Objective Cadmium (Cd) contamination in paddy soils of southern China poses a serious threat to food security and human health, highlighting the urgent need for cost-effective and environmentally friendly remediation technologies. This study aimed to prepare carbon-iron composite nanomaterials (C-Fe NMs) using tea waste and iron salts and investigate their stabilization effect and mechanisms on Cd in paddy soil.

Methods Carbon-iron composite nanomaterials were synthesized via a green method using tea waste extract and iron salts, followed by calcination at 600°C to enhance crystallinity and stability. A 60-day soil incubation experiment was conducted with three treatments: control, tea waste extract group, and C-Fe NMs group (1% addition). The content of bioavailable Cd was determined by DTPA extraction–ICP-MS. The material’s structure was characterized by XRD, SEM, FTIR and XPS, and soil microbial community structure and function were analyzed by high-throughput sequencing.

Results The C-Fe NMs treatment reduced bioavailable Cd content by over 60%, significantly higher than the tea waste extract group (P ＜ 0.05). After calcination, the material formed a core–shell structure with Fe_1-xS as the main phase, coated with a carbon layer and iron oxides, which effectively inhibited core oxidation and improved material stability. Soil bacterial richness and diversity increased significantly, and microbial functional potential related to xenobiotic degradation, membrane transport, and energy metabolism was notably enhanced.

Conclusion Carbon-iron composite nanomaterials effectively stabilized Cd in soil through multiple mechanisms including adsorption, reduction, and co-precipitation. The core–shell structure exhibited excellent stability and environmental compatibility. This material shows great potential for remediating Cd-contaminated paddy fields, providing a theoretical and technical basis for developing green remediation technologies.