Differential Nitrous oxide emission and microbiota succession in constructed wetlands induced by nitrogen forms

Nitrous oxide (N₂O) emission during the sewage treatment process is a serious environmental issue that requires attention. However, the N₂O emission in constructed wetlands (CWs) as affected by different nitrogen forms in influents remain largely unknown. This study investigated the N₂O emission profiles driven by microorganisms in CWs when exposed to two typical nitrogen sources (NH₄⁺-N or NO₃^--N) along with different carbon source supply (COD/N ratios: 3, 6, and 9). The results showed that CWs receiving NO₃^--N caused a slight increase in total nitrogen removal (by up to 11.8 %). This increase was accomplished by an enrichment of key bacteria groups, including denitrifiers, dissimilatory nitrate reducers, and assimilatory nitrate reducers, which enhanced the stability of microbial interaction. Additionally, it led to a greater abundance of denitrification genes (e.g., nirK, norB, norC, and nosZ) as inferred from the database. Consequently, this led to a gradual increase in N₂O emission from 66.51 to 486.77 ug-N/(m²·h) as the COD/N ratio increased in CWs. Conversely, in CWs receiving NH₄⁺-N, an increasing influent COD/N ratio had a negative impact on nitrogen biotransformation. This resulted in fluctuating trend of N₂O emissions, which decreased initially, followed by an increase at later stage (with values of 122.87, 44.00, and 148.59 ug-N/(m²·h)). Furthermore, NH₄⁺-N in the aquatic improved the nitrogen uptake by plants and promoted the production of more root exudates. As a result, it adjusted the nitrogen-transforming function, ultimately reducing N₂O emissions in CWs. This study highlights the divergence in microbiota succession and nitrogen transformation in CWs induced by nitrogen form and COD/N ratio, contributing to a better understanding of the microbial mechanisms of N₂O emission in CWs with NH₄⁺-N or NO₃^--N at different COD/N ratios.