Heavy Metal Bioaccumulation by Maize Grown on a Ferralsol Amended with Urban-Based Biosolid Wastes

Organic waste materials as soil amendments are one of the topical approaches applauded for achieving sustainable agriculture world-over. The objective of this study was to investigate the effect of urban-based biosolid waste (UBBW) application on heavy metals (Cr, Cu, Zn and Pb) bioaccumulation by maize (Zea mays L.) plants. A pot experiment was conducted three times, using an acid Ferralsol from Makerere University Agricultural Research Institute, Kabanyolo (MUARIK) in Uganda. Treatments included the application of three types of UBBW, namely sewage, brewery and abattoir, each applied independently at the rates of 0, 50 and 100 g per pot filled with 4 kg soil. This was equivalent to 0, 2.5 and 5.0 metric tonnes of dry materials per hectare. Phosphorus fertiliser was also applied at 0, 0.795 and 1.591 g P per pot, equivalent to rates of 0, 25 and 50 kg P ha-1. The brewery waste applied at rates ≥ 2.5 t·ha-1 and phosphorus at 25 kg P ha-1 resulted in shoot Cu concentrations below the World Health Organisation (WHO) safe limit (73.3 mg·kg-1); and Zn slightly above the WHO safe limit (99.4 mg·kg-1). In contrast, the concentrations of chromium in the maize plants were well above the WHO safe limit (2.3 mg·kg-1), irrespective of the applied type of UBBW. Shoot metal bioaccumulation followed the order zinc > copper > chromium, with Pb being below the detection limit. The safest UBBW was abattoir waste; while the least environmentally suitable was sewage waste. It is clear that irrespective of the type of UBBW, their application to Ferralsol causes less bioaccumulation of Pb and Cr in maize plants compared to Zn and Cu.