Atmospheric and Microbial Nitrate Contributions to Streams across a Regional Nitrogen Deposition Gradient

Abstract:

Chronically elevated atmospheric nitrate deposition has increased nitrate export from forests worldwide. This problem is particularly evident in the eastern U.S., where elevated stream nitrate concentrations and export from forested watersheds has led to the suggestion that some forests may be at or nearing a state of nitrogen saturation. To investigate the utility of nitrate stable isotopes in assessing the nitrogen saturation status of forests, we measured monthly δ¹⁵N, δ¹⁸O, Δ¹⁷O, and concentrations of nitrate in precipitation and stream water from reference watersheds at Coweeta (North Carolina), Fernow (West Virginia), and Hubbard Brook (New Hampshire) Experimental Forests from August 2012 to July 2013. Long-term mean nitrate deposition ranges from 11 kg ha^-1 yr^-1 to 17 kg ha^-1 yr^-1 and is significantly different (p<0.05) among the sites. Nitrate concentrations and isotopic compositions of precipitation did not differ significantly among the sites during the study. Seasonal trends in δ¹⁸O and Δ¹⁷O of nitrate values were also similar among sites, and were indicative of seasonal variation in dominant NO_x oxidation pathway. The study sites differed significantly with respect to stream nitrate concentration (p<0.05) and isotopic composition (p<0.05). The high deposition site (Fernow) had the highest mean stream water nitrate concentration during the study period but the lowest percentages of atmospheric nitrate in monthly samples and on an annual average basis. In contrast, the low deposition site (Coweeta) had the lowest mean stream nitrate concentrations during the study and the highest mean percentage of atmospheric nitrate in the stream. Unprocessed atmospheric nitrate was also present in Coweeta stream samples during every month that isotope analyses were conducted for this site. Among these watersheds, stream nitrate concentration was negatively related to the proportion of unprocessed atmospheric nitrate in streams (R²=0.23; p<0.05). We will explore potential explanations for the trends and differences in stream nitrate concentrations and isotopes, and present a conceptual model of ecosystem nitrogen saturation based on our observations within these watersheds.