A Study On The Influence Mechanism Of Ecological Restoration In Lakeshore Wetlands On Denitrification And Its Strengthening Technology

Lakeshore wetlands,acting as junctions linking terrestrial and aquatic ecosystems,have many key ecological functions including nitrogen（N）removal.However,the hydrology of lakeshore wetlands is usually damaged by anthropogenic activities such as dam construction,impoldering,etc.,and the damaged hydrology may induce the degradation of wetlands.In order to restore the degraded wetlands,numbers of projects have been implemented globally.The coupled nitrification-denitrification process has been recognized as the primary mechanism behind permanent N removal in wetlands,which converts ammonium（NH₄⁺）to nitrate（NO₃^-）via nitrification and ultimately loss to the atmosphere as nitrous oxide（N₂O）or dinitrogen（N₂）via denitrification.Nitrification and denitrification are microbial-mediated processes which can be impacted by hydrological restoration.In this study,we used the global data to examine the response of sediment denitrification in wetlands to hydrological restoration through Meta-analysis.Thereafter,we conducted field surveys on the lakeshore wetlands around the Dianchi Lake to study the response of sediment N removal rates to hydrological reconnection.In order to discriminate the primary factors affecting sediment N removal rates,we conducted field surveys on the Chuanfang River.Ultimately,we tested the influence of carbon（C）and sulphur（S）addition on the N removal rates through two mesocosm experiments.The results of our study are as follows:1.The results of overall Meta-analysis showed that the unamended denitrification rates（UDNR）were incresed by 58%after hydrological restoration while the potential denitrification rates（PDNR）were decreased by 30%.The results of categorical Meta-analysis suggested that various wetland types responded differently to hydrological restoration.In addition,clime,hydrology and plant can affect denitrification in wetlands through regulating nutrient concentrations and bacterial activity in sediment.In the global scale,the hydrological restoration usually stimulates the N removal rates in wetlands,while the impact on specific wetland types is different.2.We compared variation in sediment N removal rates to assess the restoration benefits of N removal in a series of recently hydrologically restored lakeshore wetlands,including ponds and bottomlands,of Dianchi Lake.The results showed that nitrification rates were generally higher in the ponds,while denitrification rates were higher in the bottomlands.Hydrological reconnection stimulated the development of several sediment properties critical for N cycling rates in the ponds,including increases in sediment C and N contents;however,bottomland reconnection increased sediment moisture and decreased sediment C and N contents likely due to erosion by wind-induced wave action.Correspondingly,hydrological reconnection significantly increased the sediment N removal rates in ponds but decreased the sediment N removal rates in bottomlands over time.Path analyses revealed that substrate characteristics,including moisture and C and N availability,were the critical drivers regulating wetland N removal rates.These results imply that the restoration targets could not be met simply by hydrological reconnection.Future wetland restoration requires further understanding of the relationship between changes in sediment properties and biogeochemical processes.3.We conducted field surveys on Chuanfang River,an urban river with low C:N,to examine the N removal rates,including nitrification and denitrification,at different river sites along the river reach.We also measured various environmental factors to distinguish their relative contribution to the variance of N removal rates.Our results showed an increasing trend in sediment N removal rates from source to downstream sites along the river reach,whereas the concentrations of N in water showed a declining tendency.Variance partitioning and path analysis suggested that water physicochemical properties,water nutrients,sediment characteristics,and submerged vegetation were the primary factors determining sediment N removal rates.Especially,the results of path model and Pearson correlation analysis indicated that the dissolved oxygen in water,C and N in sediment and submerged vegetation had positive effects on the N removal rates,whereas excessive water N loading had negative effects.Our study indicates the great potential of river ecosystem for N removal,while underline the restriction of low sediment C for nitrification and denitrification.4.We set a carbon packed bed reactor system to examine the response of N removal rates in low C:N water to different organic C addition.Simultaneously,we set an in situ mesocosm experiment in lakeshore wetland of Dianchi Lake and added biochar and elemental S into the sediment to study the influence of C and S addition on sediment N removal rates and associating functional gene.The results showed that the N removal rates were higher with slow-release organic C than quick-acting organic C.Although biochar could not significantly promote the sediment nitrification and denitrification,it could rapidly adsorb the ambient free N and reduce the N concentration in water.The elemental S could promote the sediment denitrification rates and associating functional gene abundance because it was typically coupled with the denitrification process.The results of experiments suggest that exogenous C and S additions can be considered as methods for ecological restoration of wetlands.Our study revealed the influence mechanism of hydrological restoration on the N removal rates in wetlands,we found the primary limiting factors for nitrification and denitrification,and we also designed experiments for strengthening the denitrification process.In short,our study can provide theoretical basis and technical support for ecological restoration of wetlands.