Atmospheric nitrogen desposition to the Neuse River watershed: Fluxes, sources and spatiotemporal variability

Atmospheric deposition of nitrogen (AD-N), as wet and dry deposition of dissolved inorganic nitrogen (DIN: NO3- , NH3/NH4+) and dissolved organics, contributes >20% of the total externally-supplied or "new" N flux to the waterways of the Neuse River basin (North Carolina). Excessive nitrogen (N) loading to N-sensitive waters such as the Neuse River Estuary has been linked to changes in microbial and algal community composition and function (harmful algal blooms), hypoxia/anoxia, and fish kills. In a three year study from June 1996 to June 1999, the weekly wet deposition of inorganic and organic N was calculated at eleven sites on a northwest-southeast transect in the watershed. The annual mean total (wet + dry) AD-N flux was calculated to be 1732 mg/m2/yr. Nitrate was the most prevalent chemical species in deposition, followed by ammonium and organics. Seasonally, the spring (March--May)and summer (June--August) months contained the highest total weekly N deposition; this does not appear to be driven by precipitation amount. There was spatial variability in AD-N deposition; in general, the upper portion of the watershed receives the lowest annual deposition. Estimates of watershed N retention and in-stream riverine processing revealed that this flux contributed from 27--58% of the total "new" N flux to the estuary, with direct deposition to the estuary surface accounting for 10% of the total "new" N flux. Sites in the middle portion of the basin had a significantly lower delta 15NH4+, suggesting that they were more impacted by NH3 from animal waste than sites in the upper and lower portions of the basin. Meteorological modeling of air parcel back trajectories did not correlate well with AD-N flux from those parcels, but there was some evidence to suggest that a large portion of this AD-N may have originated outside of North Carolina, making managing this nutrient flux a regional rather than a state issue. AD-N is an important source of "new" N to the Neuse River Estuary that should be included in effective nutrient mitigation and management efforts.