Effects of Biochar on Freezing Temperature and Unfrozen Water Content in Pb‐Contaminated Seasonally Frozen Soils: Implications for Mitigating Land Degradation

ABSTRACT Freeze–thaw processes are important drivers of land degradation in cold regions, and soil freezing temperature and unfrozen water content are key parameters for characterizing soil hydrothermal conditions and water phase transitions during freezing. However, in the context of heavy metal pollution, there remains a lack of systematic understanding regarding biochar's (BC) regulatory role in the freezing behavior of seasonally frozen soil and the dynamic changes in unfrozen water content. Based on freezing and freeze–thaw experiments, this study investigated the effects of biochar on soil freezing temperature and unfrozen water content under various conditions, with the results subjected to statistical analysis. The findings showed that dry density and Pb content exert bidirectional regulatory effects on the freezing temperature and unfrozen water content of biochar‐amended soils. Under low initial water conditions, the biochar addition ratio is the primary factor affecting soil unfrozen water content. Specifically, 1% and 3% biochar additions significantly increase soil unfrozen water content; however, as biochar content increases further, its enhancing effect gradually diminishes. Under high initial water content, soil unfrozen water content is jointly regulated by biochar content, initial water content, and lead content. Correlation analysis indicated that lead content and initial water content exhibited significant positive correlations with unfrozen water content, while biochar content showed a weak negative correlation. ANOVA further revealed a significant interaction between biochar content and initial water content, indicating a synergistic regulatory effect, whereas the interaction between biochar and lead content was not significant. These findings reveal the influence patterns of biochar under multifactor coupling on the freezing processes of seasonally frozen soil and changes in moisture status. The results provide a theoretical basis for biochar‐assisted remediation of Pb‐contaminated soils and the mitigation of land degradation in seasonally frozen regions.