Iron mediated redox regulation decouples sulfadiazine depletion from crop residue risk under arsenate Co-exposure: A sustainable micronutrient strategy for pharmaceutical contaminant mitigation

Pharmaceutical residues in agricultural systems increasing concerns because apparent contaminant depletion from environment does not necessarily indicate reduced crop residue risk. Here, Brassica chinensis L. was used to investigate how As (50 and 100 μM) affects the uptake, transformation and residue behavior of SDZ (60 μM), and whether Fe supplementation (50 μM), applied as a micronutrient-based mitigation strategy, could reduce these risks through redox regulatory mechanisms. SDZ-As co-exposure increased apparent SDZ dissipation from exposure solution from 44.9% to 54.3%. However, this decline in external SDZ availability was accompanied by greater SDZ accumulation in plant tissues, with root and leaf residues increasing by up to 90.7% and 47.1%, indicating that external depletion alone may underestimate internal residue risk. LC-MS showed that As altered the SDZ transformation profile, with additional higher molecular weight conjugates potentially contributing to increased internal residue retention. In contrast, Fe supplementation increased apparent SDZ depletion to 58.6% while reducing SDZ residues in tissues, whereas Fe supplementation restored tissue Fe status and reinforced POD centered redox responses. Factor-resolved transcriptomic analysis separated As driven stress amplification from Fe associated regulation, revealing Fe related, H 2 O 2 catabolic, redox and peroxidase linked responses. Functional perturbation experiments with SHAM and HRP further supported the contribution of POD related processes to Fe mediated reductions. These findings identify an Fe homeostasis-POD regulatory axis linking micronutrient status, plant redox capacity and antibiotic fate and provide a mechanistic basis for developing Fe based nutrient management approaches to reduce crop antibiotic residue risks under antibiotic and metalloid co-contamination.