Groundwater-transported dissolved organic nitrogen exports from coastal watersheds

We analyzed groundwater-transported nitrogen (N) exports from 41 watershed segments that comprised 10 Cape Cod, Massachusetts watersheds to test the hypotheses that chemical form of N exports is related to land use and to length of flow paths through watersheds. In the absence of human habitation, these glacial outwash–plain watersheds exported largely dissolved organic N (DON) but at relatively low annual rate. Addition of people to watersheds increased rates of both total dissolved N (TDN) and DON export through groundwater. Percent of TDN as DON in groundwater was negatively related to path length of groundwater through aquifers, but %DON was not significantly related to population density on the watersheds. DON was often the dominant form of N exported from the watersheds, even at high population densities. Our results suggest that natural sources are not entirely responsible for organic N exports from watersheds, but, instead, a substantial portion of anthropogenic N introduced to watersheds is exported as DON. This finding is in disagreement with previous results, which suggest that anthropogenic N is exported from watersheds largely as NO 3 and that DON exported from watersheds is from natural sources. Eutrophication associated with increasing land-derived nitrogen (N) loads is perhaps the greatest agent of change that alters coastal waters (Howarth et al. 2000). Because of this important role, efforts have been made to quantify and model N inputs to watersheds, retention and losses within watersheds, and exports from watersheds to receiving waters. Much has been learned in recent years about N sources and patterns of N cycling at watershed (Valiela et al. 1997), regional (Alexander et al. 2002), and global scales (Galloway 1998). However, the roles of dissolved organic N (DON) and of groundwater remain as two of the largest unknowns in watershed N budgets.