Effects Of Xiaolangdi Dam On The Spatiotemporal Dynamics Of Nitrate In Riparian Wetlands Water Along The Lower Yellow River

Riparian wetlands along the Yellow River serve as an important part of the ecological civilization construction of the Yellow River basin,playing a key role in water conservation,water purification,climate regulation and biodiversity maintenance.Nitrate pollution in water has attracted global attention.In terms of wetlands,high levels of nitrate content in water will lead to the decrease of self-purification capacity of water,resulting in water quality deterioration,hypoxia,and even eutrophication.The high levels of nitrate content in wetlands water will eventually change the wetland function and affect the surrounding ecosystem.The Xiaolangdi Dam(XLD),located at the exit of the last gorge in the middle reaches of the Yellow River,is one of these projects.The water-sediment regulation scheme(WSRS)of XLD has a critical influence on hydrological processes of the lower reaches.Therefore,to clarify the spatiotemporal dynamics of nitrate in riparian wetlands water and its response to hydrological changes of the Yellow River caused by WSRS is meaningful for reasonable watershed management.Combined with the hydrochemical and isotope characteristics of the Yellow River water,canal water,regional precipitation and groundwater,this study takes riparian wetlands along the Yellow River as in the suspended river section the research object,adopts the Piper diagram,B ayesian mixed model(SIAR model),and redundancy analysis to analyze the sources,change characteristics and the main transport pathways of nitrates in the riparian wetlands water.Further,this study analyzes the main driving factors caused by wetland nitrate change in different hydrological periods.The main conclusions are as follows:(1)There was a close connection between the riparian wetlands water and the Yellow River in different hydrological periods.Result showed that there are differences in the hydrological parameters of the Yellow River and the physicochemical properties of wetland water in different hydrological periods.Among them,the pH and dissolved oxygen(DO)content in wetlands water had significant differences due to the influence of human activities(P<0.05).Piper diagram showed that the almost all wetlands have the same hydrochemical types with the Yellow River water in different hydrological periods.Before the WSRS period,hydrochemical types showed the Ca-Mg-Cl type in these waters,which converted into CaHCO3,Na-Cl types and Ca-HCO3 type during and after the WSRS period,respectively.Combined with the lateral seepage mechanism of the lower Yellow River,the results indicated that there was a strong hydrological connection and water recharge from the Yellow River to riparian wetlands.(2)There were spatial and temporal differences in nitrate content,nitrogen and oxygen isotope values in riparian wetlands in different hydrological periods.There was no statistical difference(P>0.05)in the spatial scale of nitrate in riparian wetlands water because they were both influenced by the Yellow River,while there was a significant difference(P<0.05)in the temporal scale of nitrate in riparian wetlands water due to the intra-annual hydrological variations of the Yellow River.Moreover,denitrification in riparian wetlands water was weak and had a small effect on nitrogen and oxygen isotopes in nitrate(15N-NO3-,18O-NO3-).(3)The traceability results of nitrate in the riparian wetlands water showed that contribution of major sources of nitrate in wetland water inside and outside the Yellow River dykes was different because of precipitation and human activities.Before the WSRS period,the main sources of nitrate in the riparian wetlands water inside the Yellow River dykes were chemical fertilizers(34%),soil nitrogen(32%),domestic sewage and manure(32%),while it in the riparian wetlands water outside the Yellow River dykes was the soil nitrogen(41%).During the WSRS period,the main source of nitrate in riparian wetland water inside and outside the Yellow River dykes was fertilizer(46%,35%),which converted into chemical fertilizers(44%,50%)after the WSRS period.(4)In different hydrological periods,the migration pathways of nitrate in the riparian wetlands were different.Results showed that the main transport pathway of nitrate in riparian wetland water inside and outside the Yellow River dykes changed from Yellow River water(37%and 45%)before the WSRS period to groundwater(51%and 45%)after the WSRS period,which is mainly attributable to the rising groundwater level caused by the hydrological changes of the Yellow River and regional precipitation.(5)RDA showed that the main influence factor for the dynamic change of nitrate before and during the WSRS periods were Ca2+,altitude,air temperature and K+.The dynamic change of nitrate after WSRS had a strong positive correlation with DO and HCO3-,and a strong negative correlation with air temperature,altitude and pH.RDA of the contribution ratios of different nitrate sources showed that the hydrological status of the Yellow River was the main factor caused by the temporal and spatial changes in the contribution ratios of precipitation,chemical fertilizers and soil nitrogen in riparian wetlands water.In addition,the changes in the contribution ratio of soil nitrogen,domestic sewage and manure were affected by groundwater and canal water,respectively.