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 20 the Southern Appalachian Nitrogen Deposition Study (SANDS), in which a combination of measurements and field-scale modeling were 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 25 in aerosol, total alkyl nitrates, and total peroxy nitrates), micrometeorology, canopy structure, and biogeochemistry. Total annual deposition was  6.6 kg N ha -1 yr -1 , which is on the upper end of N r critical load estimates recently developed for similar ecosystems in nearby Great Smoky Mountains National Park. Of the total (wet + dry) budget, 50.7% was contributed by reduced forms of N r (NH x = ammonia + ammonium), with oxidized and organic forms contributing  41.6% and 7.7%, respectively. Our results indicate that reductions in NH x deposition would be needed 30 to achieve the lowest estimates (  3.0 kg N ha -1 yr -1 ) of N r critical loads in southern Appalachian forests. investigates the N deposition budget in a remote in We combine measurements of wet deposition, speciated air concentrations of N r , micrometeorology, biogeochemistry and forest 85 canopy structure with in-situ inferential dry deposition modeling to develop an annual, speciated, total N deposition budget, including net and component NH 3 fluxes as well as dry and wet organic N deposition. Seasonal and annual total N deposition fluxes are presented in the context of long-term trends in air concentrations and wet deposition of inorganic N species. Spatial representativeness is characterized using measurements of air concentrations of the primary inorganic N species and previous wet deposition measurements along an elevation gradient across the 90 topographically complex forested basin. Three-dimensional wind components were measured by sonic anemometer (Model 81000, R.M. Young Company, Traverse City, MI) above the forest canopy on the EFT. Momentum and kinematic heat fluxes were determined by eddy covariance (EC) from sonic data. For EC calculations, raw 10-Hz sonic data were processed into hourly averages after block average detrending and 2D coordinate rotation 235 (Novick et al., 2013). Air temperature and relative humidity were measured at the top of the tower (EC155, Campbell Scientific, Logan, UT) and 2/3 canopy height (HMP-45, Vaisala, Helsinki, Finland). Photosynthetically active radiation (PAR; LI-190, LI-COR Biosciences, Lincoln, NE) and upward and downward, shortwave and longwave radiation (CNR 4, Kipp & Zonen, Delft, The Netherlands) were measured at the top of the tower. Surface wetness was measured in the canopy crown, understory and at the ground using leaf wetness sensors (Model 237, Campbell 240 Scientific). Soil volumetric water content (VWC) averaged over 0–30 cm depth was measured in four locations around the tower using time domain reflectometry probes (CS616, Campbell Scientific). Soil temperture was measured at four depths (5, 20, 35, and 55 cm) in two locations near the tower using thermistors. For missing data, linear interpolation was used to fill short gaps (1–4 h). Longer gaps were filled by substitution using the average hourly diel profile calculated for each month. Micrometeorological data were used for inferential dry deposition modeling as 245 described below. profile was then applied to each biweekly AMoN air concentration, temporally scaling the NH 3 concentration by time of day while maintaining the measured biweekly AMoN concentration. Gap filling of AMoN data was not required. Comparisons of NH 3 measurements are briefly discussed in Supplemental Section S1. Hi-Vol measurements of speciated particulate N were only conducted during intensive periods to assess the relative 405 contributions of inorganic and organic fractions to total water-soluble N. The CASTNET particulate NH 4+ and NO 3-were used to provide a continuous 12-month time series of air concentration for annual deposition modeling. Concentrations of Hi-Vol and CASTNET measurements were shown to be comparable (Supplemental Section S1). For the annual time series, particulate organic nitrogen (PON) concentration was estimated based on speciated measurements during intensives, which showed that inorganic N accounts for ~88% of WSTN on average. Weekly 410 average PON concentration was estimated from the weekly CASTNET measurements assuming NH 4+ + NO 3-represents 88% of total particulate nitrogen and PON represents the balance (12%). Weekly concentrations were then expressed at the hourly time scale for modeling. Gap filling of CASTNET data was not required.