Nitrogen removal rates in a frigid high-altitude river estimated by measuring dissolved N 2 and N 2 O.

Rivers are important sites of both nitrogen removal and emission of nitrous oxide (N2 O), 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 N2 O of the Yellow River in its source region of the Tibetan Plateau, a frigid high-altitude environment. We measured the dissolved N2 and N2 O 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 N2 O ranged from 10.4 to 15.4 nmol N2 O L-1 . Excess dissolved N2 (△N2 ) ranged from -8.6 to 10.5 μmol N2  L-1 , while excess dissolved N2 O (△N2 O) ranged from 2.1 to 8.3 nmol N2 O L-1 ; they were relatively low compared with most other rivers in the world. However, N2 removal fraction (△N2 /DIN, average 21.6%) and EF5r values (N2 O - 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 EF5r values increased with altitude. Estimated fluxes of N2 and N2 O 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 N2 O emission from cold high-altitude rivers should be considered in the global nitrogen budget.