LONG-TERM TRANSFORMATIONS AND REDISTRIBUTION OF POTENTIALLY TOXIC HEAVY-METALS IN ARID-ZONE SOILS INCUBATED .1. UNDER SATURATED CONDITIONS

Solid-phase transformations of Cd, Cu, Cr, Ni and Zn, added as soluble salts at several levels to two arid-zone soils, were studied over a p eriod of one year. The soils were maintained under a saturated-paste r egime and sampled periodically. A selective sequential dissolution pro cedure was employed to determine the changes in metal distribution amo ng six operationally defined solid-phase fractions. A function, U-ts w as introduced to measure the fractional attainment of equilibrium of t he soils following a peturbation. The direction and rate of redistribu tion of the added metals in the soils were affected by the nature of t he metal the soil properties and the metal loading level. Cd added to the soils was transferred from the exchangeable (EXC) into the carbona te (CARE) fraction. When soluble Cu, Cr, Ni and Zn were added at low l oading levels, metals were transferred from the reducible oxides(RO) b ound and easily reducible oxides (ERO) bound fractions and the EXC fra ction, into the CARE fraction. However, at the higher loading level, m etals were transferred from the EXC and CARE fractions into the organi c matter bound (OM), ERO and RO fractions. The U-ts function approache d lower values as incubation continued but remained removed from 1. Th e overall flux of metals among fractions was the combined result of th e readjustment of the metals in the native soil to changing conditions due to saturation, and the transfer of added soluble metals to the le ss labile fractions.