Understanding of mechanisms in nitrous oxide (N 2 O) 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 N 2 O 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 N 2 O emission from subsurface CW with NH 4 +-N (CW-A) or NO 3 −-N (CW–B) wastewater. The experimental results show that NH 4 +-N and NO 3 −-N removal efficiencies from CW mesocosms were 49.4% and 87.6%, which indirectly lead to N 2 O emission fluxes of CW-A and CW-B maintained at 213 ± 67 and 462 ± 71 μg-N/(m2·h), respectively. Correlation analysis of nitrogen conversion dynamic indicated that NO 2 −-N accumulation closely related to N 2 O emission from CW. Aquatic NH 4 +-N could up-regulate plant biomass accumulation by intensifying citric acid cycle, glycine-serine-threonine metabolism etc., resulting in more nitrogen uptake and lower N 2 O emission/total nitrogen (TN) removal ratio of CW-A compared to CW-B. Although the abundance of denitrifying bacteria and N 2 O reductase nosZ in CW-B were significantly higher than that of CW-A, after fed with mixed NH 4 +-N and NO 3 −-N influent, N 2 O fluxes and N 2 O 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 N 2 O reductase nosZ determines N 2 O emission from CW. Hence, the selection of nitrate-loving plants will play an important role in inhibiting N 2 O emission from CW. [Display omitted] • N 2 O emitted from NO 3 −-N treatment was higher than that of NH 4 +-N treatment in CW. • NO 2 −-N accumulation closely related to N 2 O emission from subsurface CWs. • Plant biomass plays an important role in low ratio of N 2 O emission/TN removal. • N form rather than N 2 O reductase nosZ determines N 2 O fluxes of subsurface CWs. [ABSTRACT FROM AUTHOR]