Comparative efficacy of individually and combined application of compost, biochar, and bentonite on Ni dynamics in a calcareous soil

Abstract. In Iran, Ni-contaminated calcareous soils pose a significant environmental risk, yet effective remediation strategies for these specific conditions remain underexplored. While organic and inorganic amendments are commonly used, their comparative efficacy and potential synergistic effects in combined applications for Ni immobilization are not well-established. To address this, an incubation study investigated the individual and combined effects of municipal solid waste compost (M), its biochar (R), and bentonite (B) on Ni stabilization in a calcareous soil at three Ni-contamination levels (0, 150, and 300 mg Ni kg−1). Sequential extraction and DTPA-release kinetics demonstrated that R was the most effective treatment, significantly reducing labile Ni by transforming it into the residual fraction. This is likely due to its alkaline pH, ash, and phosphorus content, which promote Ni precipitation. In contrast, M increased soil Ni bioavailability. The results revealed that combinations (M+B, R+B, R+M) offered no synergistic advantage. The main finding was that singly-applied municipal solid waste biochar is a superior amendment for Ni immobilization, providing a more efficient and practical remediation strategy for contaminated calcareous soils without the need for complex combined treatments.