Nitrogen removal rates in a frigid high-altitude river estimated by measuring dissolved N₂ and N₂O

Rivers are important sites of both nitrogen removal and emission of nitrous oxide (N₂O), a powerful greenhouse gas. Previous measurements have focused on nitrogen-rich temperate rivers, with cold, low-nitrogen river systems at high-altitude receiving less attention. Here, nitrogen removal rates were estimated by directly measuring dissolved N₂ and N₂O of the Yellow River in its source region of the Tibetan Plateau, a frigid high-altitude environment. We measured the dissolved N₂ and N₂O using N₂:Ar ratio method and headspace equilibrium technique, respectively. Dissolved N₂ in the river water ranged from 337 to 513 μmol N₂ L⁻¹, and dissolved N₂O ranged from 10.4 to 15.4 nmol N₂O L⁻¹. Excess dissolved N₂ (△N₂) ranged from −8.6 to 10.5 μmol N₂ L⁻¹, while excess dissolved N₂O (△N₂O) ranged from 2.1 to 8.3 nmol N₂O L⁻¹; they were relatively low compared with most other rivers in the world. However, N₂ removal fraction (△N₂/DIN, average 21.6%) and EF_5r values (N₂O − N/NO₃ − N, range 1.6 × 10⁻⁴–5.0 × 10⁻²) were comparable with many other rivers despite the high altitude for the Yellow River source region. Furthermore, the EF_5r values increased with altitude. Estimated fluxes of N₂ and N₂O to the atmosphere from the river surface ranged from −67.5 to 93.5 mmol N m⁻² d⁻¹ and from 4.8 to 93.8 μmol N m⁻² d⁻¹, respectively, and the nitrogen removal from rivers was estimated to be 1.87 × 10⁷ kg N yr⁻¹ for the Yellow River source region. This is the first report of nitrogen removal for a frigid high-altitude river; the results suggest that N removal and N₂O emission from cold high-altitude rivers should be considered in the global nitrogen budget.