Atmospheric Nitrate Processing in Forested Watersheds Along a Nitrogen Deposition Gradient

Chronically high rates of atmospheric nitrate deposition can result in elevated nitrate export to streams draining forested ecosystems. This problem is particularly evident in the eastern U.S., where high stream nitrate concentrations and export have been reported in many forests throughout the region. This has led to the suggestion that some forests in the eastern U.S. may be at or nearing a state of nitrogen saturation. Nitrogen saturation theory holds that when nitrogen inputs to an ecosystem exceed plant and microbial demand, the system will become nitrogen saturated, with excess nitrate leaching to streams. To investigate the utility of nitrate stable isotopes in assessing the nitrogen saturation status of forested watersheds, we measured d15N, d18O, Δ17O, and concentrations of nitrate in precipitation and stream water on a monthly basis (August 2012 - July 2013) at Coweeta (North Carolina), Fernow (West Virginia), and Hubbard Brook (New Hampshire) Experimental Forests. These sites form a nitrogen deposition gradient with long-term mean nitrate deposition ranging from 10.8 kg ha-1 yr-1 to 17.3 kg ha-1 yr-1. All sites were significantly different with respect to long-term mean nitrate deposition (p<0.0001). During the study period, precipitation nitrate concentration and isotopic composition did not differ significantly among the study sites. Seasonal trends in precipitation Δ17O of nitrate were similar among all sites and were indicative of seasonal variation in dominant NOx oxidation pathway. In contrast to precipitation nitrate dynamics, the study sites differed significantly with respect to stream nitrate concentration (p<0.0001) and isotopic composition (p<0.0001). The study watershed at Fernow (WS4) received the highest long-term mean rates of atmospheric nitrate deposition and also had the highest mean stream water nitrate concentration during the study period. However, this site exported the lowest mean percentage of unprocessed atmospheric nitrate. In contrast, the study watershed at Coweeta (WS34) received the lowest long-term mean rate of atmospheric nitrate deposition and had the lowest mean stream water nitrate concentrations during the study period. However, this watershed exported the highest mean percentage of unprocessed atmospheric nitrate during the study period. Among these three sites, there was a negative relationship between stream water nitrate concentration and the percentage of unprocessed atmospheric nitrate exported to streams. We will explore potential explanations for the observed trends in precipitation and stream water nitrate concentrations and isotopic compositions, and discuss the utility of nitrate stable isotopes in assessing watershed nitrogen saturation status.