Heavy metal concentration levels in selected arable agricultural soils in South Western Nigeria

Abstract

The application of micronutrient supplements such as fertilizer, animal manure and sewage sludge in bid to enhance soil productivity and crop yield, poses the risk of heavy metals elevation and nutrients depletion in arable lands. Arable soils were collected from farmlands in use for the cultivation of food produce in selected farming communities in South Western Nigeria. The soils were air-dried, sieved to give <2 mm grain size and digested according to standard methods. The concentrations of Lead (Pb), Cadmium (Cd), Zinc (Zn) and Copper (Cu) were measured using flame atomic absorption spectrophotometer (FAAS). The concentration of Zn ranged from 12.98 to 38.94 µg/g, and was the highest amongst the measured heavy metals. This was followed by Cu, 6.17 to 20.87 µg/g and then Pb, 0.79 to 8.35 µg/g, while the concentration of Cd, 0.26 to 5.75 µg/g was the least. Geo-statistical analysis on the concentrations of the measured heavy metals revealed a significant (p < 0.05) spatial correlation (γ) with the soil pH and organic carbon (OC) (Pb, γ = 0.74; Cd, γ = 0.67; Zn γ = 0.57; Cu, γ = 0.52) and (Cd, γ = 0.79; Zn γ = 0.59; Cu γ = 0.51), respectively. The OC levels predispose soils to the retention of soil heavy metals and micro/macro-nutrients depending on the nature of OC-metal interaction. The concentrations of the heavy metals were below the European Economic Community (EEC) maximum allowable concentrations (MACs) in agricultural soils, except for Cd. About 56% of the farmland soils contained more than 3 mg/kg of Cd, while about 44% were within the 1 to 3 µg/g EEC MAC for Cd in agricultural soils. Although, the concentration levels of the measured heavy metals in the soils do not appear to be of serious concern, the study results provides evidence of gradual accumulation of Cd in farmlands