National wet and dry deposition monitoring networks are the foundation for our understanding of atmospheric nitrogen deposition to landscapes.  However, it has recently been shown that wet and dry deposition measured at National Trends Network (NTN) and CASTNET sites in the Northeastern U.S. are more heavily influenced by nitrate originating as NO_x emitted from regional power plants than vehicular emissions (Elliott et al. 2007; 2009).  Although vehicular NO_x emissions comprise the single largest NO_x source in the U.S. (54%), these recent studies suggest the fate of vehicular NO_x in the environment may be much different than that of power plant NO_x emissions. Here, in two ongoing studies, we examine more closely the fate of vehicular NO_x emissions using the isotopic composition of gaseous reactive nitrogen, plants, and soils along a gradient perpendicular to a major interstate.  We further examine the role of atmospherically deposited nitrogen as a factor in urban surface water quality by examining nitrate isotopes in surface waters collected from Nine Mile Run, an urban stream in Pittsburgh, Pennsylvania (USA) during base- and storm-flow conditions. 

Results/Conclusions

A 400 m gradient was established perpendicular to a rural section of a heavily trafficked interstate.  Deployment of passive diffusion samplers along the gradient indicates up to 7 kg ha yr higher total N deposition near the roadway.  The influence of this elevated N deposition, originating as NO_x from tailpipes, is evidenced in the isotopic composition of plants, soils, and gaseous reactive N species.  Ultimately, the dry deposition of these gaseous N species onto impervious surfaces is routed to surface waters via culverts and stormwater drains during precipitation events.  Consequently, we examine the influence of atmospheric reactive nitrogen deposition on urban surface water quality using nitrate isotopes collected from Nine Mile Run, a 1600 ha watershed in Pittsburgh characterized by 38% impervious surface cover and elevated nitrate concentrations.  Storm flow sampling indicates up to 40% of streamwater nitrate during high flow regimes originates from atmospheric sources. Together, these studies indicate that: 1) we are underestimating the total load of atmospheric N deposited to landscapes; 2) this load is not uniform in pattern, but rather heavily concentrated near highways and urban areas; and 3) the implications of this spatial distribution need to be considered as a factor influencing urban surface water quality.