Cadmium (Cd) not only significantly threatens crop growth and food safety but also degrades soil quality and reduces microbial activity. Hydrochar is a carbon-rich material derived from biomass via hydrothermal carbonisation, due to its large specific surface area and abundant surface functional groups, shows great potential for immobilisation remediation of Cd-contaminated soils. We determined hydrochar’s (1) optimal application conditions for soil Cd immobilisation and (2) regulatory effects on foxtail millet growth and development, photosynthesis, antioxidant enzyme activity, resistance gene expression and Cd accumulation. Hydrochar was prepared at two temperatures (240 and 160 °C) and applied at two concentrations (1% and 2%). Hydrochar application reduced soil available Cd by 4.59–10.09% and shifted Cd fractions from acetic acid-extractable to residual forms. Soil organic matter (SOM) and microbial biomass carbon (MBC) increased. Hydrochar application increased plant antioxidant enzyme activity, downregulated SiNRAMP5 and SiHMA2 and upregulated SiHMA3, SiPCS1 and SiMT2. Hydrochar prepared at 240 °C and applied at a 2% concentration exhibited the highest efficiency in immobilising Cd, enhancing plant photosynthesis and antioxidant defence, activating soil microorganisms and increasing SOM and MBC, offering a promising remediation strategy for weakly alkaline Cd-contaminated farmland in northern China.

Passivation is a feasible approach for remediating heavy metal-contaminated soil. However, how passivation stability depends on material type, dosage, and plant cultivar remains unclear. Phosphate rock powder (PR) and bentonite (BN) were applied for passivation at three dosages, and the passivation activity was evaluated on high- and low-Cd-accumulating wheat cultivars. Phosphate rock powder (PR) and BN decreased the available Cd content in the soil by 11.52–26.65% and 11.08–35.00%, and in the wheat grains by 7.28–49.94% and 14.14–57.61%, respectively. PR₃ and BN₃ enhanced wheat yield by 12.77–21.31% and 12.23–18.67%, respectively. The passivation activity of both materials increased with increasing dosage. The optimal ranges for effective, stable Cd passivation were 0.29–0.61 and 3.85–8.46 t ha⁻¹ for PR and BN, respectively. Path analysis revealed that PR acts mainly through increases in soil available phosphorus and associated changes in Cd fractions, whereas BN acts primarily through the soil cation exchange capacity; grain Cd was chiefly associated with reactive Cd fractions. The different Cd accumulation capacities of wheat cultivars affected the passivation effects of PR and BN. The soil of Jimai22 showed significantly lower EXC-Cd and Carb-Cd and significantly higher FeMnOz-Cd than Zhoumai18. Moreover, soil pH was higher for Jimai22 than for Zhoumai18. These results suggest that combining the selection of suitable passivation materials, optimising the dosage and planting low-Cd-accumulating cultivars is an effective strategy for maintaining Cd passivation in weakly alkaline soils.