Water and nitrogen coupling regulated community regeneration invaded by Spartina alterniflora in a costal salt marsh

Global change can easily cause the structure and function of wetland ecosystems with low species diversity to be damaged by alien species. Former studies on <em>Spartina alterniflora</em> invading coastal salt marsh wetlands only focused on the effect of aboveground communities, ignoring the potential regeneration value of soil seed banks. The successful naturalization of alien species regulated by limiting factors is a bet-hedging of the niche and fitness differences between the invasive species and the native species. Through investigating the sampling plots with different aboveground coverage of<em> S. alterniflora</em> communities, representing the different invasion stages, we studied the structure and composition of the aboveground community and the soil seed banks during <em>S. alterniflora</em> invasion, in response to the soil properties and water and nitrogen addition. The <em>S. alterniflora</em> community coverage reshaped its functional traits during naturalization. Increase of <em>S. alterniflora</em> community coverage limited its height and chlorophyll. The dominant competitive advantage of <em>S. alterniflora</em> was mainly affected by the aboveground biomass, which was regulated by soil NH4+-N and moisture content. Although the species richness were same in the soil seed banks under the <em>S. alterniflora</em> communities with different coverage, <em>S. alterniflora</em> seeds maintained its specific competitive dominance due to its highest proportion in the total seed banks. The niche breadth of<em> S. alterniflora</em> and the niche overlap between<em> S. alterniflora</em> and <em>Tripolium pannonicum</em> was the highest under low aboveground coverage, while the niche overlap between <em>E. crusgalli</em> and <em>T. pannonicum</em> was the lowest, showing the independence of the aboveground coverage. The soil seed bank germination experiments showed that the plant density of alien species <em>S. alterniflora</em> decreased when the soil nitrogen concentration exceeded 1g/kg, while the plant density of native species<em> E. crusgalli</em> and <em>T. pannonicum</em> decreased when the water depth above the soil surface exceeded 2 cm. The results further showed that nitrogen-water coupling altered the plant niche and fitness of the exotic and native species germinating from the soil seed banks and their coexistence, which implied that nitrogen-water coupling affected the potential regeneration patterns of <em>S. alterniflora</em> invaded communities in the coastal salt marsh of eastern China.