Biochar is widely used to remediate heavy metal-contaminated soils; however, it can increase C limitation in soil microorganisms, potentially compromising soil ecological sustainability. Effective strategies are, therefore, needed to reduce heavy metal bioavailability while maintaining microbial activity and soil fertility. To this end, we aimed to evaluate the effects of different straw–biochar ratios on heavy metal availability, microbial nutrient limitation, C cycling and soil fertility and to identify an optimal remediation strategy for Cd- and Zn-contaminated soil. Cd- and Zn-contaminated soils were amended with varying straw–biochar ratios (S4C0, S3C1, S2C2, S1C3, S0C4 and control) and incubated for 90 days. Soil heavy metal bioavailability, microbial nutrient limitation, CO₂ emissions, enzyme activities and organic C stability were analysed. Compared with the control, the straw–biochar treatment at a 1:3 ratio (S1C3) reduced diethylenetriaminepentaacetic acid–extractable Cd and Zn by 30.22–32.81% and 21.31–23.06%, respectively. S1C3 significantly improved soil organic C content and stability, enhanced available nutrients (N, P and K), increased microbial biomass and elevated enzyme activities related to C, N and P acquisition and alleviated microbial C limitation. The optimized straw–biochar ratio (1:3) provides a novel integrated approach for remediating heavy metal-contaminated agricultural soils while simultaneously enhancing soil C stability, nutrient availability and microbial activity. This strategy offers a practical foundation for sustainable farmland management.