In this comprehensive study, the evolution of soil and biochar in competitive heavy metal adsorption mechanisms following natural ageing is meticulously examined. The research spans a two-year period, focusing on the interaction between three soils and two biochars buried in the field. Competitive adsorption characteristics and mechanisms in four systems (Cd, Cd+Ni, Cd+Cu, and Cd+Ni+Cu) were scrutinized. Notably, the physicochemical properties, adsorption capacity, and mechanisms exhibited the most significant changes within the initial 0.5 years, emphasizing the dynamic nature of the soil-biochar interplay.

Results underscored that biochar’s properties and adsorption capacity diminished with ageing, while soils exhibited an opposite trend. After 0.5 years of co-ageing with acidic soil, the Cd(II) adsorption capacity of modified biochar decreased significantly in the ternary system, contrasting with an increase in acidic soil. Mineral mechanisms’ contributions dwindled noticeably, while non-mineral mechanisms were only slightly affected by ageing.

The study’s environmental implication stresses the global concern regarding heavy metal pollution in soils, often stemming from industrial and agricultural activities. Biochar emerges as a promising in situ remediation agent, but the intricate interaction between soil and biochar after natural ageing remains unclear. The findings suggest a crucial insight: when implementing biochar for remediating heavy metal-contaminated soils, application should precede planting crops by at least six months to facilitate optimal contact and reaction with the soil. This nuanced understanding of the ageing process enriches the knowledge base for sustainable soil remediation practices.