Emissions of nitrous oxide (N2O) from a tidal, freshwater river, the Hudson River, New York.

Rivers receive a significant fraction of the anthropogenic nitrogen applied to the world's watersheds. Environmental conditions in rivers should be conducive to the formation of N2O, and recent models suggest that rivers could constitute up to 25% of the anthropogenic contribution of N2O to the atmosphere. Few direct measurements exist, however, of N2O flux between rivers, especially large rivers, and the overlying atmosphere. We measured the concentration of N2O over a 2-year period in a large, tidal, freshwater river. We coupled these measurements with a physical model of gas exchange based on inert gas tracer additions to this river and computed the flux of N2O to the atmosphere. The tidal, freshwater Hudson River is persistently supersaturated in N2O with respect to the atmosphere, with average partial pressure of N2O (pN2O) of 0.58 muatm or about 185% of atmospheric equilibrium. At all times during a 2-year cycle and at all locations sampled along a 200 km stretch of the river, the river was a net source of N2O to the atmosphere. We estimate that the tidal, freshwater Hudson River contributes 0.056 g of N2O-N m(-2) to the atmosphere annually. Despite relatively high concentrations of NO3 in the Hudson River, the tidal, freshwater river is a minor source of N2O in comparison to other rivers for which estimates exist and to components of its own watershed. The river itself accounts for only 1.3% of the total N2O contribution to the atmosphere that occurs in the Hudson watershed.