Kinetics of Ligand-Controlled Release of Zinc in Acid Sulfate Paddy Soils

Abstract

The elevated solubility of zinc (Zn) in acid sulfate paddy soils can limit rice production and pose a risk of environmental pollution. However, little attention has been paid to the ligand-controlled release of Zn in these soils. Here we quantified the rate of ligand-controlled Zn release in Thai acid sulfate paddy soils, using ethylenediaminetetraacetic acid as the extractant. Sequential extractions were performed to obtain quantitative information on Zn fractions contributing to the ligand-controlled mechanisms. The amount of Zn released within 192 h varied significantly (8–43 mg kg−1), which corresponded to 12%–40% of the total soil Zn, indicating that Zn solubility in most soils was relatively low and that Zn mainly occurred as residual phases. The kinetics of Zn release was well described by the power function model (r = 0.65–0.99, median = 0.87). The magnitude of initial Zn release (coefficient a) was significantly (P < 0.05) related to the aqua regia-soluble Zn. Easily mobile Zn, organically bound Zn, and Zn associated with Fe and Mn oxides also contributed to the ligand-controlled release mechanisms to various degrees. Our results provide a systematic understanding of Zn fractions and release from acid sulfate paddy soils, the dynamics of which have a significant influence on the availability, phytoextraction, and mobility of Zn in terrestrial and engineered environments.