Lotic intersite nitrogen experiment II: rates andmechanisms of nitrate retention in streams from reaches tolandscapes

Hamilton, S.K. and the multisite LINX team

Presented at the All Scientist Meeting (2004-10-08 )

Human activities such as fossil fuel combustion, agricultural fertilizer application and legume cultivation are adding large amounts of fixed nitrogen to terrestrial and aquatic ecosystems. This added nitrogen causes a number of serious environmental problems such as acidification of soil and water, harmful algal blooms, and impairment of drinking water supplies. Despite the importance of these problems, we have a poor understanding of the fate of this added nitrogen. Studies of large river basins show that most of the added nitrogen is not exported to the oceans but has disappeared from our accounting somewhere within these basins. Recent research has indicated that uptake and removal by plants and microbes in streams and rivers might account for some of the missing nitrogen. In particular, it appears that small streams in the headwaters of river basins are most important in removing nitrogen. To gain a better understanding of the role that small streams play in removing nitrogen from water and preventing it from polluting downstream ecosystems, experiments are being performed in 72 streams in eight different regions of the United States. To determine how humans affect the ability of streams to take up and remove nitrogen, streams in agricultural and urban areas as well as in more undeveloped watersheds are included in the study. A naturally occurring but uncommon isotope of nitrogen (15N) is added to stream water as nitrate (the most abundant form of fixed nitrogen in streams) over a 24-hour period. The fate of the added 15N is tracked by collecting samples of water and different forms of stream organic matter downstream from the addition point. The experimental results are then used to develop a general model of nitrogen retention in streams that includes uptake by organisms and permanent removal by the production of nitrogen gas. These results are extended to much larger spatial scales by combining our stream model with information on nitrogen loading and land use in one large river basin in each of the eight study regions to predict nitrogen concentrations at the river mouth. These predictions are tested by field measurements of stream nitrogen concentrations at different locations within each river basin. The ultimate goal of the research is to provide land managers and planners with a better understanding of the importance of streams in controlling the nitrogen loading to lakes and coastal ecosystems, and to show how humans can preserve or enhance these ecosystem services provided by streams.

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