Abstract
Assessing nutrient critical load exceedances requires complete and accurate atmospheric deposition budgets for reactive nitrogen (Nr). The exceedance is the total amount of Nr deposited to the ecosystem in excess of the critical load, which is the amount of Nr input below which harmful effects do not occur. Total deposition includes all forms of Nr (i.e., organic and inorganic) deposited to the ecosystem by wet and dry pathways. Here we present results from the Southern Appalachian Nitrogen Deposition Study (SANDS), in which a combination of measurements and field-scale modeling was used to develop a complete annual Nr deposition budget for a deciduous forest at the Coweeta Hydrologic Laboratory. Wet deposition of ammonium, nitrate, nitrite, and bulk organic N were measured directly. The dry deposited Nr fraction was estimated using a bidirectional resistance-based model driven with speciated measurements of Nr air concentrations (e.g., ammonia, ammonium aerosol, nitric acid, nitrate aerosol, bulk organic N in aerosol, total alkyl nitrates, and total peroxy nitrates), micrometeorology, canopy structure, and biogeochemistry. Total annual deposition was ~6.7 kg N ha-1 yr-1, which is on the upper end of Nr critical load estimates recently developed for similar ecosystems in the nearby Great Smoky Mountains National Park. Of the total (wet + dry) budget, 51.1% was contributed by reduced forms of Nr (NHx=)ammonia+ammonium)NHx=ammonia+ammonium)<math> <mfenced> <mrow> <msub><mrow><mtext>NH</mtext></mrow> <mrow><mi>x</mi></mrow> </msub> <mo>=</mo></mrow> </mfenced> <mtext>ammonia</mtext> <mspace></mspace> <mo>+</mo> <mspace></mspace> <mtext>ammonium</mtext> <mo>)</mo></math> , with oxidized and organic forms contributing ~41.3% and 7.6%, respectively. Our results indicate that reductions in NHxNHx<math> <msub><mrow><mtext>NH</mtext></mrow> <mrow><mi>x</mi></mrow> </msub> </math> deposition would be needed to achieve the lowest estimates (~3.0 kg N ha-1 yr-1) of Nr critical loads in southern Appalachian forests.