Biochar mitigates polyethylene and oxytetracycline stress in Amaranthus tricolor by improving soil properties, reducing oxidative damage, and moderating CO2 emissions

Microplastics (MPs) and antibiotics (ATs) frequently co-occur in agricultural soils, but their combined impacts on soil–plant systems and potential remediation options remain unclear. This 60-day pot experiment tested whether maize biochar (20 t ha-1) can alleviate the effects of polyethylene (PE, 10 g kg-1) and oxytetracycline (OTC, 50 mg kg-1) on soil properties, antioxidant enzyme activities, and A. tricolor growth. Co-exposure to PE and OTC significantly reduced soil nutrient availability, suppressed root development by 23%, and decreased aboveground biomass by 45% relative to the control, while pollutant treatments enhanced antioxidant enzyme activities, indicating induced oxidative stress rather than improved defense alone. Biochar application increased soil pH, cation exchange capacity, and nitrogen availability (NO3--N and NH4+-N by 22% and 19%, respectively), and partially mitigated contaminant-induced declines in photosynthesis, root traits, and biomass. Consistent with the CO2 measurements, biochar alone elevated short-term soil CO2 emissions by 49% and 27% at 30 and 60 days, respectively, whereas PE+OTC reduced CO2 fluxes by 55% and 29%, with biochar partly offsetting this suppression rather than uniformly lowering CO2. These findings suggest that corn straw biochar can alleviate phytotoxic effects of PE-OTC co-contamination and support crop production by improving soil nutrient status and moderating oxidative stress responses, while simultaneously altering soil carbon dynamics. From an applied perspective, biochar shows promise for managing co-contaminated vegetable soils, but future work should resolve underlying mechanisms especially pollutant bioavailability, microbial processes, and antibiotic resistance genes to guide safe, field-scale implementation.