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

Abstract

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 Nr 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 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 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 similar to 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 N-r (NHx = ammonia + ammonium), with oxidized and organic forms contributing similar to 41.3 % and 7.6 %, respectively. Our results indicate that reductions in NHx deposition would be needed to achieve the lowest estimates (similar to 3.0 kg N ha(-1) yr(-1)) of Nr critical loads in southern Appalachian forests.