Long-term Ecosystem Research Highlights Fate of Nitrogen in Rainfall

Summary

Department of Energy studies on Walker Branch Watershed in the Department’s Oak Ridge (Tennessee) National Environmental Research Park showed that stream ecosystems can help prevent nitrogen pollutants from reaching downstream lakes, estuaries, and the ocean. Fossil fuel use is increasing the amount of nitrogen in rainfall in many parts of the United States, and inputs of this nitrogen to aquatic ecosystems can result in harmful algal blooms and drinking water contamination. Combining computer simulation and data from 12 years of field measurements, scientists at Oak Ridge National Laboratory found that biological organisms in streams removed about 20% of the nitrate nitrogen entering the stream from the watershed, thus reducing the concentration of nitrate exported downstream. The removal of nitrate nitrogen was highly seasonal; it was greatest in autumn (due to uptake by bacteria and fungi growing on newly fallen leaves trapped in the stream) and in early spring (due to high rates of uptake by algae before the stream becomes heavily shaded by new leaves in the deciduous forest overhead). These results are consistent with studies at the Hubbard Brook Experimental Forest (New Hampshire) and elsewhere showing that streams can reduce the downstream transport of nitrate nitrogen, and demonstrate the important role of streams in preventing high nitrate export and the eutrophication of downstream aquatic ecosystems. This study was recently documented in the journal Biogeochemistry.