Seasonal shifts in dissolved organic matter composition drive the formation of nitrogenous disinfection byproducts via a carbon-nitrogen coupling mechanism in source water.

Dissolved organic matter (DOM) composition plays a critical role in disinfection byproducts (DBPs) formation, yet the molecular mechanisms governing the formation of highly toxic nitrogenous DBPs (N-DBPs) remain unclear, particularly under seasonally varying conditions. This study conducted continuous monthly fixed-point sampling for 1 year at a large drinking water source lake in Taihu, and simulated chlor(am)ination disinfection of the water samples. A total of 16,383 molecular formulas were identified, while seasonal variations were reflected by changes in the abundance and distribution of specific molecular groups. Protein-like and nitrogen-containing compounds were enriched in summer, whereas lignin-like and more aromatic compounds dominated in winter. Despite moderate total DBPs levels, winter samples exhibited the highest Chinese hamster ovary (CHO) cell LC50 values, with HANs contributing over 60% of toxicity. Molecular-level analysis revealed that HANs formation was not directly associated with nitrogen-containing DOM, but instead strongly correlated with lignin-like CHO compounds. Combined with positive relationships between HANs and ammonia nitrogen (p < 0.05), these results suggest a potential carbon-nitrogen coupling mechanism underlying the formation of nitrogenous disinfection byproducts, in which lignin-derived carbon structures act as key reactive precursors, while nitrogen availability regulates the incorporation pathways leading to N-DBPs. This mechanism explains the decoupling between total DBPs concentration and toxicity and highlights the critical role of DOM molecular composition and nutrient conditions in controlling DBPs risks. The study also reveals that targeted control measures for DBPs in drinking water and risk assessment are necessary in different seasons.