Spartina alterniflora and invasive Phragmites australis stands have similar greenhouse gas emissions in a New England marsh

• P. australis and S. alterniflora had similar CO2, CH4 and N2O emissions.
• P. australis had cooler, drier sediments, with lower nutrient concentrations.
• P. australis had greater aboveground biomass, and a lower root:shoot ratio.
• Fluxes of N2O were near zero, and efflux of CH4 was generally low.
• Management for P. australis should not be based on greenhouse gas fluxes.

Salt marshes are critically important for a range of ecosystem services including nutrient filtration and carbon sequestration. Despite this, salt marshes are subjected to a variety of anthropogenic impacts, many with negative consequences. One impact is the introduction and subsequent proliferation of invasive species. An exotic strain of Phragmites australis has invaded salt marshes on the U.S. East Coast, affecting biodiversity, hydrology, and biogeochemistry. We quantified the difference between P. australis and native S. alterniflora in greenhouse gas emissions, net primary productivity, and biogeochemical parameters in sediment and porewater. Although we observed differences between the species (P. australis had greater biomass, drier and cooler sediments, and lower porewater nutrient concentrations), we observed no differences in greenhouse gas emissions. Vegetated and unvegetated plots had similar fluxes of methane (CH4) and nitrous oxide (N2O), but very different fluxes of carbon dioxide (CO2) due to photosynthesis. Fluxes of the three gases examined were related to sediment temperature and live biomass. In addition, CH4 fluxes were linked to salinity while N2O fluxes were linked to photosynthetically active radiation. CO2 fluxes varied as expected over the seasons. Rates of CH4 production were generally low and typical of salt marshes, but showed a wide variation demonstrating the high spatial heterogeneity of CH4 production. The marsh in this study was not a significant sink or source of N2O, although small fluxes of N2O were detected on occasion. This study confirms that salt marshes with relatively low nitrogen impacts are not important contributors of N2O to the atmosphere.