Potential Ecological Impact Of Physical Control Of Spartina Alterniflora And The Reuse Of Treatment Waste In Coastal Wetlands

As a perennial salt marsh grass,Spartina alterniflora（S.alterniflora）is native to the coastal salt marsh areas along the east coast of North America and the Gulf of Mexico.Since S.alterniflora was introduced into coastal areas of China in the 1970s,it has seriously threatened the native species richness and coastal wetland ecosystem,and led to the degradation of coastal wetlands due to its strong invasion ability.In recent years,the control of S.alterniflora has been carried out in coastal areas of China.In the application process,the chemical control method has the risk of chemical pollution,while the biological control method is easy to cause the second invasion of alien species.As an environmentally friendly control strategy for S.alterniflora,physical control treatments,including mowing（M）,"mowing+flooding"（MF）,and"mowing+ploughing"（MP）,have been widely used in controlling S.alterniflora in coastal wetlands.Environmental disturbances inevitably caused by physical control may affect biogenic elements migration and transformation at the water-soil interface and soil bacterial community structure in coastal wetlands,while there is still a lack of systematic and in-depth research on this issue.As a high-quality biomass resource,S.alterniflora has high photosynthetic efficiency,growth rate,population density,productivity and large community biomass.The utilization of S.alterniflora resources,such as the preparation of functional biochar（BC）to restore degraded coastal wetlands,could reduce the prevention and control cost,and promote the sustainable control and management of S.alterniflora,as well as maintain the biodiversity and restore the ecological function of coastal wetlands.In this study,tidal cycle simulation devices were constructed to examine the migration and transformation mechanism of biogenic elements in coastal wetland soil at the water-soil interface under different S.alterniflora physical treatments（M,MF and MP）,and reveal the effects of management on soil bacterial community from the perspective of microbiology.At the same time,the waste of S.alterniflora was used to prepare P and Mg modified biochar（BC）to amend the coastal wetland soil and restore indigenous plant Suaeda salsa（S.salsa）community,and the effect mechanism was explored.The main results obtained were as follows:（1）Compared with M treatment,MF and MP treatments completely cleaned up the S.alterniflora without regeneration during the experimental period.However,flooding,ploughing,and the death of S.alterniflora during the treatment process could alter the soil microenvironment,which resulted in the changes of soil organic matter（SOM）,dissolved organic carbon（DOC）,ammonia nitrogen（NH4⁺-N）,total nitrogen（TN）,total phosphorus（TP）and salinity.The changes of soil microenvironment increased the release of endogenous nutrients in coastal wetland.MF treatment respectively increased the releases of NH4⁺-N,TP,and the average concentration of DOC by 37.3%,10.1%and 52.5%.MP treatment respectively promoted the releases of NH4⁺-N,nitrate nitrogen（NO3^--N）,nitrite nitrogen（NO2^--N）and the average concentration of DOC by 13.7%,19.6%,66.7%and36.8%.Water quality results and PARAFAC analysis of dissolved organic matter（DOM）confirmed that the release of endogenous nutrients and their components might increase the risk of eutrophication in offshore waters.（2）Three physical control treatments for S.alterniflora changed the soil microenvironment,which affected the structure and composition of bacterial community.Mantel test analysis and structural equation model analysis showed that the changes of biotic elements and their direct effects caused by MF and MP treatments significantly changed the structure of bacterial community in soil.Continuous flooding and higher salinity in MF treatment negatively affected bacterial community structure by creating soil microenvironment with poor spatial heterogeneity,while decomposition of S.alterniflora and ploughing in MP treatment significantly increased the richness and diversity of bacterial community through sufficient nutrient supply and suitable environment.The co-occurrence network of bacterial community showed that the activities and interactions of bacteria in MP treatment were more active and denser than those in MF treatment.（3）P-Mg modified BC materials（PA-Mg-BC and DAP-Mg-BC）were successfully prepared from the waste straw of S.alterniflora,which were proved to be potential soil amendments for coastal wetlands.The results of XPS,FTIR and XRD indicated that the modified BC formed a stable Mg-P complex with low solubility（Mg2P2O7）.The sustained release kinetics of P further proved that the modified BC delayed the rapid release of P,and the addition of Mg O improved the tolerance of modified BC to the competitive anions SO4^2-and CO3^2-during the release of P.In addition,compared with unmodified BC,the yields as well as their C/N and C/H atomic ratios of the two modified BC materials were higher,indicating that the pretreatment of adding P source and Mg O could improve the polarity and reduce the aromatization of modified BC materials.（4）Pot experiments showed that proper application of PA-Mg-BC and DAP-Mg-BC could significantly improve soil quality in coastal wetland.The contents of soil organic carbon（SOC）,DOC,TN,TP,available phosphorus（AP）,andβ-glucosidase activity were significantly increased（p<0.05）,the DOM components were significantly improved,as well as the soil p H and salinity decreased to varying degrees.In addition,the addition of modified BC materials improved the soil microenvironment to a certain extent,which provided a favorable condition for bacterial growth resulting in the increase of bacterial community diversity.RDA analysis showed that there was a negative correlation between SOC,TN,AK,DOC,TP,AP,β-glucosidase and salinity in soil with modified BC materials addition,confirming that PA-Mg-BC and DAP-Mg-BC prepared in this experiment could alleviate environmental stress and beneficial to soil bacteria.The addition of modified BC materials increased the relative abundances of Bacillus,Thalassobacillus,Marinobacter,and Salinimicrobium,which playing important roles in resisting environmental stress and affecting carbon/nitrogen cycle in soil,indicating the composition of soil bacterial community had changed in a benign way.The modified BC materials could improve soil quality and soil environment,and promote the growth of S.salsa through enhancing the absorption of nutrients by plants,which have great potential for vegetation restoration in the future.This study could provide a scientific basis for the prevention and control of S.alterniflora in coastal wetlands,and also provide a feasible method for the ecological restoration of coastal wetlands and the resource utilization of S.alterniflora.