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 determ ined by their partitioning between solution and solid-phase and their furth er redistribution among solid-phase component. The wetting-drying moisture regime is one of the most important factors in controlling the physical, ch emical, and biological properties of irrigated soils. Solid-phase redistrib ution 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 y ear. The heavy metals were fractionated into six operationally defined frac tions. During the long-term process in the two soils, Cr was transferred fr om the carbonate fraction (CARB) into the organic matter fraction (OM), and Cu, Ni, and Zn moved from the exchangeable (EXC) and CARE fractions into t he reducible oxide (RO), OM, easily reducible oxide (ERO), and residual fra ctions (RES) with time. The soil moisture regime strongly affects metal red istribution. Compared with a field capacity regime, soils at the wetting-dr ying cycle and saturated paste regimes had higher metal reactivity, resulti ng in the more complete movement of metals toward stable fractions, especia lly in the loessial soil and for Ni, Zn, and Cu.