Nitrogen Status In Surface Water And Nitrogen Utilization Mechanisms Of Aquatic Plants In City Environments

The continuous increase in anthropogenic reactive nitrogen（N）emissions results in eutrophication of waters in many parts of the world,especially in urban areas.Nitrogen-containing pollutants were directly emitted by human activities into urban waters.As a result,the quality of waters in the urban areas and the connected downstream is deteriorated.However,there is still a lack of detailed studies of nitrogen status in both mobile and non-mobile waters in many urban areas.The mechanism of N biogeochemical process in aquatic system,especially in mobile waters is difficult to study,and there are many uncertainties about the N transformation mechanisms of waters and the utilization mechanisms of N by plants.Therefore,the chemical composition of N in urban mobile and non-mobile waters was focused in this study.The mechanism of nitrogen utilization and the response to the variation of N of aquatic plants in the urban surface waters were also explored combining with different subtypes of aquatic plants and the basic principle of N isotope geochemistry.The main conclusions are as follows:After the investigation of Nanming River in Guiyang,it was found that the water quality was from grade I to worse than Grade V,the concentrations of ammonium（NH₄⁺）,nitrate（NO₃^-）and dissolved organic nitrogen（DON）were significantly higher（p<0.05）and the proportion of NH₄⁺to TN increased when the water passed through the city.It indicated that the main N source is domestic sewage.The nitrogen content of aquatic plants significantly increased in nitrogen pollution area,indicating that the nitrogen absorption capacity of aquatic plants is increased in response to water nitrogen concentration increase.The nitrogen isotope values(δ¹⁵N)of aquatic plants did not change significantly,and there was no significant difference among plant species,indicating that theδ¹⁵N of aquatic plants cannot fully respond to the changes of nitrogen status in the waters.This may be due to the different mechanism of N utilization under the different N status in rivers.Plants mainly use NO₃^-in the upstream with low N concentration,and use NH₄⁺and NO₃^-in the downstream with high N concentration.Through the investigation of a number of non-flowing water bodies in Tianjin City,it is found that there is a great difference in water quality among the water bodies,from Grade I to worse than Grade V.The proportion of NH₄⁺to TN is increased in high-nitrogen waters,which indicates the main nitrogen source is sewage.The total nitrogen content of aquatic plants increased with the increase of inorganic nitrogen in surface water,which indicated that the nitrogen absorption of aquatic plants increased under high nitrogen environment.However,δ¹⁵N value of submerged,floating and emergent plants did not respond to changes of water nitrogen concentration,indicating that nitrogen status change of water could not be considered by using nitrogen isotope value of aquatic plants because its nitrogen utilization mechanism difference.Plants mainly use NO₃^-in low water N concentration,but aquatic plants may use NH₄⁺and NO₃^-in high nitrogen concentration.This study reveals the response mechanism of nitrogen utilization strategies of aquatic plants to nitrogen pollution of urban surface water and provides a scientific basis for nitrogen pollution assessment of urban surface water and plant restoration measures.