Cadmium bioavailability in acidic soils under bean cultivation: role of soil additives

Continuous application of phosphate fertilizer and bio-solids leads to cadmium (Cd) accumulation in agricultural soil. Acidic soil environment makes the situation worsen due to the conversion of higher soil Cd to plant available form. Therefore, identifying effective and economical methods to reduce the bioavailability of soil Cd is imperative. This pot experiment was conducted in an acidic soil (pH 5.69) with three different soil additives [biochar, plant growth-promoting rhizobacteria (PGPR) and ZnSO4] under three levels (6, 12 and 18 mg kg−1) of Cd on common bean (Phaseolus vulgaris L. cv. Falguni). Biochar and ZnSO4 were mixed with soil at 5.5 and 0.1 Mg ha−1, respectively, before seed sowing, while PGPR was applied as a seed coating. A significant reduction in plant morpho-physiological and biochemical parameters and soil chemical and biological properties was recorded with increasing Cd levels. Biochar, PGPR and ZnSO4 addition resulted in substantial improvement in the studied plant and soil parameters in Cd-treated soil by significantly reducing the bioavailability of soil Cd. ZnSO4 addition recorded improved (51%) plant growth in terms of total biomass with lower (57% shoot and 42% reduction in root) plant accumulation of Cd. The highest reduction in bioavailability of soil Cd was recorded with the application of biochar (34–97% reduction) compared with ZnSO4 (14–89%). Though application of PGPR reduced the Cd bioavailability to 94% at the lowest contamination, it increased Cd bioavailability by 2% at the highest level of contamination. Thus, biochar application (5.5 Mg ha−1) as soil additive is an effective means to mitigate Cd toxicity in acidic soils.