USGS Studies of Denitrification and Nitrogen Removal in Streams

New Data and Methods for Predicting Nitrogen Removal in Streams
Reveal Important Effects of Land Use and Stream Hydrology

Results of two new studies led by USGS scientists are available in a special issue of the journal Biogeochemistry, which highlights findings from a workshop sponsored by the National Science Foundation on new approaches to modeling denitrification—an ecologically important bacterial process that converts reactive nitrogen compounds to inert nitrogen gas.

The two studies, conducted in collaboration with scientists in universities and other federal agencies, report on physical, biochemical, and land-use factors that can affect the spatial variability and seasonality in nitrogen removal by denitrification in agricultural and forested streams.

The findings show that the fraction of nitrogen removed by denitrification depends upon the nitrogen concentration in a stream, and, specifically, that relatively small proportions are removed in streams with elevated concentrations (see the sidebar illustration for two watersheds with contrasting land use).

Unfortunately, nitrogen concentrations are highest in many agricultural streams in the winter and spring. This is the time when many streams also have high flows and export the largest amounts of nitrogen to coastal estuaries and other receiving waters. Excessive nitrogen can result in the growth of large amounts of algae and a zone of low dissolved oxygen (hypoxia), which can stress aquatic life in receiving waters, such as the northern Gulf of Mexico and Chesapeake Bay.

The new studies may contribute to more accurate model-based watershed assessments, which currently assume that the proportion of nitrogen removed by denitrification is unaffected by the levels of nitrogen concentration in streams. In addition, the findings provide scientific support for targeting nutrient management strategies in watersheds with relatively high nitrogen levels, where in-stream denitrification is expected to be least efficient.

Other findings in the USGS led studies highlight seasonal variability in denitrification and nitrogen removal, and the importance of land use and hydrology, such as related to stream flow and water velocity, on nitrogen concentrations and removal.

The articles are authored by Böhlke and others ("Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrogen concentration in the upper Mississippi River basin, USA") and by Alexander and others ("Dynamic modeling of nitrogen losses in river networks unravels the coupled effects of hydrological and biogeochemical processes").

For questions, contact Richard Alexander, (703)648-6869 or John Karl Böhlke, (703)648-6325.

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