Spatiotemporal patterns of different forms of nitrogen in a coastal mangrove wetland invaded by Spartina alterniflora

The invasion by smooth cordgrass (Spartina alterniflora) has a serious impact on ecosystem function and the ecological environment in the coastal region of subtropical China. However, little is known about the fluctuation of different forms of nitrogen in mangrove wetlands invaded by cordgrass. In this study, we compared the spatial and temporal distribution of different forms of nitrogen with or without invasion by S. alterniflora in a coastal wetland in the Jiulong River estuary, Fujian province, China. The results revealed that the variation of nitrogen concentration in the wetland was closely related to the main intertidal vegetation type, season and sampling location. Organic nitrogen accounted for the majority of the deposited nitrogen and exhibited the highest values in the mangrove zone. The content of ammonium nitrogen and nitrate nitrogen was higher in cordgrass zone and lower in the mangrove, reaching a maximum in summer. Total nitrogen concentration was significantly influenced by the tidal water level which was highest in high tide zone and lowest at low tide. Invasion of S. alterniflora significantly altered nitrogen stocks in the sediment, including the contents of total nitrogen, total organic nitrogen, ammonium nitrogen and nitrate nitrogen.