Nitrogen loading dynamics across a rural to urban gradient suggest diverse hydro-climate mediated transport mechanisms

Efforts to reduce non-point source pollution derived from urban watersheds requires that we understand the spatial and temporal patterns in both source and transport of different loads. To do this, we have utilized high temporal resolution in-situ spectrometry to monitor the nitrogen loading dynamics from five small watersheds spanning the rural to urban gradient in the Research Triangle region of North Carolina.

In-situ spectrometry revealed strong patterns in event NO₃^-, TDN, DOC, and SO₄^2- concentration with development patterns and wetness gradients. Total annual dissolved nitrogen loading increased with development intensity and the stormflow portion of loading also increased with development intensity. However, preliminary flow separation suggests that baseflow dissolved nitrogen loading was greater annually than stormflow for all watersheds. Event dissolved nitrogen loading increased during wet periods with the peak in N loading generally shifting later in the hydrograph with increased antecedent wetness. Low intensity rainfall and moderate antecedent wetness, conditions that support infiltration, were strong predictors of total event NO₃^- loading. Secondary nitrogen peaks on the falling limb of the event hydrograph were an important contributor to urban watershed loading and can result in elevated post storm baseflow loading. These preliminary results suggest that event driven subsurface transport of nutrients and baseflow derived loading are more significant than surface runoff for our five study watersheds. This suggests that management methods that promote infiltration may not reduce the total loading to stream networks, but instead transfer loading from high to lower flow. Combining infiltration-based management methods with effective stream restoration will be imperative to reducing loading to downstream waterbodies.