Direct Biochar–Root Interactions May Alter Cadmium Sequestration at the Interface: A Hydroponic Study

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.