Co-Regulations of Spartina alterniflora Invasion and Exogenous Nitrogen Loading on Soil N(2)O Efflux in Subtropical Mangrove Mesocosms

Both plant invasion and nitrogen (N) enrichment should have significant impact on mangrove ecosystems in coastal regions around the world. However, how N(2)O efflux in mangrove wetlands responds to these environmental changes has not been well studied. Here, we conducted a mesocosm experiment with native mangrove species Kandelia obovata, invasive salt marsh species Spartina alterniflora, and their mixture in a simulated tide rotation system with or without nitrogen addition. In the treatments without N addition, the N(2)O effluxes were relatively low and there were no significant variations among the three vegetation types. A pulse loading of exogenous ammonium nitrogen increased N(2)O effluxes from soils but the stimulatory effect gradually diminished over time, suggesting that frequent measurements are necessary to accurately understand the behavior of N-induced response of N(2)O emissions. With the N addition, the N(2)O effluxes from the invasive S. alterniflora were lower than that from native K. obovata mesocosms. This result may be attributed to higher growth of S. alterniflora consuming most of the available nitrogen in soils, and thus inhibiting N(2)O production. We concluded that N loading significantly increased N(2)O effluxes, while the invasion of S. alterniflora reduced N(2)O effluxes response to N loading in this simulated mangrove ecosystem. Thus, both plant invasion and excessive N loading can co-regulate soil N(2)O emissions from mangrove wetlands, which should be considered when projecting future N(2)O effluxes from this type of coastal wetland.