Distributions and fluxes of nitrous oxide in lower reaches of Yellow River and its estuary: Impact of water-sediment regulation

Abstract The Yellow River is the second largest river in China and is well-known for its high sediment load. Since 2002, water-sediment regulation has been performed annually to scour the silted river channel in the lower reaches and to promote release of sediment from the large reservoirs. Here we present a comprehensive study of the variations in distribution and emission of dissolved N 2 O in the lower reaches of the Yellow River and its estuary based on: (i) monthly sampling at a lower-river station (Kenli) from November 2008 to December 2009, (ii) daily monitoring at this station during the water-sediment regulation event in summer of 2009, and (iii) field surveys of the estuary before, during, and after the regulation event. N 2 O concentrations in the lower reaches of the Yellow River ranged from 8.78 to 24.26 nmol/L, and had high values in winter and spring. N 2 O flux from the Yellow River to the Bohai Sea was about 2.27 × 10 5  mol/year. Water-sediment regulation had a strong impact on N 2 O distribution and transportation in the lower Yellow River. A sharp increase of N 2 O (8-fold) occurred at the beginning of water-sediment regulation, and this excessive N 2 O was likely from stimulation of nitrification in the water column. A total of 55.9% of the annual N 2 O input from the Yellow River to the Bohai Sea occurred during water-sediment regulation, but the corresponding water discharge during this period accounted for only 26.9% of total runoff. N 2 O concentrations in the lower reaches of the Yellow River and its estuary were almost all super-saturated, and this region acted as a net source of atmospheric N 2 O. High N 2 O saturations and air-sea fluxes were present in the Yellow River estuary during water-sediment regulation. These results indicate that water-sediment regulation in the Yellow River has a great impact on the estuarine distribution and atmospheric emission of N 2 O, and that this effect lasts for several weeks.