Purpose <p>Calcium-iron composite modified biochar (FeCaBC) exhibits excellent pollutant adsorption capacity, but its synergistic effects on cadmium (Cd)-contaminated soil remediation and greenhouse gas emissions remain unclear. This study aimed to clarify the impacts of FeCaBC on soil available Cd, crop Cd uptake, soil organic carbon (SOC), and soil CO₂ emissions in Cd-contaminated soils.</p> Materials and methods <p>Soil incubation, CO₂ emissions, and potting experiments were integrated to evaluate the performance of FeCaBC. Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and structural equation modeling (SEM) were used to elucidate the underlying mechanisms.</p> Results and Discussion <p>FeCaBC application significantly improved soil nutrient availability, increasing available P (114%-186%), available K (48%-84%), and ammonium N (20%-32%). At 0.5%–2% application rate, FeCaBC reduced soil available Cd by 37%-52% and ryegrass Cd content by 10%-31% (<i>P</i> &lt; 0.05), primarily via promoting Cd transformation from exchangeable to residual fractions. Concurrently, 0.5%-2% FeCaBC increased SOC by 130%-134% and inhibited soil CO₂ emissions by 30%-37% (<i>P</i> &lt; 0.05). FTIR, XPS, and SEM results indicated that FeCaBC facilitated synergistic Cd immobilization and carbon sequestration through two pathways: (1) surface functional groups mediating Cd complexation and ion exchange; (2) regulating soil physicochemical properties, nutrient cycling, and oxygen-containing functional group proportions to form stable organic complexes.</p> Conclusions <p>FeCaBC is an effective soil amendment that synchronously remediates Cd-contaminated soil and enhances carbon sequestration by reducing soil available Cd and crop Cd uptake while inhibiting soil CO₂ emissions and increasing soil organic carbon content, making it promising for sustainable remediation of contaminated agricultural soils. </p> Graphical Abstract <p></p>