Effects Of Nitrogen And Microplastics On Seedling Establishment And Population Dispersal Of Coastal Plant

Coastal wetland plants are in a transitional intertidal ecosystem,and play an important ecological role in providing ecological barriers,carbon sinks,and protecting biodiversity.Due to the impact of human activities,the effects of eutrophication and emerging microplastic pollution on ecosystem structure and function in coastal areas has received extensive attention.A theoretical basis for the population development and wetland management under the trend of continuous pollution of coastal wetlands could be provided by studying the effects of nitrogen（N）and microplastics（MPs）pollution on plant seedling establishment and population dispersal in coastal wetlands.In this study,the typical invasive seedling Spartina alterniflora and the native seedling Rhizophora stylosa and Aegiceras corniculatum in coastal wetlands were selected as the research objects.The survival,morphological indicators,carbon content and nitrogen content of coastal plant seedlings were analyzed by setting 3 levels of N concentration（0,750,1500 mg TN/kg）and 3 levels of MPs abundance（0,4000,8000 items/kg）in the mesocosm experiment.In addition,by constructing a population dispersal model based on Cellular Automata（CA）coupled with Spartina alterniflora seedling establishment and N concentration changes,scenario simulations were carried out in combination with different environmental conditions（other physical and biological factors）and initial growth conditions（seedling survival and clonal growth）.The response law of Spartina alterniflora population dispersal rate was analyzed under nitrogen pollution conditions.The results showed that:（1）Under the present experimental conditions,changes in MPs abundance had no significant effect on the seedling survival of the invasive Spartina alterniflora and the native Rhizophora stylosa and Aegiceras corniculatum.Emergence rate of Spartina alterniflora was significantly increased by nitrogen input,but survival rate of the three seedlings was decreased.The death of mangrove seedlings was mainly due to the delayed development of their hypocotyls,while the death of Spartina alterniflora seedlings was due to the phenomenon of withering.During the seedling establishment,Growth rates of the three seedlings were significantly reduced by the increase of N concentration,and the growth process of coastal plants was inhibited.（2）Morphological characteristics of coastal seedlings mainly responded to changes in N concentration,but did not significantly respond to changes in microplastic abundance.With the increase of N concentration,the lengths of shoot and root,as well as the number of taproots and leaves,were significantly decreased in all three seedlings;Hypocotyl elongation and diameter increase of the mangrove seedlings were significantly reduced,while the number of withered leaves of Spartina alterniflora was remarkably increased;Root/shoot biomass ratio of Spartina alterniflora seedling was not significantly changed,while belowground biomass investment of the mangrove seedlings was remarkably increased.The growth and development of coastal seedlings was overall inhibited by N input.（3）Nutrient distribution of coastal seedlings was significantly altered by changes in N concentrations in the environment.With the increase of N input concentration,total nitrogen（TN）content in soil environment was significantly increased,and C:N ratio in the soil was decreased.After soil nitrogen was absorbed by the coastal seedlings,TN content of the seedlings both in the root and shoot were significantly increased.Thus,C:N ratio of the seedlings was significantly decreased,and N was accumulated in the seedling root.（4）The spread rate of Spartina alterniflora population showed a nonlinear decreasing trend with the increase of N concentration.Effects of the population dispersal based on seedling colonization would be amplified by other disturbance factors.When peak seedling density was less than 1 no./m~2,the population dispersal was sensitive to changes in N concentration,and the decrease in the overall dispersal rate was relatively large.Under the current nitrogen pollution scenario,if the N concentration was increased by 0.25,0.5 and 1g/kg,overall spread rate of Spartina alterniflora would be reduced by 20%,40%and 70%,respectively.