Interactive effects of bacteria-loaded biochar on the physiological responses of Brassica rapa var. chinensis in the Pb and Zn contaminated soil

• Bacteria-loaded biochar significantly immobilizes Pb and Zn in contaminated soil, reducing exchangeable fractions by up to 98–99% and increasing residual fractions substantially. • The amendments enhance Brassica rapa growth by improving chlorophyll content and antioxidant enzyme activities (SOD, POD), mitigating oxidative stress. • Bacteria-loaded biochar treatment optimizes photosynthetic parameters, including stomatal conductance and intercellular CO2 concentration. • Malondialdehyde (MDA) content, a stress biomarker, is reduced by up to 41.6% under high Pb/Zn stress, indicating effective alleviation of metal-induced stress.