The Effects Of Vegetation Succession And Human Activities On The Material Circulation Of Salt Marshes

The coastal salt marsh is located on the interface between terrestrial ecosystem and marine ecosystem.It is a special type of wetland with functions of "source to sink",which is of great significance in the global material cycle alongside its sensitivity to human activities.Salt marsh vegetation plays an important role in the salt marsh ecosystem and has a major impact on the salt marsh material cycle.The succession process of the salt marsh vegetation will lead to changes in the structure and function of the salt marsh wetland,which will have an impact on the material circulation of the salt marsh.Taking the salt marsh in the south bank of Hangzhou Bay as the study area,this work investigates the effects of vegetation succession and human activities(reclamation)on the material circulation of salt marshes.In this study,a series of sediment cores and surficial sediments were collected in different vegetation zones and reclamation area from the intertidal flat of the Andong Shoal,the south bank of Hangzhou Bay.The grain size parameters,the content of Total Carbon(TC),Total Nitrogen(TN),Total Organic Carbon(TOC),210Pb radioactivity and water content,underground biomass and other related parameters were measured in laboratory.The sedimentation rate,contribution percentage of TOC and organic carbon burial rate were estimated using the data obtained by cores.On the basis of this,the effects of vegetation succession and reclamation on sediment composition,deposition rate and carbon-nitrogen cycles were analyzed.The results show that when the salt marsh vegetation shifted from the Scripus mariqueter to the Spartina alterniflora,the sediment became finer,as a result of vegetation mediation.Overall,the sediments changed from relatively coarse sediments on the bare mudflat to the relatively fine sediments on the upper marsh.The median grain size of surface sediments in the reclamation area were found similar to those in the natural baremudflat,but median grain size of core sediments was between the natural bare mudflat and the Scripus mariqueter marsh.The sedimentation rate was in the order of Scripus mariqueter(2.81 cm·yr-1)>Spartina alterniflora(2.36 cm·yr-1)>ecotone zone(2.05 cm·yr-1)>mudflat(1.59 cm·yr-1)>reclamation area(1.2 cm yr-1).Among them,the Scripus mariqueter marsh,which is closest to the bare mudflat,showed the highest sedimentation rate due to vegetation trapping on sediments.The sedimentation rate of Spartina alternifloraflat was lower than that of the Scripus mariqueter marsh,although Spartina alterniflora was generally more efficient than the Scripus mariqueter in trapping sediments.This was because Spartina alterniflora marsh is located on the upper marsh,with a limited sediment input.The reclamation area had the lowest sedimentation rate,due to the rapid decrease of sediment supply after reclamation.The carbon and nitrogen contents of surface sediments showed:1)TC and TN obeyed the law of"Spartina alterniflora>reclamation area>Scripus mariqueter>ecotone zone>bare mudflat.2)TOC data revealed a spatial variation of"Spartina alterniflora>reclamation area>bare mudflat>Scripus mariqueter>ecotone zone".The carbon and nitrogen contents in the cores showed a different pattern.The natural salt marsh displayed a landward pattern:with the landward changes of elevation and vegetation zonation,the carbon and nitrogen content of the sediment cores increased.TOC content in the reclamation area was considerably lower than that of the tidal flat.It showed the trend of carbon release after reclamation.Based on C/N ratio,the sediments of the mudflat was identified to have sea source characteristics,whilst the saltmarsh sediments showed terrestrial characteristics due to the input of aboveground biomass,and the source was also more diverse than the bare mudflat.system to estimate the contribution of TOC by vegetation species and reclamation.The results showed that the contribution percentage of TOC by Scripus mariquete was estimated to be 18.85%,the ecotone zone area was 27.10%,and the Spartina alterniflora is 45.48%.The TOC content in the reclamation area has been greatly reduced,and the carbon storage has been converted to carbon release.The burial rates of organic carbon within the study area showed"Spartina alterniflora(159 g·m-2·a-1)>Scripus mariquete(140 g·m-2·a-1)>ecotone zone(1 19 g·m-2·a-1)>tidal flat(63 g·m-2·a-1)".This forms a low value of organic carbon burial rate between Spartina alterniflora and Scripus mariquete,indicating the difference between the alien species Spartina alterniflora and the native pioneer species,Scripus mariquete,in the way of organic carbon burial.In general,in coastal salt marshes,the influence of vegetation species and reclamation is remarkably different.When vegetation succeeded from Scripus mariquete to Spartina alterniflora,sediment particles became finer,and the carbon and nitrogen content increased considerably to enhance carbon storage capbility.In contrast,the reclamation activity caused the coarsening of sediment particles,and the carbon and nitrogen contents increased in the surface sediments but rapidly decreased in the vertical direction,resulting in a decrease in carbon storage capbility.