Dissimilatory nitrate reduction processes and corresponding nitrogen loss in tidal flow constructed wetlands

Nitrate reduction processes play significant roles in the retention and removal of nitrogen from water in anaerobic soils of constructed wetlands. The microbial mechanisms regulating nitrate reduction in artificial tidal wetlands remain poorly understood. The current study explored three microbial nitrate reduction processes in two tidal wetland mesocosms, PA (planted with Phragmites australis) and NP (unplanted) based on molecular and stable isotopic analyses. Potential rates of anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and denitrification ranged between 2.54 and 8.77, 0.12–0.93 and 40.11–387.94 nmol N₂ g⁻¹ ds (dry soil) d⁻¹, with relatively higher values obtained in PA than NP. Denitrification contributed to 93.78–97.71% of total nitrate reduction, compared to 1.99–5.94% and 0.23–0.40% via anammox and DNRA, as 94.04–98.00% of nitrogen gas was produced through denitrification, with the rest via anammox. The absolute abundances of bacterial 16S rRNA and nitrogen functional genes were 3.22–9.11 × 10⁹ and 0.8 × 10⁴–1.1 × 10⁸ copies g⁻¹ ds, respectively. Proteobacteria, Chloroflexi, Acidobacteria, Nitrospirae, and Actinobacteria were the dominated phylum in artificial tidal wetlands, accounting for over 70%. The relative abundances of denitrification and anammox organisms varied from 0.97% to 3.60% and from 0.02% to 0.14%, respectively. Pearson correlation analyses indicate that nitrate played a vital role in the competition between DNRA and denitrification, while nitrite was the major factor regulating anammox. Collectively, these findings improve our understanding of nitrate removal mechanisms in tidal flow constructed wetlands, providing a sustainable solution to the nitrate pollution problem in the Yangtze River Estuary.