Leaching of Heavy Metals from Rice Fields with Different Irrigation Management

To reveal the impact of irrigation management on the release and leaching of soil metals (Cu, Zn, Pb, Cr, and Cd), deep percolation rate and metals contents in soil solutions were observed in rice fields with nonflooding controlled irrigation (NFI) and flooding irrigation (FI) treatments. The contents of Cu and Cr in the deep solutions were safe according to the environmental quality standard for groundwater, but contents of other metals might lead to groundwater contamination, especially for Cd. The release of metals in surface soil was increased for NFI because the wetting-drying cycles in NFI fields resulted in less reluctant and high decomposition and mineralization of soil organic matter in surface soil, and consequently enhanced the release of soil metals into solutions. Seasonal metals leaching losses in NFI fields were 44.9-53.8% lower than in FI, due to the large reduction in both deep seepage rates and metals concentrations in deep soil solutions. Higher release of metals in NFI surface soils might lead to higher bioavailability of micronutrients (Cu and Zn) to crops, but higher risks in toxic metals (Pb, Cr, and Cd) uptakes.