ABSTRACT

The study was conducted to assess the potential risks of heavy metal (Cu, Cd, Pb, Fe and Mn) toxicity in soils and plants associated with copper fungicides application in cocoa plantations at Ajata Ibeku and Itunta Ibere in Abia State, Nigeria. Soil and plant samples (Theobroma cacao, Chromolaena odorata, Costus afer, Musa species, and Centrosema pubescens) were collected in a 2 × 3 factorial experiments fitted into randomized complete block design. Standard approved procedures in line with the Association of Applied Chemist (AOAC) as well as the American Society for Testing and Methods (ASTM) were applied in the determination of heavy metal (Cu, Cd, Pb, Fe and Mn). Also, standard procedures for the sequential extraction were used to fractionate the metals into operationally defined forms.  The results of the study showed that the tested copper fungicides (Kocide 101, Nordox, Ridomil, and Mackecknie) contained varying amounts of Cu, Cd, Pb, Fe, and Mn. The order of magnitude of total and fractionated elements (Cu, Cd, Pb, Fe, and Mn) in the studied cocoa plantation soils were as follows: 50 > 25 > 10 years. The order of magnitude of metal forms were Cu: Mn-Fe oxide > residual > water-soluble > organically bound > exchangeable complex; Cd: residual > water-soluble > Mn-Fe oxide > exchangeable complex > organically bound; Pb: exchangeable complex >Mn-Fe oxide > water-soluble > organically bound > residual: Fe; Mn-Fe oxide > residual > exchangeable complex > organically bound > water-soluble; and Mn: residual > Mn-Fe oxide > exchangeable complex > water-soluble > organically bound. Total soil Cu at Ajata Ibeku plantation (40.10, 33.70 and 17.20 mg/kg respectively for 50-, 25-, and 10-years old plantations) were significantly higher at p ≥ 0.05 than those of Itunta Ibere with Cu levels of 18.80, 15.80, and 7.90 mg/kg for plantation ages of 50, 25 and 10 years respectively. Cu content of consumable plant tissues was within the safe limit (40 mg/kg) given by WHO/FAO. Cd contents of plant tissues of 1.89 – 42.28 mg/kg were above the European Union and World Health Organization safe limit of 0.20 mg/kg Cd. Cd and Pb were not detected in Musa tissues at the study locations, and Pb was below the detectable limit in cocoa bean at Itunta Ibere. These data were below the European Commission Codex and WHO/FAO permissible limits. Iron was predominantly high in plant tissues than other elements with the highest Fe concentrations of 152.01 to 770.96 and 303.87 to 1,006.39 mg/kg in Centrosema pubescens shoots and roots respectively. Fe contents of cocoa beans and Musa fruit were 206.95 – 693.51 and 15.48 – 41.52 mg/kg respectively. Cu had the highest soil to plant transfer factor (TF), Fe and Mn were relatively high while Pb had the least value. Cd and Pb recorded zero transfer factors for tissues of Musa species. Pollution assessment showed that metals pollution index order was Pb > Cd > Cu >Mn> Fe at Ajata Ibeku and Pb > Mn > Cd > Cu > Fe at Itunta Ibere; geo-accumulation index Pb> Cd > Cu > Mn > Fe at Ajata Ibeku and Pb > Mn > Cd > Cu > Fe at Itunta Ibere; and bioaccumulation index Pb > Cd > Mn > Cu > Fe both at the study locations. The results of the study showed that Cd and Pb, and Mn to a large extend present serious ecological concern in the study areas.