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

In Iran, a considerable proportion of agricultural soils are contaminated with various heavy metals (HMs), including nickel (Ni), necessitating remediation to mitigate their transfer into the food chain. However, there remains a scarcity of research on the effectiveness of applying organic and inorganic materials, either individually or in combination, for Ni immobilization in contaminated calcareous soils. To address this gap, an incubation experiment as completely randomized design with three replications was conducted to compare the effect of different soil amendments, either individually or combined (municipal solid waste compost (M), bentonite (B), municipal solid waste compost biochar (MB), M+B, MB+B, MB+M each applied at 2 % wt.) on Ni immobilization in a calcareous soil with three Ni contamination levels (0 (Ni0), 150 mg kg-1 (Ni1) and 300 mg kg-1 (Ni2). The study employed analytical techniques such as SEM-EDX, XRD, FTIR, sequential extraction, and DTPA-release kinetics to assess the efficiency of these amendments on stabilizing Ni in the soil. Elevating Ni levels from Ni0 to Ni2 increased Ni concentrations across all soil fractions, especially in Fe/Mn oxides (FeMnOx) and organic matter (OM). All amendments except M enhanced Ni immobilization by converting more labile fractions (WsEx, Car, FeMnOx) into residual (Res) form. While combined amendments were not more effective than single treatments, MB was the most efficient in stabilizing Ni. MB also exhibited the lowest 'a' and highest 'b' values attributed to the power function kinetics model, indicating superior Ni desorption reduction. These finding are likely due to its alkaline pH, ash content, and phosphorus content, which facilitate Ni precipitation. In contrast, M increased Ni desorption by raising its bioavailability (WsEx and Car fractions). The combined application of biochar (MB) with either bentonite (B) or compost (M) did not exhibit synergistic effects on the immobilization of Ni in the soil. In conclusion, the independent application of municipal solid waste-derived biochar appears to be a potentially effective amendment for enhancing Ni immobilization in contaminated calcareous soils.