Role of different organic amendments on Cd and Pb bioavailability, uptake, and growth by Ipomoea aquatica

• Amendments shifted Cd and Pb from mobile to stable forms, reducing plant uptake. • Absorption of Cd and Pb in the plant reduced by 53-92 % and 32-67 %, respectively. • Chicken manure biochar showed maximum decrease in Cd and Pb bioavailability in soil. • Amendments increased soil pH and decreased BCF and TF thus enhancing plant growth. • Mobility of Cd was higher than Pb, showing higher risk to plants and humans. Heavy metal contamination in acid soil is a potential risk for crop growth and food security. Organic amendments can decrease metal toxicity and ameliorate soil; however, their comparative effectiveness has not been well defined. This experiment evaluated the effectiveness of four organic amendments, including sugarcane trash biochar, chicken manure biochar, used tea waste, and eggshell powder, in improving soil properties, reducing Cd and Pb bioavailability, and promoting the growth of Ipomoea aquatica (water spinach). A pot experiment was conducted using soil contaminated with Cd (18 mg kg -1 ), Pb (1125 mg kg -1 ), or both, with a 5 % amendment rate. Chicken manure biochar exhibited the best improvement on soil organic matter, cation exchange capacity, pore structure, specific surface, and essential nutrient contents (N, P, Fe), contributing favorable conditions for plant growth and mitigating metal mobility among all treatments. It decreased DTPA-extractable Cd and Pb by 62 % and 89 %, respectively, and increased metals’ conversions from labile to less labile forms. Its high porosity, functional groups, Fe amount, and alkalinity contributed to promoting its metal adsorption, co-precipitation, and complexation over other amendments. Consequently, shoot Cd and Pb uptakes were decreased by 92 % and 74 %, respectively, and significantly reduced bioconcentration and translocation factors were found. The shoot and root biomasses were increased up to 22- and 15-fold, respectively, due to toxicity alleviation and benefits derived from better nutrient availability. Other amendments also improved soil quality and reduced metal uptake but to a lesser extent. Overall, chicken manure biochar was the most effective and sustainable amendment to remediate metal-contaminated acidic soils.