Influence of dissolved organic carbon and initial moisture on zinc sorption by two arid soils

Sorption of added zinc to irrigated soils in al id regions is an important process that may control the availability of zinc to growing plants. Two so il surface samples varying in clay, organic matter, and calcium carbonate c ontent were selected from central and southwestern regions of Saudi Arabia and prepared in order to give different initial moisture contents ranging f rom air dried to 100% of field capacity. The sorption experiment was conduc ted using Zn concentrations ranging from 5 to 25 mg L-1, prepared from ZnSO 4 either in distilled water or in solutions containing 75 mg L-1 dissolved organic carbon (DOC). Results indicate that the amount of Zn sorbed in the presence of DOC was relatively high compared with the absence of DOC and Zn retention was strongly affected by the initial soil moisture content. Also , equilibrium Zn concentrations were quite low, while Zn retentions were hi gh in all treatments. Data of Zn sorption were described by the Freundlich isotherm, and two linear portions were found in most cases. In the absence of DOC, retentions of added Zn were controlled by the available exchange si tes and/or the precipitation of Zn as sparingly soluble forms. Zn ions in t he presence of DOC were able to form soluble-Zn organic complexes that adso rb on the soil surfaces. The Extent of such behavior was related to the var iations in clay, organic matter, and calcium carbonate contents as well as the initial moisture of the soil. Results indicate that addition of DOC red uces the amount of extractable Zn from either soil Zn or the sorbed Zn by a mmonium bicarbonate diethylenetriamine pentaacetic acid (AB-DTPA). More tha n 80% of the sorbed Zn was extracted by AB-DTPA, and the percentage of extr acted/sorbed Zn decreased with the increase in sorbed Zn. The obtained resu lts give evidence that initial moisture content and addition of DOC reduce the extractability of applied inorganic Zn by AB-DTPA extract in arid soils .