Enhancing wastewater denitrification in constructed wetlands: microbial mechanisms driven by lotus leaf-based carbon

The performance of constructed wetlands (CWs) in treating low C/N domestic wastewater is often constrained by limited carbon availability for denitrification. This study proposes an innovative approach using alkali-pretreated lotus leaves as a biodegradable, slow-release carbon source to enhance denitrification efficiency and uncover the underlying metabolic and microbial mechanisms. Results demonstrated that wet lotus leaves-especially those wet-4% NaOH-released carbon more efficiently than dried leaves, with a peak rate of 25.93 mg g^-1·h^-1 (calculated as ΔCOD/Δt normalized to wet mass and measured within the first 20 h of the static experiment). This pretreatment also mitigated nitrogen and phosphorus leaching risks. Supplementation with 400 g of 4% NaOH-treated wet lotus leaves (CW2) significantly improved the TN removal efficiency, reaching a peak of 91.89 ± 3.56% during days 1-16, with an overall average of 85.63% ± 7.72%. Mechanistically, tryptophan-like DOM signals (EEM) suggest that small-molecule soluble organics released from plant-derived carbon sources can be rapidly taken up and utilized by denitrification-associated microorganisms, thereby supporting the denitrification process. Microbial characteristics analysis revealed that CW2 enriched microbial richness (Shannon = 6.96; Chao 1 = 2015.9) and shifted community composition toward denitrification-favorable taxa. Notably, Proteobacteria abundance increased from 21.5% to 38.5%, accompanied by the proliferation of key genera including Pseudomonas and Janthinobacterium. The taxa associated with nitrifiers, denitrifiers, and anammox bacteria collectively contributed to a more robust nitrogen removal pathway. These findings suggest that the supplementary carbon source-by regulating dissolved oxygen distribution, supplying bioavailable carbon, and establishing spatially structured redox gradients-strategically modulates microbial ecology and functional metabolism, offering a low-cost and sustainable solution for enhancing nitrogen removal in constructed wetlands.