Atmospheric deposition of reactive nitrogen to a deciduous forest in the southern Appalachian Mountains

Assessing nutrient critical load exceedances requires complete and accurate atmospheric deposition budgets for reactive nitrogen (N_r). The exceedance is the total amount of N_r deposited to the ecosystem in excess of the critical load, which is the amount of N_r input below which harmful effects do not occur. Total deposition includes all forms of N_r (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 N_r 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 N_r fraction was estimated using a bidirectional resistance-based model driven with speciated measurements of N_r 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 N_r 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 N_r $\left({\text{NH}}_x=\right)\text{ammonia}+\text{ammonium})$ , with oxidized and organic forms contributing ~41.3% and 7.6%, respectively. Our results indicate that reductions in ${\text{NH}}_x$ deposition would be needed to achieve the lowest estimates (~3.0 kg N ha^-1 yr^-1) of N_r critical loads in southern Appalachian forests.