Spatial Distribution and Relative Enrichment of Some Upper-Group Trace Elements in Rhizosphere of Highly Anthropized and Rapidly Developing Tropical Environment

Abstract

Most upper-group (Groups 13-17) trace elements are toxic including those already confirmed as micronutrients considering the very thin margin between their adequacy and toxicity in the biosphere. The concentrations, distribution and relative enrichment were studied of some of these elements in 0-50-cm soil layer at Ikwo in southeastern Nigeria representing largely disturbed and rapidly evolving ecosystems of the humid tropical region. Samples was from the north (N), south (S), east (E) and west (W) zones with pronounced mining and agricultural activities, including a reference zone at the centre (F). The samples were taken from fallow lands in the dry season. The concentrations of the trace elements were determined using Inductively Coupled Plasma Atomic Emission Spectroscopy. Enrichment factor was calculated as the ratio of each element to that of Fe (as a reference element) in the soil. The elements had their highest concentrations in F-zone and lowest in N-zone, with top-bottom ranges as 21.00-10.75, 1.83-0.93, 10.90-5.58, 4.05-2.05, 4.97-2.54, 2.20-1.12, 17.75-9.09, 26.74-13.69, 4.41-2.26, and 1.89-0.96 mg kg–1 for Se, As, B, Al, Si, S, Sn, Sb, I and Br, respectively. Thus mining and agricultural activities, rather than leading to the build-up of these elements, tend to contribute towards reducing their levels in the rhizosphere. Enrichment factors indicated moderate-to-insignificant levels of the majority of the elements in the soils. Generally, the elements showed similar distribution and enrichment patterns across the five zones. These results suggest that, at their current modes and intensities in the humid tropics, mining and agricultural activities apparently may not pose any ecological hazards due to these trace elements.