Influence of Dissolved Organic Carbon and Initial Moisture on Zinc Sorption by Two Arid Soils

Abstract

Sorption of added zinc to irrigated soils in arid regions is an important process that may control the availability of zinc to growing plants . Two soil surface samples varying in clay , organic matter , and calcium carbonate content were selected from central and southwestern regions of Saudi Arabia and prepared in order to give different initial moisture contents ranging from air dried to 100 % of field capacity . The sorption experiment was conducted using Zn concentrations ranging from 5 to 25 mg L 1, prepared from ZnSO4 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 high 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 sites and / or the precipitation of Zn as sparingly soluble forms. Zn ions in the presence of DOC were able to form soluble - Zn organic complexes that adsorb on the soil surfaces . The extent of such behavior was related to the variations in clay , organic matter , and calcium carbonate contents as well as the initial moisture of the soil . Results indicate that addition of DOC reduces the amount of extractable Zn from either soil Zn or the sorbed Zn by ammonium bicarbonate diethylenetriamine pentaacetic acid ( AB DTPA ). More than 80 % of the sorbed Zn was extracted by AB-DTPA , and the percentage of extracted / sorbed Zn decreased with the increase in sorbed Zn . The obtained results give evidence that initial moisture content and addition of DOC reduce the extractability of applied inorganic Zn by AB-DTPA extract in arid soils.