Interactive effects of aquatic nitrogen and plant biomass on nitrous oxide emission from constructed wetlands.

Understanding of mechanisms in nitrous oxide (NO) emission from constructed wetland (CW) is particularly important for the establishment of related strategies to reduce greenhouse gas (GHG) production during its wastewater treatment. However, plant biomass accumulation, microbial communities and nitrogen transformation genes distribution and their effects on NO emission from CW as affected by different nitrogen forms in aquatic environment have not been reported. This study investigated the interactive effects of aquatic nitrogen and plant biomass on NO emission from subsurface CW with NH-N (CW-A) or NO-N (CW-B) wastewater. The experimental results show that NH-N and NO-N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to NO emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 μg-N/(m·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO-N accumulation closely related to NO emission from CW. Aquatic NH-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower NO emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and NO reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH-N and NO-N influent, NO fluxes and NO emission/TN removal ratio in CW-A were extremely close to that of CW-B, suggesting that nitrogen form rather than nitrogen transformation microbial communities and NO reductase nosZ determines NO emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting NO emission from CW.