Alteration of contamination threat due to dilution effect on metal concentration in maize-wheat biomass on sludge amended clayey soil

Abstract

Industrial sludge often contains considerable amount of organic matter and plant nutrients to enhance crop production. However, its utilization in agriculture is viewed with concern as it also enhances the entry of toxic heavy metals into the agroecosystem. A field study was conducted to assess the potential of sludge generated from a soft beverage industry in cereal crops after critical analysis of benefits and contamination risks. The treatments were control, recommended doses of major fertilizers applied to both maize and following wheat crops, and organic amendments, viz., cattle dung manure and sludge at graded rates (2-50 t/ha) applied only to maize crop. Growth, yield, and heavy metal concentrations in plant parts were measured. Sludge application rates at ≥ 20 t/ha had significant direct as well as residual effects on crops in terms of enhancing their growth parameters and grain yields in comparison to the direct effects of fertilizer applications. It also enhanced Cu, Cd, Pb, and Zn concentrations in vegetative biomass of both crops even with the lowest rate of application, but had lower or little effect on their concentrations in grains. Sludge facilitated accumulation of metals in vegetative biomass of maize as indicated by increase in dynamic factor of bioaccumulation (BA_dyn) to > 1. Relative uptake of added metals by maize biomass increased with increasing sludge rate up to 10 t/ha, but decreased significantly at the highest application rate. Heavy metals concentration in biomass due to increasing rates of sludge application was the result of a trade-off between their "increasing entry in soil-plant system" and "dilution in biomass" due to enhanced crop growth. Strategy for safe application of this contaminated sludge in agroecosystem was discussed through analysis of heavy metals transfer characteristics in soil-plant system. The study indicates that conjoint application of lower rates of both sludge and N fertilizer can minimize risk of heavy metals contamination while ensuring higher crop yields.

Keywords: Bioaccumulation factor; Cattle dung manure; Cereal; Risk minimization; Translocation factor; Waste recycling.