Redistribution of heavy metals in arid-zone soils under a wetting-drying cycle soil moisture regime

Bioavailability, toxicity, and mobility of heavy metals in soils are determined by their partitioning between solution and solid-phase and their further redistribution among solid-phase component. The wetting-drying moisture regime is one of the most important factors in controlling the physical, chemical, and biological properties of irrigated soils. Solid-phase redistribution of Cu, Cr, Ni, and Zn, added as soluble salts to two arid-zone soils incubated under a wetting-drying cycle moisture regime, was studied for 1 year. The heavy metals were fractionated into six operationally defined fractions. During the long-term process in the two soils, Cr was transferred from the carbonate fraction (CARB) into the organic matter fraction (OM), and Cu, Ni, and Zn moved from the exchangeable (EXC) and CARE fractions into the reducible oxide (RO), OM, easily reducible oxide (ERO), and residual fractions (RES) with time. The soil moisture regime strongly affects metal redistribution. Compared with a field capacity regime, soils at the wetting-drying cycle and saturated paste regimes had higher metal reactivity, resulting in the more complete movement of metals toward stable fractions, especially in the loessial soil and for Ni, Zn, and Cu.