Site: Arctic LTER
The site of a nitrogen isotope tracer experiment at the Arctic LTER site on the North Slope of Alaska.

A tracer approach to investigation of the nitrogen (N) cycle of streams, first developed at the Arctic LTER, has transformed scientific understanding of the nitrogen cycle and food web structure in flowing waters. By adding a continuous drip of 15N-NH4 (ammonium) or 15N–NO3 (nitrate) to a stream and then sampling the downstream transport, uptake and recycling of nitrogen over distance and time, scientists can determine how the ecosystem stores, transforms and recycles the nutrient. For example, in the first applications of this technique in the Kuparuk River ARC scientists found that ammonium that enters the river as seepage is assimilated by microbes within a distance of 800 meters (Peterson et al. 1997). By analyzing samples of algae, insects and fish for 15N tracer content, scientists were able to determine the key connections in the food web and measure how rapidly nitrogen was taken up and released by organisms at all trophic levels.

The technique is now widely applied to understand how streams in biomes throughout North America transport and process nitrogen (Peterson et al. 2001). For example, researchers found that the primary control of the uptake distance for ammonium was stream discharge and that nitrate uptake distances averaged five to10 times longer on average than ammonium uptake distances. Small headwater streams typically transformed by uptake, nitrification and denitrifrication, more than half of their inorganic nitrogen inputs within the first few kilometers of stream transport.

Denitrification, which converts nitrate to nitrogen gas, is a key process for controlling the eutrophication in streams and downstream lakes and estuaries. A nationwide study of denitrification in 72 streams demonstrated that both total biotic uptake of nitrogen and denitrification increase with increasing stream nitrate concentration, but that the efficiency of nitrate removal declines with increasing concentration (Mulholland et al. 2008). Thus by overloading streams with nitrate in runoff, disproportionate increases in both nitrate export and subsequent downstream nitrogen pollution ensue.

Another product of the denitrification reaction in streams is the formation of nitrous oxide (N2O), a potent greenhouse gas. Results from many studies worldwide have demonstrated that nitrous oxide emissions from rivers account for at least 7% of the global anthropogenic nitrous oxide budget, an amount more than twice the recently published estimates of the Intergovernmental Panel on Climate Change (IPCC).

Graph for
The relationship between stream discharge (Q) and the average distance travelled by a dissolved ammonium molecule prior to uptake (Sw).