Quantifying Direct Contributions of Atmospheric Nitrate to Streams during Snowmelt

Stephen D. Sebestyen
USDA Forest Service,
Grand Rapids, MN USA

At an upland forest in northeastern Vermont, USA where atmospheric nitrogen deposition is chronically elevated, we quantified sources of water and nitrate to elucidate the effects of atmospheric wet deposition of nitrate on stream nitrate concentrations during snowmelt events. Using high-frequency stream water samples, isotopic tracers, and end-member mixing analysis, we determined that the high concentration of stream water nitrate during snowmelt was a mix of nitrate from the melting snowpack and nitrate that was flushed to the stream from nitrified sources in the landscape. The presence of isotopically distinct atmospheric nitrate in streams indicated that atmospheric sources may quickly move to streams and short-circuit biogeochemical processes that typically retain nitrogen in the landscape. Overall, the direct input of nitrate from atmospheric sources (7 to 10% of the stream nitrate outflow from the catchment) was small. However, our study highlights how a combined isotopic and hydrological approach can be used to detect and quantify the direct effects of atmospheric nitrogen pollution on forest streams.