Vegetation alteration and nitrogen inputs (via run-off, atmospheric deposition, and wastewater) as a result of anthropogenic activities strongly affected the emission of N 2 O from coastal marshes. To gain insight into impacts of the invasion of Spartina alterniflora and N deposition on N 2 O fluxes, mesocosms experiments were conducted to measure N 2 O emissions from marshes vegetated with S. alterniflora and a native Phragmites australis, with or without exogenous N at the rates of 2.7gNm −2 , respectively. Mean N 2 O fluxes during the growing season in S. alterniflora mesocosms without N addition was 9.36μgm −2 h −1 , significantly higher than 6.79μgm −2 h −1 in P. australis mesocosms. The stimulatory effects could be attributed to higher plant biomass of S. alterniflora providing more labile organic C to the rhizosphere for nitrobacteria and denitrifying bacteria, and to more oxygen transported to the rhizosphere facilitating coupled nitrification and denitrification. N deposition increased N 2 O fluxes in S. alterniflora and P. australis mesocosms by 13.5% and 48.2%, respectively, suggesting that exogenous N significantly promoted N 2 O emissions from coastal marshes. Compared to mesocosms without N fertilization, the increase rate of total N accumulation and above-ground biomass under N addition were 63.1% and 28.0% in the S. alterniflora mesocosms, whereas 26.7% and 15.3% in the P. australis mesocosms, respectively. This meant stronger competition of S. alterniflora with soil microorganisms for the available N, leading to lower increment of N 2 O fluxes in the S. alterniflora mesocosms under N addition. Thus, it could be concluded that both the invasion of S. alterniflora and atmospheric N deposition dramatically stimulated N 2 O emissions from coastal marshes, and that N 2 O fluxes in the S. alterniflora marshes weakly responded to N addition.