Assessment of the effects of cadmium and lead on pH and cation exchange capacity of soil under different plant canopy in the tropical wet and dry climate

The effects of heavy metals pollution on agricultural produce can not be over emphasize. To estimate the effect of heavy metal on pH and Cation Exchange Capacities of soil on incubation, relationships between availability of metals in soil after contamination were investigated for a range of soils and metals. Three concentrations (0 mg/kg, 2 mg/kg and 5 mg/kg) of lead and cadmium were added as nitrate solution as single and combine treatments to six soil samples under different plant canopy. The soils metal were incubated at field capacity for 8 weeks under 25 0 C. The exchangeable bases in soils were determined in IM ammonium acetate (pH 7.0) extract by FAAS and pH by pH meter with a combination electrode. The treatment is arranged in randomised complete design each in triplicates. The pH decreased from 7.02 to 6.70 and 6.63.  Exchange Capacities decreased from range of of 6.62 to range of 4.71 and 3.10 C mol kg 1 under single and combined treatments respectively for the six locations. These results enable us to understand natural attenuation of metal contamination and also to assess the risk of soil contamination by determining effects of metals reactions with CEC and pH in 8 weeks incubated soils after artificial contamination. (c) JASEM http:/dx.doi.org/10.4314/jasem.v19i4.13 KEYWORD: heavy metals, Incubation process, pH and CEC  Soil contamination with heavy metals has been a big issue worldwide (Alloway 1995; Nriagu 1996) It has been carried out intensively to investigate contaminated soils for evaluating the level of contamination in relation to human health and/or environmental conservation. It is also very important to investigate the process of soil contamination itself with special attention to available fractions because it directly relates to the management and prevention of soil contamination. Food chain contamination by heavy metals has become a burning issue in recent years because of their potential accumulation in biosystem through contaminated water, soil and air (Alloway 1995; Nriagu 1996) Heavy metals mobility is closely related to metal solubility, which is further regulated by adsorption, precipitation and ion exchange reactions in soils. It is very important to investigate the process of soil contamination itself with special attention to available fractions because it directly relates to the management and prevention of soil contamination. However, such predictions under field conditions suffer from much uncertainty. This is partially due to the difficulty in assessing the effects of dynamic soil solution chemistry on trace metal speciation (Arowolo et al. 1999) However, changes in soil solution chemistry, such as pH, redox potential and ionic strength, may also significantly shift the retention processes of heavy metals by soils (Gerringa et al. 2001) These effects may be further complicated by ligand competition from other cations (Amrhein et al, 1994; Norrstrom and Jacks, 1998) Soil redox status varies temporally and spatially. In a surface soil, it is influenced by rainfall, bioactivity, and in particular changes in land use. The importance of maintaining an adequate soil nutrition supply to crops in order to maximize agricultural output has long been recognized and the routine application of plant residue to agricultural land have become an integral part of agriculture in developing countries (Kalavrouziotis et al. 2009) However, continued long term application of plant residue can lead to accumulation of organic nutrients in surface horizons which will likely influence of soil properties on kinetics of heavy metals release under different land use. Therefore, a better understanding of the the effect of incubation period of heavy metals reaction with the soil properties like pH and CEC seem to be an important issue of present day research on risk assessments.  The study was conducted at the Federal University of Agriculture along Alabata road, Abeokuta (Latitude 7 o 15'N, Longitude 3 o 25'E) within a forest savanna Assessment of the effects of cadmium and lead on pH and cation