Maize (Zea mays L.) Plants Alter the Fate and Accumulate Nonextractable Residues of Sulfamethoxazole in Farmland Soil

The fate of sulfonamide antibiotics in farmlands is crucial for food and ecological safety, yet it remains unclear. We used [phenyl-U-14C]-labeled sulfamethoxazole (14C-SMX) to quantitatively investigate the fate of SMX in a soil–maize system for 60 days, based on a six-pool fate model. Formation of nonextractable residues (NERs) was the predominant fate for SMX in unplanted soil, accompanied by minor mineralization. Notably, maize plants significantly increased SMX dissipation (kinetic constant kd = 0.30 day–1 vs 0.17 day–1), while substantially reducing the NER formation (92% vs 58% of initially applied SMX) and accumulating SMX (40%, mostly bound to roots). Significant NERs (maximal 29–42%) were formed via physicochemical entrapment (determined using silylation), which could partially be released and taken up by maize plants. The NERs consisted of a considerable amount of SMX formed via entrapment (1–8%) and alkali-hydrolyzable covalent bonds (2–12%, possibly amide linkage). Six and 10 transformation products were quantified in soil extracts and NERs, respectively, including products of hydroxyl substitution, deamination, and N-acylation, among which N-lactylated SMX was found for the first time. Our findings reveal the composition and instability of SMX-derived NERs in the soil–plant system and underscore the need to study the long-term impacts of reversible NERs.