Research Emissions of Nitrous Oxide (N2O) from a Tidal, Freshwater River, the Hudson River, New York JONATHAN J. COLE* AND NINA F. CARACO Institute of Ecosystem Studies. Box AB, Millbrook, New York 12454
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 µatm 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.
Introduction Because N2O is both a greenhouse gas and regulator of atmospheric ozone, its rising concentration in the atmosphere has received considerable attention (1, 2). N2O is a byproduct of two different bacterial transformations of N: dissimilatory reduction of NO3 (denitrification) and the oxidation of NH4 (nitrification (3)). While these reactions occur in many environments, the conditions in aquatic ecosystems are often ideally suited to promote both denitrification in hypoxic or anoxic water and sediments and nitrification in well-oxygenated surface waters. The anthropogenic input of dissolved inorganic N (DIN ) NO3 + NH4) into many aquatic systems increases the potential for both of these reactions. In fact, much of the increase in atmospheric N2O is thought to be derived ultimately from anthropogenic disturbances * Corresponding author phone: (845)677-5343; fax: (845)677-5976; e-mail:
[email protected]. 10.1021/es0015848 CCC: $20.00 Published on Web 02/06/2001
2001 American Chemical Society
of the N cycle, especially the increased application of N fertilizers for agricultural crops (2, 4). Rivers, and river dominated estuaries, which receive about 20% of this N fertilizer, globally (5) may be especially important hot spots for the production of N2O. Based on a global land use model, Seitzinger and Kroeze (6) estimate that global N2O production from rivers alone could be as large as 1.8 Tg N y-1 or more than 30% of the anthropogenic production of N2O on land. For a number of reasons this estimate has a large uncertainty (6), but the model assumes that there is a direct connection between the input of DIN to rivers and the output of N2O gas. While the potential for rivers to contribute substantial N2O to the atmosphere is high, there have been few direct measurements of the flux of N2O to the atmosphere in rivers, especially in large rivers (e.g. ref 7). In this study we measured the export of N2O from an N-enriched river, the tidal, freshwater portion of the Hudson River, to the atmosphere over an 18-month period.
Study Site The Hudson River drains approximately 33 500 km2, and most of its watershed is in New York State. The tidal portion of the Hudson River extends for 240 km north of New York City (River km 0) to Albany, NY (River km 240). The southern 40 km is oligohaline; the northern 200 km consists of tidal freshwaters (conductivity
