long-term (10 years) rice cultivation history (RAL). A total of 360 heavy metal concentration data were used to assess heavy metal levels. The acid volatile sulfide (AVS) and the simultaneously extracted metals (SEM) were analyzed to compare the toxicity of heavy metals in sediments/soils at different reclamation stages. The ratios of SEM-Hg/THg (total mercury) and SEM-As/TAs (total arsenic) were used to assess the effect of Fe/S on the stability of Hg and As. Correlation matrix analysis and multiple linear regression models were performed to investigate the correlations between heavy metals and soil physicochemical properties. Results and discussion The cycle of leaching and drying in EA could effectively remove heavy metals for further use. And, there were no significant variations in sediment/soil properties and heavy metals between EA and RAS, suggesting a weak influence of short-term farming activities. However, due to long-term human cultivation activities and the influence of soil total organic carbon (TOC), electrical conductivity (EC), clay size fraction content, and redox conditions, the concentrations of Zn (95.27 mg·kg −1 ), Cu (26.42 mg·kg −1 ), Ni (39.93 mg·kg −1 ), Fe (6.34%), and As (9.80 mg·kg −1 ) significantly increased in RAL compared with NW, EA, and RAS. In NW and RAL, insoluble sulfide had a strong immobilization effect on As, while Fe(III) (oxyhydr)oxides were responsible for the stabilization of Hg in EA and RAS. Fe/S speciation, soil moisture content (MC), and TOC were key factors controlling the mobility and biotoxicity of Hg and As. And, these effects mainly occurred in 0–15 cm of surface soils. Conclusion Paddy or