Pyrolysis Temperature-Dependent Biochar Regulates Lead Immobilization and Antioxidant Defense in Wedelia trilobata for Contaminated Soil Remediation

This study investigated the effects of rice straw biochar produced at 450 °C (RBC450) and 700 °C (RBC700) on Pb immobilization, soil properties, and the physiological and molecular responses of Wedelia trilobata (L.) grown in Pb-contaminated soil. Biochar was applied at 1%, 2.5%, and 5% (w/w) to soil contaminated with 150 mg kg−1 Pb. Changes in soil physicochemical properties, Pb availability and accumulation in plant tissues, physiological performance, oxidative stress indicators, antioxidant enzyme activities, and antioxidant-related gene expression were evaluated. Both biochars reduced soil-available Pb and Pb accumulation in roots and shoots, with RBC700 showing greater Pb immobilization. Biochar application also improved soil pH, electrical conductivity, organic carbon, nitrate nitrogen, and available phosphorus. In plants, biochar enhanced catalase and peroxidase activities while reducing malondialdehyde accumulation and reactive oxygen species production, indicating alleviation of Pb-induced oxidative stress. qRT-PCR analysis further showed modulation of SOD-, POD-, and CAT-related antioxidant responses under Pb stress, with RBC450 treatment strengthening antioxidant defense at the transcriptional level. Overall, rice straw biochar reduced soil-extractable Pb and modified soil physicochemical properties and antioxidant responses in W. trilobata, with RBC700 showing greater effectiveness in decreasing Pb extractability. However, these changes did not consistently restore all physiological parameters under Pb stress.