Determination of bioavailable concentrations of copper and Zinc in farm soils in Kaduna metropolis, Nigeria

Abstract

This study assessed the mobility and bioavailability of Cu and Zn in soils of vegetable farms located within the city of Kaduna, Nigeria vis-à-vis the soil physicochemical properties.The city was divided into 20 zones of approximately equal areas. Soil samples were taken over a period of three months from each zone for analysis of soil physicochemical properties and Cu and Zn concentrations in sequential fraction of the soil sample. Composite samples of soil were collected from vegetable farms and gardens in each zone and also from two rural villages, about 30 km south-west of the city as control. Sequential fractionation of soil samples were carried out using modified Tessier method, followed by analysis of fractions for Cu and Zn concentrations using Atomic Absorption Spectrophotometer. Mean of total concentration(μg/g) and percentage bioavailability were Cu 51.2 ±27.7 (8.1%) and Zn 238 ±89 (23.0%), respectively. Correlation analysis showed that soil pH, % organic carbon, % clay, cation exchange capacity, and metal speciation in soil influence metal mobility and bioavailability in the city soil. The mean concentrations of Cu and Zn in soil in the study site were generally higher than those of the control site, revealing some levels of pollution of the city soil by these metals. Also, the mean concentrations of Zn in some zones were significantly higher (p < 0.05) than established world standards such as WHO/FAO maximum permissive limit of 300μg/g in agricultural soil. The high concentrations of Zn in the soil of the vegetable farms in some zones and the high bioavailability calls for concern as this could pose a potential risk to the consumers of the vegetables grown on these farms.

 Keywords: Bioavailability, Copper, Zinc, Soil, Vegetable Farms, Kaduna City, Nigeria