The status and dynamics of some trace elements in a savanna soil under long-term cultivation.

A recent study in the savanna suggested that the surface soils might be contaminated with Ti, V, U and Th because of increasing phosphate fertilizer applications. In this study, we determined the dynamics of Ga, V, U, Nb, Y, La, Ce and Nd in a savanna soil that has received NPK fertilizers, farmyard manure (FYM) and FYM + NPK for 50 years using an adjacent uncultivated or native site as a reference. The concentrations of the elements in the soils were determined by X-ray fluorescence. Mass balance calculations indicated that Nb, U, Y, La, Ce and Nd increased by between 8 and 61% in the cultivated soil compared with the uncultivated natural site. Long-term application of FYM increased the masses of Nb (16%), Th (40%), La (26%), Ce (44%) and Nd (54%). There were no discernible changes in the masses of the trace elements that could be unambiguously associated with phosphate fertilizer applications. It was, thus, concluded that physical accumulation as leaching losses and crop removal depleted soluble ions, stabilization by organo-mineral complexes and aeolian deposition are the probable soil enrichment mechanisms of these trace elements during cultivation.