The co-contamination of arsenic (As) and cadmium (Cd) in paddy soils threatens rice safety, yet synergistic mitigation strategies using silicon (Si) and ferrous sulfate (FeSO₄) remain underexplored. This study integrated hydroponic and soil pot experiments to evaluate Si-FeSO₄ interactions on As/Cd accumulation and rice growth. Hydroponic trials employed 21-day-old rice seedlings exposed to 0.5 mg As(III)/Cd(II) L⁻¹ with/without 70 mg Si L⁻¹ and 30–70 mg Fe L⁻¹, followed by sequential harvesting at 14 and 21 days. Soil experiments utilized co-contaminated paddy soil (50 mg As kg⁻¹ and 1.2 mg Cd kg⁻¹) amended with Si (80 or 400 mg kg⁻¹) and Fe (100 or 1000 mg kg⁻¹), with pore water dynamics monitored over 120 days. Hydroponic results demonstrated that 70 mg Si L⁻¹ combined with 30 or 70 mg Fe L⁻¹ enhanced shoot biomass by 12–79% under As stress, while simultaneously reducing shoot As concentrations by 76–87% and Cd concentrations by 14–33%. Iron plaque induced by FeSO₄ exhibited contrasting adsorption behaviors: hydroponic roots immobilized both As and Cd (p < 0.01), whereas roots in soil primarily retained Cd (p < 0.05). In soil experiments, the optimal treatment of 100 mg Fe kg⁻¹ and 400 mg Si kg⁻¹ (Fe₁ + Si₂) increased grain biomass by 54%, while reducing As and Cd concentrations by 37% and 42%, respectively. However, a higher Fe dosage (Fe₂: 1000 mg kg⁻¹ Fe) paradoxically increased grain Cd concentrations. Mechanistically, Si amendment elevated soil pH (Δ + 0.72), facilitating Cd immobilization, while FeSO₄ lowered pH (Δ−0.07–0.53), increasing Cd mobility. A strong correlation between soluble Cd and plant uptake was observed (p < 0.01), while changes in As accumulation were unrelated to aqueous behavior. The optimized Si/Fe molar ratio of 7.95:1 effectively mitigated As and Cd co-accumulation, offering a dual-functional strategy for safe rice cultivation in contaminated soils.

Stabilizing soil lead (Pb) with single amendments often requires a trade-off between immobilization durability and cost-effectiveness, restricting their large-scale use. Here, we demonstrate that co-applying hydroxyapatite (HAP) with either rice straw biochar (RBP) or maize straw biochar (MBP) overcomes these limitations through synergistic soil–plant-microbe interactions. Compared to the un-amended control, the combined treatments reduced CaCl₂-extractable Pb by 93.2–95.5% and the acid-soluble Pb fraction by 39.4–41.8%. Spectroscopic analyses confirmed the direct binding of Pb to biochar surfaces, while an amendment-induced increase in soil pH further enhanced Pb immobilization. Consequently, this reduced Pb availability in the rhizosphere was associated with diminished iron plaque formation on root surfaces, ultimately decreasing root Pb accumulation. At the subcellular level in leaves, the combined treatments promoted Pb sequestration within vacuoles and reduced the organelle-bound Pb fraction by 16.1–31.9%. This subcellular shift significantly lowered the leaf-to-grain transfer factor by 49.4–54.3%. Furthermore, the combined treatments (RBP) enhanced soil bacterial alpha diversity and shifted community composition; notably, the enrichment of Bacteroidota and suppression of Desulfobacterota suggest their potential contribution to further Pb stabilization. Through these integrated mechanisms, Pb concentrations in brown rice were reduced below Chinese national food safety limits. These findings highlight that the co-application of biochar and HAP provides a cost-effective, multi-barrier strategy to mitigate soil-to-grain Pb transfer, enabling the sustainable utilization of Pb-contaminated paddy soils.