Lead (Pb) mobility in soils and hence its availability for plant uptake may be influenced by several soil properties, some of which are modified by sludge application. It is therefore imperative to understand how these modified properties affect Pb concentrations in different geochemical fractions of sludge-amended soils. Vertisols are common in arid- and semi-arid areas and are used extensively for agriculture. This study aimed at determining the relationship between selected soil properties (pH, organic matter (OM) content, cation exchange capacity (CEC), weight percent (wt %) clay, concentrations of Fe and Al) in a montmorillonitic vertisol, and the concentrations of Pb in different geochemical fractions of the vertisol before and after sludge addition. The vertisol was amended with sludge at volume/volume percent (v/v %) ratios of 5:95, 10:90, and 20:80 sludge:vertisol. Lead concentrations in the exchangeable (S1), carbonate (S2), reducible (S3), oxidizable (S4) and residual (S5) fractions of the vertisol were extracted using a five stage sequential extraction procedure. Results were subjected to multivariate analyses which included regression analyses, Bonferroni adjusted two-step cluster analyses, and principal component analyses. Lead concentrations in the different fractions followed the order S4 > S1 > S5 > S2 > S3. Lead concentration increased with sludge application rate (SAR) in all samples. Two clusters were identified through cluster analyses. In the vertisol with no sludge, CEC and pH appeared to be the most significant factors affecting segregation of Pb into the different fractions (P< 0.05) with pH explaining 80%, 84.6% and 77.4% of the variations observed in Pb concentration in the S1, S3 and S4 fractions respectively. At SAR of 5%, 10% and 20%, OM content becomes significant in determining partitioning of Pb into the different fractions with Fe becoming more important at higher sludge application rates. Findings of the study indicate that in the application of sludge to soils, the pH, OM and Fe content of the soils should be taken into consideration as they may influence the bioavailability and eventually bioaccumulation of Pb in plants.
