Aging mechanism of biochar based on fluorescence spectroscopy: Assessing soil dissolved organic matter (DOM) dynamics and Cd bioavailability

[Display omitted] •Aging treatment altered the structure of BC and the composition and abundance of its derived DOM.•Humic-like substances are the main components of DOM and have the strongest binding capacity with Cd.•Aging treatment weakened the passivation ability of BC for Cd in soil. Biochar (BC) has been widely applied in the environmental field, yet the effects of aging on BC-derived dissolved organic matter (DOM) and its influence on soil DOM post-application remain underexplored. Therefore, this study accelerates the aging of BC through dry-wet cycling (DW), freeze–thaw cycling (FT), and ultraviolet irradiation (UI) to investigate the effects of aging treatments on the release characteristics of BC-derived DOM and their impact on the remediation of Cd-contaminated soil. The results show that UI treatment significantly increased the release of BC-derived DOM (209.57 mg/L after 25 days, compared to 135.75 mg/L and 129.47 mg/L for DW and FT treatments, respectively), and reduced the alkalinity of BC. Soil incubation experiments demonstrated that S-DW and S-UI increased the bioavailability of DOM and stimulated microbial activity, whereas S-FT and S-BC treatments, which had higher humic acid content, effectively reduced cadmium bioavailability (passivation rates: S-BC 27.84 %, S-FT 23.76 %, S-DW 18.98 % and S-UI 24.81 %). Fluorescence spectroscopy analysis revealed that aging increased the humic-like substances in BC-derived DOM, affecting the composition of soil DOM and Cd immobilization. These findings emphasize the dynamic properties of aged BC in soil remediation, offering theoretical support for its long-term use in pollution control.