Dissolved organic nitrogen losses from forested watersheds across the southern Appalachians

Although regional and global models of nitrogen cycling typically focus on nitrate, dissolved organic nitrogen (DON) is the dominant form of nitrogen (N) loss from many ecosystems.  Recent work suggests that chronic N pollution can increase both the quantity and quality of watershed DON export.  As a preliminary test of this hypothesis, we performed two synoptic streamwater sampling campaigns.  Once during the forest growing season and once prior to forest canopy leafout we compared streamwater concentrations of dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC) and DON for 79 streams of the southern Appalachians (in Joyce Kilmer and Slick Rock Wilderness Area (JKSWA), the Great Smoky Mountains National Park (GSMNP), and the Oak Ridge Reservation (ORNL)).  Across JKSWA and GSMNP growing season DON concentrations were significantly positively correlated with streamwater DIN and only weakly correlated with DOC.  In contrast, pre-leafout DIN and DON concentrations were negatively correlated.  Streamwater DIN and the C:N of DOM concentrations in these streams were negatively correlated during the growing season.  At ORNL we found significant differences in streamwater DIN, DON and DOC between dolomite and shale geologies.  Across all streams DON concentrations were much higher and the C:N of DOM was much lower during the growing season than pre leaf-out. These data suggest that DON is of a different origin or under a different set of controls than the bulk DOM pool and that DIN and DON may be regulated by the same mechanistic controls during the growing season.