ABSTRACT

In this study, the seasonal variations of heavy metals concentration in the soil and plant parts at Uyo mechanic village, Uyo, Nigeria, were investigated during the two major seasons ofNigeria. Sampling soil depths were 0-15 cm, 15 - 30 cm and 30 - 45cm which represent the main feeding zones ofthe plant roots. Soil and Plants samples collected from two mechanic workshops were treated and digested using standard wet digestion methods. The heavy metals investigated were: Cadmium (Cd), Copper (Cu), Iron (Fe), Lead (Pb) and Zinc (Zn) in the three soil depths and three plant parts namely; the leaves, stem and roots in Amaranthus spinosus, Pennisetum purpureum and Aspilia africana. The seasonal variations of heavy metals concentration in the soil samples and three plant parts of Amaranthus spinosus, Pennisetum purpurseum and Aspilia africana in the mechanic village and control site were determined using Atomic Absorption Spectrophotometer (AAS). The result of the analysis carried out showed that the levels of heavy metals were generally higher at the dry season than rainy season. The concentration of heavy metals in the mechanic village soil varies at different depths in the two seasons, the highest value of each ofthe heavy metals were observed at the depth of 0-15cm(topsoil) while the lowest value of each of the heavy metals were observed at the depth of 30- 45cm(sub-sub soil). The leaves of Amaranthus spinosus had the highest mean value for Fe(l 16.4mg/kg) in the three plants species while the stem of Pennisetum purpureum had the highest mean value for Fe(96.1mg/kg), in the three plants species and the root of Aspilia africana had the highest mean value for Fe(129.1mg/kg) in the three plants species in the dry season when compared to that of rainy season. The leaves of Amaranthus spinosus had the highest mean value for Zn(55.71mg/kg) in the three plants species while the stem of Amaranthus spinosus had the highest mean value for Zn(51.96mg/kg) in the three plant species and the root of Pennisetum purpureum had the highest mean value for Zn(59.73mg/kg), in the three plants species in the dry season when compared to that of rainy season, the leaves of Aspilia africana had the highest mean value for Pb(20.35mg/kg) in the three plants species while the stem of Aspilia africana had the highest mean value for Pb(18.67mg/kg) in the three plants species and the root of Aspilia Africana had the highest mean value for Pb(21.3mg/kg) in the three plants species in the dry season when compared to that of rainy season. The leaves ofAmaranthus spinosus had the highest mean value for Cu(9.8mg/kg) in the three plants species while the stem ofAspilia africana had the highest mean value for Cu(9.3mg/kg) in the three plants species and the root of Amaranthus spinosus had the highest mean value for Cu(9.92mg/kg) in the three plants species for dry season when compared to that of rainy season. The leaves of Amaranthus spinosus had the highest mean value for Cd(2.4mg/kg) in the three plants species while the stem ofAmaranthus spinosus had the highest mean value for Cd(2.05mg/kg) in the three plants species and the root of Amaranthus spinosus had the highest mean value for Cd(2.25mg/kg) in the three plants species for dry season when compared to that of rainy season. In general, the levels of heavy metals in the soil and plant parts were lower in the rainy season than in the dry season due to increased aeration and dilution from rainfall. Heavy Metals present in the soils depths and plant parts for Cd, Cu, Fe, Pb and Zn, varied markedly due to different anthropogenic sources of these metals coupled with seasonal effects. It was found that the bioavailability of heavy metals at different soil depths and plant parts changes with respect to season, plant species and the type ofmetal. Results also show that the mean concentrations ofthe heavy metals decreased with soil depths.