Trade-off effect between biochar and freeze-thaw cycle on cadmium (Cd) availability in the black soil: Insights from soil aggregate turnover

Given the inadequate investigation on how biochar-mediated cadmium (Cd) speciation changes in soil aggregates under freeze-thaw cycles, this study involved an array of biochar addition levels (0 %, 0.5 %, 1 %, 3 %) (B0, B0.5, B1, B3) and freeze-thaw cycles numbers (0, 5, 15, 30) (F0, F5, F15, F30) to delve into biochar's influence on soil aggregates structure and Cd morphology transformation in black soil. The results manifested the freeze-thaw cycles diminished 0.25–2 mm agglomerates, and fostered microagglomerates formation, which prompted Cd enrichment in microaggregates. Biochar amendment, notably the B1 treatment, mitigated aggregate fragmentation, and boosted soil aggregate stability. The Cd mobility was augmented by the freeze-thaw cycles, while biochar facilitated the available, acid-extractable, reducible and oxidizable Cadmium in the macroaggregates converting to the stabilized residual cadmium, so that the Cd migration factor and bioavailability could be reduced. In areas susceptible to freezing and thawing, adding biochar to the soil is of dramatic efficacy, applying 1 % biochar is a cost-effective strategy to simultaneously maintain soil structure stability and reduce Cd bioavailability. These findings highlight biochar’s substantial prospect in soil remediation of cold regions. This work contributes valuable insights into the dynamic interactions of biochar with soil aggregates and Cd under freeze-thaw cycles. • Biochar attenuates the breaking caused by freeze-thaw cycles on agglomerates. • Biochar promotes the stabilization of cadmium (Cd) in soil agglomerates. • Biochar - Cd - soil aggregates interactions revealed.