Effect of controlled-release urea on heavy metal mobility in a multimetal-contaminated soil

Controlled-release N fertilizers can affect the availability of heavy metals in the contaminated paddy soil. A soil incubation experiment was conducted to investigate the effects of prilled urea (PU), S-coated urea (SCU), and polymer-coated urea (PCU) on the solubility and availability of heavy metals Cd, Pb, Cu, and Zn in a multimetal-contaminated soil. The results showed that the application of different coated urea significantly affected the solubility and availability of heavy metals. At 5 d of incubation, the application of PU, SCU, and PCU had significantly decreased the concentrations of water-soluble and available Cd, Pb, Cu, and Zn, when compared with the control. At 60 d of incubation, the depletory effects of PU on water-soluble and available heavy metals had reduced, and the initial decrease in the concentrations of water-soluble Cd, Pb, Cu, and Zn caused by SCU had changed to an increase. The concentrations of water-soluble Pb, Cu, and Zn in the SCU-treated soil were higher than those in the control. Application with PCU led to a higher water-soluble Cu than that in the control, while the available Cd, Pb, and Zn were lower than those in the control. The effect of different coated urea was much stronger on the water solubility of the heavy metals than on their availability. The effects of controlled-release urea on the transformation of heavy metals resulted in changes in the concentrations of NH₄⁺, water-soluble SO₄^2-, and soil pH. The results further suggested that PCU could be used in dry farming operations in multimetal-contaminated acid soils.