Suitability of urban wastes for crop production in Zaria,northern Nigeria: bioavailability and geochemical fractions of potentially toxic elements

A savannah soil amended with two urban wastes and planted with sorghum (Sorghum bicolor) in a pot experiment was evaluated for chemical forms and phytoavailability of Cr, Ni and Pb. The metals were partitioned into seven operationally defined chemical fractions: mobile (F1), easily mobilisable (F2), occluded in Mn oxides (F3), organically bound (F4), occluded in amorphous Fe oxides (FeOx_am, F5), occluded in crystalline Fe oxides (FeOx_c, F6), and residual (F7). Although the application of the urban wastes increased the total concentrations of the metals in the soil, there were no corresponding increases in Cr and Ni uptake by sorghum except for Pb uptake. Waste application increased mobile Ni (F1) but decreased the residual form (F7). The organically bound Pb (F4) was increased at the expense of residual Pb (F7). With respect to Cr, waste application increased Cr form occluded in non-crystalline Fe oxides (F5) in the soil. Bioassay of the test crop indicated that the uptake of Pb by sorghum (Y-Pb) correlated significantly with Pb occluded in non-crystalline and crystalline Fe oxides (F5 and F6). Since a single application of urban wastes to this savannah soil significantly increased Pb uptake by sorghum, the consequences of long-term applications under urban and peri-urban agriculture (UPA) should be of great concern; particularly with respect to children’s exposure to Pb through consumption of food and vegetables.