Distribution characteristics of heavy metal(loid)s in aggregates of different size fractions along contaminated paddy soil profile

Soil aggregates exert a significant influence on the retention and availability of heavy metal(loid)s in soil. In this study, the concentration distribution and chemical forms of heavy metals (Cu, Zn, Cd, Pb, and Hg) and a metalloid (As) in different aggregate-sized fractions (2-0.25, 0.25-0.05, 0.05-0.002, and < 0.002 mm) along the profile (0-1, 1-5, 5-15, and 15-25 cm) of a contaminated paddy field were investigated. The results showed that the values of pH, free Fe oxides (Fe-d), bulk density, and catalase activity gradually increased, whereas the soil organic matter (SOM), cation exchange capacity (CEC), electrical conductivity (EC), microbial biomass carbon (MBC), and urease activity decreased with depth. Long-term heavy metal pollution might impact the catalase activity but showed no obvious influence on the urease activity. Additionally, there was a notable difference in physicochemical properties among the soil aggregates of various particle sizes. The 2-0.25-mm fraction aggregates contained more organic matter, whereas the highest values of CEC and Fe-d were observed in the < 0.002-mm fraction. The concentrations of all six heavy metals/metalloid decreased with depth. In different layers, Cu and Cd showed the highest concentrations in the 2-0.25 mm-fraction, followed by the < 0.002-mm fraction, whereas the highest concentrations of Zn, Pb, and As appeared in the < 0.002-mm fraction. No obvious distribution regularity was observed for Hg among the aggregates. All of the metal(loid)s had lower activity in the deeper soil layers, except for Hg. Furthermore, Cu and Cd displayed more stable forms in the < 0.002-mm fraction aggregates.