Heavy metal immobilization using biochar (BC) is different from the usual soil incubation due to the low molecular weight organic acids (LMWOAs) in the rhizosphere and is an issue worth evaluating. Therefore, the impacts of rice husk BC (5%), tartaric acid, and oxalic acid, coupled with combinations of BC and tartaric acid/oxalic acid on the transformation of cadmium (Cd), lead (Pb), and zinc (Zn) among their geochemical forms, including their bioavailability in a metal-contaminated soil, were investigated in an incubation experiment. The application of BC, low concentration of tartaric acid (2 mmol kg⁻¹ soil) (TA2), and the combined BC plus a low level of tartaric acid (BC-TA2) markedly reduced the acid-soluble and available (CaCl₂-extractable) Cd, Pb, and Zn compared to control (CK) in which BC-TA2 was found to be the most effective treatment. The trends were reversed in the case of the high concentrations of tartaric acid (>5–20 mmol kg⁻¹ soil), all levels of oxalic acid (2–20 mmol kg⁻¹ soil), and the combined BC plus high levels of tartaric acid/oxalic acid treatments. The BC-TA2 transformed the highest amounts of acid-soluble and reducible Cd, Pb, and Zn to the oxidizable and residual fractions with incubation time. The results suggested that the low concentration of tartaric acid enhanced Cd, Pb, and Zn immobilization, while the higher level of tartaric acid and all concentrations of oxalic acid increased their mobilization. In conclusion, BC-TA2 could immobilize the most heavy metals and serve as an amendment for metals’ immobilization/redistribution in contaminated soils. GRAPHICAL ABSTRACT

An incubation study was conducted to investigate the effects of adding cow manure (CM), poultry manure (PM), city waste (CW) and triple super phosphate (TSP) on extractable P in a sandy loam acidic soil (Aquepts) and a silt loam alkaline soil (Fluvents). Phosphorus was added at a rate of 200 mg P kg⁻¹ to the soil in the forms of CM, PM, CW and TSP. The soil was incubated at field capacity for 15, 30 and 60 days after which the P sources were extracted with Olsen, Kelowna, Mehlich-3, and Bray and Kurtz-II extractants. Regardless of the extractants and soils, after 15 days of incubation, the highest extractable P source was the TSP (88-97 mg kg^-1 as Olsen P), and the lowest was the CW (11-15 mg kg^-1 as Olsen P). The extractable P levels with the CM (41-48 mg kg^-1 as Olsen P) and PM (71-73 mg kg^-1 as Olsen P) were between the extractable P levels with the TSP and CW. Extractable P in the soils with the organic amendments increased slightly with increasing incubation time from 15 to 60 days. With the TSP, extractable P decreased sharply in the acidic soil, while it decreased gradually in the alkaline soil. In both soils, extraction efficiency was in the order of Bray and Kurtz-II> Mehlich-3> Kelowna>Olsen. The efficiency of the added P as measured by NaHCO₃ increased with the organic amendments, while the efficiency decreased with the TSP with days of incubation. This study indicates that soil samples taken 30 days after the addition of CM and PM in the soil might reflect the most plant-available P.