Response Of Nitrogen Dynamic Change To Storm In Urban River Of North China

In recent years,uncertainties of global climate change leads to an increase in frequency of rainstorm in various places,and consequently the urban runoff is an important factor of the nutrient dynamic in the urban riverine system.Haihe River,as an important river in Beijing-Tianjin-Hebei Metropolitan Region,is important for regional social development and ecological civilization,and promotes the development of urban social economy and culture.In order to understand the influence of rainstorm events on the nitrogen dynamics in Haihe River?Tianjin section?,four important river sections?Yangliuqing Sluice Gate,Bei'an Bridge,Haihe Bridge,Beiyunhe Bridge?were selected as monitoring stations.River water and city road runoff samples were regularly collected before and after the rainstorm.The nutrient and nitrate isotopes were analyzed.The sources of nitrate were qualitatively analyzed by nitrogen and oxygen isotopes of nitrate,and the proportional contribution potential nitrate sources was quantitatively estimatedusing SIAR?Stable Isotope Analysis in R?mixture model.The results showed that the concentration of NO₃^--N in the sample was 0.05⁷.96 mg·L^-1,which increased obviously after the heavy rainfall event and then recovered to the previous level that before the rainfall after a week;the range of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-was-3.4‰⁺33.3‰and-0.1‰⁺30.7‰,respectively,which decreased significantly during storm events and then increased to the previous level.The compositions of nitrate isotopes indicated that NO₃^-was mainly derived from soil organic nitrogen?SON?,manure&sewage?SW?,atmospheric deposition?AT?and chemical fertilizer?CF?.The results of the SIAR model indicated that contribution ratios of nitrate sources are different before and after the rainstorm:before the rainstorm,SON accounted for 31%,AT accounted for 11%,SW accounted for 28%,CF accounted for 30%;after the rainstorm,SON accounted for 25%,AT accounted for 18%,SW accounted for 35%,CF accounted for 22%.The comprehensive analysis indicated that the response of nitrogen dynamics to rainstorm varied with different degrees,which may be related to land use pattern and hydrodynamic condition.Meanwhile,mixing,biological process and the different responses to rainfall affected the dynamic changes of nitrogen of the Haihe River in Tianjin.The nitrogen loads of Haihe River increased significantly after the rainstorm,and the impact of rainstorm on river nitrogen is difficult to eliminate quickly within a short period of time.Hence,the control and treatment of water quality in the urban rainfall runoff should be taken into consideration for urban water environment quality management.