Chitosan-crosslinked microwave-engineered biochar alleviates lead stress in Brassica napus by improving soil functioning and plant performance in Pb-contaminated soil.

Lead (Pb) contamination in agricultural soils reduces crop productivity and threatens food safety, highlighting the need for in situ amendments that lower Pb bioavailability while supporting soil functioning and plant performance. This study upgraded poultry manure-derived biochar through microwave activation and genipin-crosslinked chitosan modification (MPBCH), then evaluated its performance in a greenhouse pot experiment with Brassica napus grown in field-collected Pb-contaminated soil. Six treatments were compared: untreated Pb-stressed soil (CK), chitosan (CH), poultry manure biochar (PBC), microwave-prepared biochar (MPBC), chitosan-modified PBC (PBCH), and MPBCH. The MPBCH produced the strongest overall response, increasing soil pH from 5.84 to 6.95 and reducing DTPA-extractable Pb by 62.0%. Root and shoot Pb concentrations decreased by 52.1% and 57.9%, respectively, while immobilization-related indices consistently indicated lower Pb mobility and restricted soil-to-plant transfer. MPBCH also increased β-glucosidase, phosphomonoesterase, catalase, and urease activities by 43.74%, 40.22%, 60.24%, and 40.48%, respectively. Rhizosphere bacterial analysis showed the highest Shannon diversity (4.52) and Chao richness (1882.12) under MPBCH, with clear community separation in principal coordinates analysis, significant divergence by PERMANOVA (F = 3.73, R2 = 0.599, p = 0.0005), and enrichment of bacterial biomarkers. These belowground responses coincided with improved biomass, photosynthetic pigments, gas-exchange traits, nutrient status, antioxidant defense, and lower oxidative stress markers. Transcriptomic and qRT-PCR analyses supported reduced Pb-stress status under MPBCH, with shifts in carbon metabolism, glutathione metabolism, photosynthesis, and metal transport-related genes. Multivariate integration identified MPBCH as the strongest-performing treatment across the measured Pb immobilization, plant physiological, and soil biochemical responses.