Remediation of alkaline cadmium (Cd) contaminated soil through biochar combined with organic fertilizer application

The soil environment in regions of China impacted by coal gangue pollution is significantly degraded, with cadmium (Cd) identified as one of the most prominent heavy metal contaminants. Biochar (BC) and organic fertilizer (OF) are rich in nutrients and environmentally friendly modifiers, which have been widely applied in heavy metal-contaminated soil remediation. This study used BC (1%, 2%, 3%) and OF (2.5%, 5%) in a 120d potting experiment. The results showed that BC_3%-OF_5% exhibited the most effective repair performance among all the treatment groups. The weak acid extraction Cd (Fr.1) decreased by 77.95% and the residue fraction (Fr.4) content increased by 98.97% at BC_3%-OF_5%, which demonstrated the Cd transformation from the unstable to the stable state. Moreover, the physicochemical properties of soil have been ameliorated. It led to the largest decrease in pH (9.01-8.02) and the greatest increases in SOM (25-77.28 g/kg) and CEC (3.2-9.9 cmol/kg). Meanwhile, microbial diversity analysis revealed an increase in both diversity and evenness compared to the control group (CK), and the relative abundance of Proteobacteria and Bacteroidota increased by 12.02% and 410.12% in BC_3%-OF_5%, respectively. Furthermore, genera known to have a passivating effect on Cd (Lysobacter, Luteimonas, etc.) showed increased abundance over time. In conclusion, combined remediation with BC and OF has a passivating effect on Cd, soil physicochemical properties, and microbial community structure in alkaline Cd-contaminated wheat field soils in northern China. These results provide experimental guidance and a theoretical basis for treating heavy metal pollution in farmland and related food safety and health concerns.