Abstract: Anthropogenic nitrogen and phosphate inputs into river exert heavy pressure on the river and coastal ecosystems. The riverine nitrate flux impacted by watersediment regulation scheme (WSRS) is unclear. Based on WSRS in the Xiaolangdi Reservoir in the Yellow River, we analyzed the nitrate (NO₃^-) concentrations and dual nitrate isotope ratios (δ¹⁵N-NO3 and δ¹⁸O-NO3) of river water during water regulation and sediment regulation, respectively, to clarify nitrate sources and their responses to WSRS. The results showed that during water regulation in late June 2018, the median and mean value of NO₃^- concentrations were 13.11 mg·L^-1 and 13.04±1.20 mg·L^-1 (n=11), respectively, and the median and mean values were .8‰ and 2.2‰±1.6‰ (n=11) for δ¹⁵N-NO3, and 8.8‰ and 9.2‰±2.1‰ (n=11) for δ¹⁸O-NO3, respectively. During the sediment regulation in early July 2018, the median and mean values of NO₃^- concentrations were 15.46 mg·L^-1 and 15.42±1.26 mg·L^-1 (n=10), respectively. The δ¹⁵N-NO3 had the median and mean values of 1.3‰ and 0.6‰±2.1‰ (n=10), and δ¹⁸O-NO3 had the median and mean values of 9.4‰ and 8.9‰±1.7‰ (n=9), respectively. The results of one-way ANOVA showed significant difference in NO₃^- and δ¹⁵N-NO3 values (P<0.05) between the two phases, but not for δ¹⁸O-NO3values (P>0.05). The nitrate of main stream riverine was mainly derived from soil nitrate and chemical fertilizer nitrate. During water regulation, the fractions of soil nitrate and chemical fertilizer nitrate were 15.1%±11.2% and 63.7%±10.8%, respectively. During the sediment regulation, their contributions were 11.2%±10.6% and 70.4%±11.6%, respectively. The more contributions of chemical fertilizer to riverine nitrate during sediment regulation were probably resulted from chemical fertilizer nitrate in soil due to waterlogging. We discussed the factors affecting nitrate behaviors during WSRS in the Yellow River. The increases of nitrate fluxes in the sediment regulation were due to chemical fertilizer nitrate in the soil induced by the artificial flood, with potential impacts on the downstream aquatic ecosystem in the Yellow River.

Key words: Xiaolangdi Reservoir in the Yellow River, water-sediment regulation scheme, nitrate, nitrogen and oxygen isotopes, ammonium oxidation.