Fe-impregnated biochar enhances cadmium detoxification and compartmentalization in rice

Iron-modified biochar is a promising cost-effective material for cadmium (Cd)-contaminated soil remediation, yet how the additional iron (Fe) affects Cd uptake and translocation in rice remains unclear. This study compared two Fe-modified biochars, including impregnated biochar (I-PBC) and co-pyrolyzed biochar (Fe-PBC), with raw biochar (PBC) and explored their effects on Cd accumulation in rice under low application rates (0.1%-0.2%, w/w). These rates were selected for field applicability and cost-efficiency in regional agricultural practices. Cd-contaminated acidic soil collected from southeastern China was used in the pot experiments. In the first pot experiment, I-PBC at 0.1% significantly reduced Cd concentrations in rice grains by 26.3%, 43.7%, and 13.5%, compared with the control, PBC, and 0.1% Fe-PBC, respectively. The results from a second experiment (0.1% amendments: PBC, I-PBC, FeSO₄, water-washed PBC) suggest that I-PBC enhances Cd compartmentalization in aboveground tissues by increasing Cd in the cytosolic fraction while reducing soluble Cd proportions, likely sequestering Cd into less metabolically active compartments (e.g., vesicles). Additionally, I-PBC promoted the conversion of Cd into low-toxicity Cd-oxalate complexes, as evidenced by a significant increase in the HCl-extractable Cd fraction (FHCl, predominantly Cd-oxalate) in stems. These findings demonstrate that impregnation is superior to co-pyrolysis for Fe-modified biochar in inhibiting grain Cd accumulation, with key mechanisms involving enhanced Cd chelation by oxalic acid and subcellular compartmentalization. This study highlights the potential of low-dose I-PBC for practical Cd-contaminated farmland remediation.