Design parameters and electrical conductivity shape dissolved organic matter aromaticity in full-scale constructed wetlands.

Constructed wetlands (CWs) often serve as final polishing systems before wastewater enters receiving waters, where active biogeochemical reactions can reshape dissolved organic matter (DOM) and influence various ecological processes in downstream aquatic systems. However, DOM variations and their associated factors in full-scale CWs remain poorly constrained. Combining a field survey of 57 sites across 14 full-scale CWs in Shandong, China, with laboratory experiments, this study examined changes in DOM characteristics, the key factors associated with these changes, and their potential roles. The results indicated that, despite no significant change in bulk DOC concentrations, CWs generally shifted DOM toward greater aromaticity and hydrophobicity and reduced low molecular weight neutrals (LN, <350 Da). Multiple statistical models consistently identified plant density (PD), electrical conductivity (EC), and light conditions (LC) as dominant factors associated with these shifts, with a predominant direct association (coefficient = 0.56) and a possible microbially linked indirect association (0.30). Specifically, the direct associations of PD and EC with enhanced DOM aromaticity and hydrophobicity may be attributable to plant litter inputs and the self-assembly of LN into building blocks (300-500 Da), respectively, while the indirect association may involve microbial processing that enriched or retained humic substances and aromatic formulas while depleting LN. In contrast, LC showed opposing effects, likely reflecting direct photodegradation and possible indirect microbial effects. Overall, CWs play a dual role by reducing labile DOM while potentially increasing ecological risks associated with aromatic effluents, and the pivotal roles of PD, EC, and LC highlight the need to align CW design with influent characteristics to balance these outcomes.