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

• Dissolved N2O showed seasonal variations (∼3-fold) in the lower Yellow River.
• The lower reaches and estuary of Yellow River were net sources of atmospheric N2O.
• N2O increase (∼8-fold) abruptly at the beginning of water-sediment regulation.
• Water-sediment regulation increased N2O transport (∼2-fold) and emission (∼2-fold).

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 N2O 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. N2O 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. N2O flux from the Yellow River to the Bohai Sea was about 2.27 × 105 mol/year. Water-sediment regulation had a strong impact on N2O distribution and transportation in the lower Yellow River. A sharp increase of N2O (8-fold) occurred at the beginning of water-sediment regulation, and this excessive N2O was likely from stimulation of nitrification in the water column. A total of 55.9% of the annual N2O 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. N2O 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 N2O. High N2O 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 N2O, and that this effect lasts for several weeks.

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