[Effect of Biochar Aging on Cadmium Immobilization in Soils Under Dry-wet Cycles]

To illuminate the effects and mechanisms of Cd immobilized by rice husk biochar and soil solid components under wet-dry cycles, a 30-cycle accelerated aging incubation was conducted on soils with different levels of cadmium （Cd） pollution. Results showed that the positive effects of biochar on soil pH, cation exchange capacity （CEC）, and the content of dissolved organic carbon （DOC） first increased but then decreased during incubation, with the most pronounced effects observed in light Cd-contaminated soil. At the end of incubation, soil pH, CEC, and the content of DOC increased by 0.03-0.19 units, 5.45%-18.35%, and 1.14%-7.71%, respectively. Furthermore, the entry of biochar into the soil solid components promoted the migration of Cd from the organo-mineral complexes （OMC） fraction to the particulate organic matter （POM） and mineral fractions, enhancing the adsorption and immobilization of Cd by the soil solid components. At the end of incubation, compared to the control group, the Cd content in the OMC fraction decreased by -7.74%- -23.40%, while it increased by 6.91%-19.66% in the POM fraction. Meanwhile, the immobilization rates of Cd by biochar in these two fractions reached 25.00%-29.31% and 10.63%-19.84%, respectively. The decrease in DTPA-Cd content in soil solid components was related to the increased porous structure and changes in surface properties of biochar after aging. Results showed the process of dry-wet cycles promoted the immobilization of Cd in soil. For light and medium Cd-contaminated soils, the application of biochar effectively reduced the contents of DTPA-Cd through enhancing the adsorption of active Cd by soil solid components and improving soil physical and chemical properties. For heavy Cd-contaminated soils, the former mechanism was the primary immobilization mechanism. Overall, this study contributes to reveal the interaction mechanisms between aged biochar and soil solid components under dry-wet alternation, and offers novel insights into the effects of biochar aging on heavy metal stabilization.