Assessment of potentially toxic metals and physico-chemical properties of soils of a Sandstone landscape in Cross River State, Nigeria

Abstract

The presence of metals in concentrations that exceed safe thresholds have the capacity to endanger food production and human health security through bioaccumulation in animals and plants. The antagonistic effects of the potentially toxic metals (PTMs) may potentially cause a decline in soil fertility which eventually results in crop growth decline, impaired development and decreased productivity. This study seeks to evaluate the concentration of PTMs in soils of sandstone origin in a tropical rainforest in Cross River State, Nigeria. A profile pit was sunk in each of crest, middle slope and valley-bottom along a landscape in the Agoi Ibami-Mfamosing area. The landscape extended from the Agoi Ibami area to Mfamosing. Soil samples were collected and subjected to physico-chemical analyses and heavy metals extraction by aqua regia mixture of 3 parts of HCl and 1 part of HNO₃. The extract was analyzed using inductively coupled plasma optical electron spectroscopy (model iCAP 7000). The particle sizes were dominated by sand with amounts exceeding 600 g kg^-1, while bulk density was lower than 1.6 Mg m^-3 and soil pH ranged from strongly acid to neutral (5.3-7.2) across all sampled locations. Amongst the potentially toxic metals studied, arsenic (As), boron (B), barium (Ba), beryllium (Be), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), antimony (Sb), vanadium (V) and zinc (Zn) were reported in the study area, however, only Be (1.85-4.16 mg kg^-1), Cd (0.21-1.88 mg kg^-1) and Sb (1.87-4.48 mg kg^-1) exceeded the required threshold across the entire study area. Lead concentration was found to exceed the required threshold only in Agoi Ibami. Hence, Be, Cd, Sb and Pb are most likely to be a threat to the activities of micro and macro fauna and flora in the area. There is therefore the need to embark on a full-scale evaluation with larger sample size to ascertain the status of potentially toxic metals’ concentration in the Cross River State tropical rainforest soils.