Response of methane emission to invasion of Spartina alterniflora and exogenous N deposition in the coastal salt marsh

Spartina alterniflora exhibits great invading potential in the coastal marsh ecosystems. Also, nitrogen (N) deposition shows an apparent increase in the east of China. To evaluate CH4 emissions in the coastal marsh as affected by the invasion of S. alterniflora and N deposition, we measured CH4 emission from brackish marsh mesocosms vegetated with S. alterniflora and a native plant, Suaeda salsa, and fertilized with exogenous N at the rates of 0 and 2.7 g N m−2, respectively. Dissolved porewater CH4 concentration and redox potentials in soils as well as aboveground biomass and stem density of plants were also monitored. The averaged rate of CH4 emission during the growing season in the S. alterniflora and S. salsa mesocosms without N application was 0.88 and 0.54 mg CH4 m−2 h−1, respectively, suggesting that S. alterniflora plants significantly increased CH4 emission mainly because of higher plant biomass rather than stem density compared to S. salsa, which delivered more substrates to the soil for methanogenesis. Exogenous N input dramatically stimulated CH4 emission by 71.7% in the S. alterniflora mesocosm. This increase was attributable to enhancement in biomass and particularly stem density of S. alterniflora driven by N application, which transported greater photosynthesis products than oxygen into soils for CH4 production and provided more pathways for CH4 emission. In contrast, there was no significant effect of N fertilization on CH4 emission in the S. salsa mesocosm. Although N fertilization significantly stimulated CH4 production by increasing S. salsa biomass, no significant increase in stem density was observed. This fact, along with the low gas transport capacity of S. salsa, failed to efficiently transport CH4 from wetlands into the atmosphere. Thus we argue that the stimulatory or inhibitory effect of N fertilization on CH4 emission from wetlands might depend on the gas transport capacity of plants and their relative contribution to substrates for CH4 production and oxygen for CH4 oxidation in soil.