The fate and transformations of zinc added to soils

Abstract

A new sequential extraction procedure to remove specifically adsorbed forms of trace metals and easily reducible manganese (Mn) oxide fractions was used to study the fate and transformations of zinc (Zn) added to soils. Most of the endogenous Zn in field soils (75–87%) was found to exist in a residual fraction which is considered to be silicates, while the Zn added as a fertiliser in the field soils was found predominantly in an EDTA-extractable fraction and in association with iron (aluminium) [Fe (Al)] and Mn oxides. The Zn recently added to soils was found to be more in the reactive forms (water-soluble plus exchangeable and EDTA-extractable Zn) than the Zn added to field soils in association with long-term Zn application. With time, the EDTA-extractable Zn transformed into the unreactive forms (Zn associated with Fe (Al) and Mn oxides). The processes could be described by a diffusion equation. The apparent diffusion rate coefficients were found to be in the order of 10^–10–10^–11/s. The diffusion activation energy (E_a) was found to be 67 kJ/mol. The diffusion of Zn cations into microporous solids is probably a rate-limiting process. The transformation of reactive Zn into unreactive Zn was enhanced by elevated temperatures and by drying and rewetting. The drying and rewetting effect at relatively high temperature may be important in the processes which lead to decreases in the availability of Zn to plants.