The distribution and geochemical assessment of trace elements from the semi-arid to sub-humid savanna of Nigeria

The results of a study of the distribution and origin of trace elements in the surface soils of the Nigerian savanna are reported. Composite surface soil samples representing the four savanna zones: Southern Guinea, Northern Guinea, Sudan and Sahel and the major soil parent materials: aeolian deposit, Basement complex, sedimentary, basaltic rocks were examined. Instrumental Neutron Activation Analysis was used to determine four trace elements, aluminum (Al), titanium (Ti), vanadium (V), dysprosium (Dy) commonly found in plant and animal tissues. The values obtained for all the samples were comparable to the range of values reported by FOREGS for topsoils worldwide. On the average, the Nigerian savanna soils contained about 4.6 % Al, 0.5 % Ti; 67 mg V kg(-1) and 4.5 mg Dy kg(-1). Except for soils of the semi-arid Sahel region, with very low values, elemental values reported in this study are within the tolerable range in soils, indicating the unlikelihood of problems associated with either deficiency or toxicity of these trace elements. Clay, organic carbon and free iron oxides which are the principal constituents of the soil colloid correlated positively and significantly with all the elements. Soil contamination assessment using contamination factor (c (f)) and geoaccumulation index (I (geo)) revealed no contamination by the elements studied and are likely to have originated from the soil parent materials. I (geo) values were 0.01 for V and 0.23 for Dy for all the locations indicative of near similar levels of pollution within the Nigerian savanna. Geochemical balance evaluation indicates a depletion of V with reference to levels in the earth crust while Dy enrichment in a few locations like Kanji could be ascribed to likely higher values in soil parent materials from such locations. All the elements were significantly influenced by the soil parent materials while only V and Dy were influenced by ecological zones. In the management of these elements in the Nigerian savanna, manipulation of these three soil properties holds the key to the successful and sustainable management of these elements in the soils.