CaO-modified Hydrochar Reduces Soil Cadmium Bioavailability by Altering Soil Properties, Shifting Bacterial Community, and Promoting Microbial Metabolisms

The efficient remediation of cadmium (Cd)-contaminated soil is of great significance for reducing the threats to ecosystem and human health. At present, the carbon-rich hydrochar with abundant oxygen-containing functional groups as environmental remediation material gains numerous attention due to its cost-effectiveness and sustainability. A pot experiment was performed to investigate the Cd immobilization efficiency and microbial response in soil with the presence of pristine corn stalk hydrochar and CaO-modified hydrochars. Results showed that compared with pristine hydrochar, fresh biomass of plant shoot was promoted by 0.904-15.6% with CaOmodified hydrochar amendment, and the bioavailable Cd content and correspoding Cd uptake by shoot were decreased by 8.19-16.2% and 4.18-17.0%, respectively. Additionally, the relative abundance of Actinobacteriota benefiting Cd immobilization in soil was elevated from 30.3% to 34.4-44.6% after CaO-modified hydrochar amendment. The microbial metabolisms (e.g., carbohydrate metabolism, amino acid metabolism, and citrate cycle) were also enhanced by CaOmodified hydrochar, contributing to reduce Cd bioavailability. Redundancy analysis revealed that bacterial community structure in rhizosphere soil was significantly influenced by soil dissolved organic carbon and electric conductivity (p<0.05). Overall, the cost-effective CaOmodified hydrochar with reduced phytotoxicity was a promising remediation material for Cd immobilization in metal-contaminated soil.