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

• Published in: Biogeosciences Vol. 20; no. 5; pp. 971 - 995
• Main Authors: Walker, John T, Chen, Xi, Wu, Zhiyong, Schwede, Donna, Daly, Ryan, Djurkovic, Aleksandra, Oishi, A Christopher, Edgerton, Eric, Bash, Jesse, Knoepp, Jennifer, Puchalski, Melissa, Iiames, John, Miniat, Chelcy F
• Format: Journal Article
• Language: English
• Published: Germany Copernicus GmbH 09.03.2023; Copernicus Publications
• Subjects: Aerosols; Ammonia; Ammonium; Ammonium compounds; Analysis; Atmospheric chemistry; Biogeochemistry; Budgets; Deciduous forests; Deposition; Ecosystems; Estimates; Forests; Hydrology; Load; Measurement techniques; Micrometeorology; Mountains; National parks; Nitrates; Nitric acid; Nitric acids; Nitrogen; Nitrogen deposition; Nutrient loading; Trends; Wet deposition; Appalachian Mountains; United States > US; North America
• ISSN: 1726-4170; 1726-4189; 1726-4189
• DOI: 10.5194/bg-20-971-2023

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