Distribution and emission of NO in the largest river-reservoir system along the Yellow River.

Rivers and reservoirs are affected by human activities and are sources of the greenhouse gas nitrous oxide (NO). Concentrations of NO in the middle and lower reaches of the Yellow River and Xiaolangdi Reservoir, China, were measured in June and December 2017. Fluxes were estimated by boundary layer method to explore their controlling factors, especially the impact of damming and reservoir operation. In the middle and lower reaches of the Yellow River, NO concentrations in surface waters were 26.65 ± 14.67 nmol L in summer and 21.16 ± 5.35 nmol L in winter. In comparison, the concentrations of NO in the reservoir were 32.94 ± 17.32 nmol L in summer and 23.73 ± 5.60 nmol L in winter. The longitudinal distribution of NO along the river exhibited different patterns with surface NO decreasing downstream towards the dam in summer but increasing in winter. Vertical profiles of NO concentrations in the reservoir showed an increase with depth in summer but were almost vertically uniform in winter. In winter, NO that had accumulated in the bottom water in summer was transported to the surface by vertical mixing and released into the atmosphere. Dissolved oxygen (DO), water temperature, and in situ biological production were the main factors affecting the distribution of NO. The mean emissions rates of NO from the surface waters were 13.7 ± 8.8 μmol m d in summer and 13.2 ± 7.6 μmol m d in winter. Approximately 1.31 × 10 mol NO was released from the reservoir surface in 2017, which represents 0.12% of the annual NO emissions from global reservoirs. The construction of dams increased NO emission from the lower reaches of the river by 4.53 × 10 mol and 1.22 × 10 mol due to the discharge of the bottom water and the water and sediment regulation, respectively. This study demonstrates that the construction of dams and reservoir operation practices have made the Xiaolangdi Reservoir a key area for NO emissions.