Nitrogen removal rates in a frigid high-altitude river estimated by measuring dissolved N and NO.

Rivers are important sites of both nitrogen removal and emission of nitrous oxide (NO), 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 N and NO of the Yellow River in its source region of the Tibetan Plateau, a frigid high-altitude environment. We measured the dissolved N and NO using N:Ar ratio method and headspace equilibrium technique, respectively. Dissolved N in the river water ranged from 337 to 513 μmol N L, and dissolved NO ranged from 10.4 to 15.4 nmol NO L. Excess dissolved N (△N) ranged from -8.6 to 10.5 μmol N L, while excess dissolved NO (△NO) ranged from 2.1 to 8.3 nmol NO L; they were relatively low compared with most other rivers in the world. However, N removal fraction (△N/DIN, average 21.6%) and EF values (NO - N/NO - N, range 1.6 × 10-5.0 × 10) were comparable with many other rivers despite the high altitude for the Yellow River source region. Furthermore, the EF values increased with altitude. Estimated fluxes of N and NO to the atmosphere from the river surface ranged from -67.5 to 93.5 mmol N m d and from 4.8 to 93.8 μmol N m d, respectively, and the nitrogen removal from rivers was estimated to be 1.87 × 10 kg N yr 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 NO emission from cold high-altitude rivers should be considered in the global nitrogen budget.