Atmospheric nitrogen inputs and losses along an urbanization gradient from Boston to Harvard Forest, MA

Urbanization alters nitrogen (N) cycling, but the spatiotemporal distribution and impact of these alterations on ecosystems are not well-quantified. We measured atmospheric inorganic N inputs and soil leaching losses along an urbanization gradient from Boston, MA to Harvard Forest in Petersham, MA. Atmospheric N inputs at urban sites (12.3 +/- A 1.5 kg N ha(-1) year(-1)) were significantly greater than non-urban (5.7 +/- A 0.5 kg N ha(-1) year(-1)) sites with NH4 (+) (median value of 77 +/- A 4 %) contributing thrice as much as NO3 (-). Proximity to urban core correlated positively with NH4 (+) (R-2 = 0.57, p = 0.02) and total inorganic N inputs (R-2 = 0.61, p = 0.01); on-road CO2 emissions correlated positively with NO (3) (-) inputs (R-2 = 0.74, p = 0.003). Inorganic N leaching rates correlated positively with atmospheric N input rates (R-2 = 0.61, p = 0.01), but did not differ significantly between urban and non-urban sites (p > 0.05). Our empirical measurements of atmospheric N inputs are greater for urban areas and less for rural areas compared to modeled regional estimates of N deposition. Five of the nine sites had NO (3) (-) leached that came almost entirely from nitrification, indicating that the NO3 (-) in leachate came from biological processes rather than directly passing through the soil. A significant proportion (17-100 %) of NO (3) (-) leached from the other four sites came directly from the atmosphere. Surprisingly, the four sites where atmospheric sources made up the largest proportion of leachate NO3 (-) also had relatively low N leaching rates, suggesting that atmospheric N inputs added to terrestrial ecosystems can move to multiple sinks and losses simultaneously, rather than being lost via leaching only after abiotic and biotic sinks have become saturated. This study improves our understanding of atmospheric N deposition and leaching in urban ecosystems, and highlights the need to incorporate urbanization effects in N deposition models.