Effects of invasion of Spartina alterniflora and exogenous N deposition on N2O emissions in a coastal salt marsh

Vegetation alteration and nitrogen inputs (via run-off, atmospheric deposition, and wastewater) as a result of anthropogenic activities strongly affected the emission of N2O from coastal marshes. To gain insight into impacts of the invasion of Spartina alterniflora and N deposition on N2O fluxes, mesocosms experiments were conducted to measure N2O emissions from marshes vegetated with S. alterniflora and a native Phragmites australis, with or without exogenous N at the rates of 2.7 g N m−2, respectively. Mean N2O fluxes during the growing season in S. alterniflora mesocosms without N addition was 9.36 μg m−2 h−1, significantly higher than 6.79 μg m−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 N2O fluxes in S. alterniflora and P. australis mesocosms by 13.5% and 48.2%, respectively, suggesting that exogenous N significantly promoted N2O 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 N2O 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 N2O emissions from coastal marshes, and that N2O fluxes in the S. alterniflora marshes weakly responded to N addition.