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 EF5r 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 EF5r 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.