| Abstract
| - Rivers receive a significant fraction of the anthropogenicnitrogen applied to the world's watersheds. Environmentalconditions in rivers should be conducive to the formationof N2O, and recent models suggest that rivers couldconstitute up to 25% of the anthropogenic contribution ofN2O to the atmosphere. Few direct measurements exist,however, of N2O flux between rivers, especially large rivers,and the overlying atmosphere. We measured theconcentration of N2O over a 2-year period in a large,tidal, freshwater river. We coupled these measurementswith a physical model of gas exchange based on inert gastracer additions to this river and computed the flux ofN2O to the atmosphere. The tidal, freshwater Hudson Riveris persistently supersaturated in N2O with respect to theatmosphere, with average partial pressure of N2O (pN2O) of0.58 μatm or about 185% of atmospheric equilibrium. Atall times during a 2-year cycle and at all locations sampledalong a 200 km stretch of the river, the river was a netsource of N2O to the atmosphere. We estimate that the tidal,freshwater Hudson River contributes 0.056 g of N2O−Nm-2 to the atmosphere annually. Despite relatively highconcentrations of NO3 in the Hudson River, the tidal, freshwaterriver is a minor source of N2O in comparison to otherrivers for which estimates exist and to components of itsown watershed. The river itself accounts for only 1.3%of the total N2O contribution to the atmosphere that occursin the Hudson watershed.
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