Tracing Nitrate Sources of the Yellow River and Its Tributaries With Nitrogen Isotope

Human activities have greatly affected the nitrogen(N) cycles in the terrestrial and aquatic ecosystems.The Xiaolangdi Reservoir on the Yellow River and the mainstream and tributaries of the River thereafter,including Qin River and Yiluo River,were selected as subjects in this study to trace potential nitrate sources of the waters with nitrogen isotope.It was found that the average nitrate content in the mainstream of the Yellow Rvier and its tributaries,Qin River and Yiluo River,was(4.77±0.95) mg·L-1,(3.45±1.71) mg·L-1 and(4.50±0.91) mg·L-1,respectively;and the average δ15N-NO-3 in the mainstream was(+3.2±4.5)‰.Dissolved nitrate in upper stream of the Yellow River in the studied area came mainly from mineralized soil organic nitrogen,and in the rivers of the lower reaches from mineralized soil organic nitrogen and fertilizers applied.The average δ15N-NO-3 was(+8.3±4.6)‰ in the Qin River.Its nitrate came mainly from atmospheric precipitation,mineralized soil organic nitrogen and fertilizers during the high water season,and from sewage water and mineralized soil organic nitrogen during the normal water season,and from growth of phytoplankton and cyanobacteria,and denitrification in enclosed waters formed after the river discontiunued its flow during the low water season.Druing the low water season,the average δ15N-NO-3 was +10.9‰ and +3.4‰ in the Luo River and the Yi River respectively.In the former,sewage was the main source of nitrate,while in the latter the fertilizers.