23 Jun 2022
23 Jun 2022
Status: this preprint is currently under review for the journal BG.

Atmospheric Deposition of Reactive Nitrogen to a Deciduous Forest in the Southern Appalachian Mountains

John Thomas Walker1, Xi Chen1,a, Zhiyong Wu1,b, Donna Schwede1, Ryan Daly1,c, Aleksandra Djurkovic1, A. Christopher Oishi2, Eric Edgerton3, Jesse Bash1, Jennifer Knoepp2,, Melissa Puchalski4, John Iiames1, and Chelcy Ford Miniat2,d John Thomas Walker et al.
  • 1U.S. Environmental Protection Agency, Office of Research and Development, Durham, NC, USA
  • 2U.S. Department of Agriculture, Forest Service, Otto, NC, USA
  • 3Atmospheric Research and Analysis, Inc., Cary, NC, USA
  • 4U.S. Environmental Protection Agency, Office of Air and Radiation, Washington, DC, USA
  • anow at: U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Durham, NC, USA
  • bnow at: RTI International, Durham, NC, USA
  • cnow at: Boulder A.I.R. LLC, Boulder, CO, USA
  • dnow at: U.S. Department of Agriculture, Forest Service, Albuquerque, NM, USA
  • retired

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 were 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 6.6 kg N ha-1 yr-1, which is on the upper end of Nr 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 Nr (NHx = ammonia + ammonium), with oxidized and organic forms contributing 41.6 % and 7.7 %, respectively. Our results indicate that reductions in NHx 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.

John Thomas Walker et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-133', Anonymous Referee #1, 19 Jul 2022
  • RC2: 'Referee's comments on bg-2022-133', Chris Flechard, 11 Aug 2022

John Thomas Walker et al.

John Thomas Walker et al.


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Short summary
Better estimates of atmospheric nitrogen (N) deposition are needed to accurately assess ecosystem risk and impacts from deposition of nutrients and acidity. Using measurements and modeling, we estimate total N deposition is 6.6 kg N ha-1 yr-1 at a forest site in the southern Appalachian Mountains, a region sensitive to atmospheric deposition. Reductions in deposition of reduced forms of N (ammonia and ammonium) will be needed to meet the lowest estimates of N critical loads for the region.