The persistence of cadmium (Cd) immobilization in acidic paddy soils is exacerbated by acidification and fluctuating redox conditions that promote Cd re-mobilization. While biochar is a promising amendment, its long-term efficacy in Cd immobilization relative to conventional lime and the underlying mechanisms remain incompletely resolved. This study tested the hypothesis that biochar’s superior effect lies in its durable enhancement of soil pH buffering capacity (pHBC), not merely in increasing initial pH. Using six acidic paddy soils amended with three biochars (corn straw, peanut straw, and seeded sunflower plate) and pH-matched lime [Ca(OH)₂] controls, we quantified pHBC changes, resistance to simulated acidification, and Cd dynamics during a flooding-drying cycle. Results showed that biochar amendments increased pHBC by 24.7–110%, significantly more than lime. Under acid stress, biochar-treated soils maintained higher pH and released 40–85% less soluble and extractable Cd than lime controls at equivalent pH range. Correlation and regression analyses established that the biochar-induced change in pHBC (ΔpHBC) was the strongest predictor of reduced Cd availability, exerting twice the influence of native soil pHBC. During the redox cycle, enhanced pHBC directly attenuated soil re-acidification upon drainage, minimizing Cd re-mobilization. Thus, the durable enhancement of soil pHBC is the central mechanism for biochar’s sustained Cd immobilization, advocating a strategic shift from transient pH adjustment to building inherent soil buffering resilience for long-term remediation security.

Biochar is widely recognized for its ability to immobilize heavy metals in soil, yet its direct effect on plant physiological metal-sequestration capacity remains poorly understood. This study explores a critical distinction between two mechanisms: direct, concurrent metal immobilization by biochar versus its capacity to physiologically precondition plants, altering their inherent metal uptake and distribution. Using a hydroponic design with pH-matched controls, the latter was isolated by preconditioning rice plants with peanut straw biochar (PSB) or corn straw biochar (CSB) and subsequently removing amendments before cadmium (Cd) exposure. Our results reveal that biochar (PSB) preconditioning may modify root architecture and surface chemistry, enhancing negative zeta potential and functional group density. This modification increased root Cd adsorption capacity by 50.1% and 142.7% within 2 h or 2.2% and 52.6% within 48 h compared to the normal and pH-adjusted controls, respectively, with shifted metal speciation toward stable complexes. However, this enhanced root sequestration coincided with an increased translocation factor, elevating shoot Cd content by 78% compared to the normal control. In contrast, CSB preconditioning showed negligible effects. Our findings suggest that biochar’s net impact on metal distribution is probably the product of two temporally distinct processes: chemical immobilization in growth media versus physiological preconditioning effects. This dual mechanism framework may explain the variability in literature reports on the effect of biochar on heavy metal uptake by plants. It also highlights the need for holistic biochar risk assessment that considers both chemical and plant physiological pathways in both soil and hydroponic systems.