The composite of chicken manure, lime, and sepiolite enhanced sulfur availability and simultaneously reduced bioavailable Cd in Cd-contaminated soil.

Although the effectiveness of various soil amendments in immobilizing cadmium (Cd) has been well studied, their impact on soil nutrient cycling, particularly sulfur (S) transformation, is poorly understood. This study evaluated the effects of six single or composite immobilizing agents, including chicken manure (OM), biochar (B), lime (L), sepiolite (SE), OL (OM + L + SE), and BL (B + L + SE) on soil Cd bioavailability and S transformation via a pot experiment. The results showed that after 120 days of remediation, OL and BL exhibited the highest Cd immobilization efficacy, with CaCl2-extractable Cd reduced by 94.7% and 95.8% (P < 0.05), respectively, compared with CK. They effectively facilitated Cd sequestration into carbonate-bound (F2-Cd) and Fe/Mn oxide-bound (F3-Cd) fractions. Throughout the remediation, all agents reshaped inorganic S distribution and significantly enhanced S availability. Notably, at the end of remediation, OL significantly increased available S (AS) by 80.8% relative to CK, and exhibited the highest arylsulfatase (ASF) activity, indicating that OL had a sustained and powerful S mineralization capacity. Additionally, L, SE, and OL significantly enriched Bacteroidota and Bacillaceae. These taxa contributed to Cd sequestration and detoxification, and Bacillaceae further facilitated organic S mineralization. At the functional level, the significant upregulation of tauD and downregulation of cysK were closely associated with AS enhancement, collectively contributing to the AS pool. Overall, the OL composite emerged as an effective and sustainable amendment, concurrently mitigating Cd toxicity, enhancing S availability, and maintaining the supply of other essential nutrients (N, P, K), thereby improving overall soil quality.