Citation:

Walker, JohnT, J. Knoepp, M. Puchalski, J. Iiames, C. Miniat, X. Chen, Z. Wu, D. Schwede, J. Bash, R. Daly, A. Djurkovic, C. Oishi, AND E. Edgerton. Atmospheric Deposition of Reactive Nitrogen to a Southern Appalachian Deciduous Forest. UNAM Centro de Ciencias de la Atmosfera Seminar Series, Mexico City, MEXICO, April 09, 2021.

Impact/Purpose:

Assessment of critical load exceedances for nutrients 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. Total deposition budgets, which may be developed from measurements, models, or a combination of the two, are often incomplete due to lack of consideration of organic forms of Nr. Reliance on models for estimating the dry deposited fraction is another important source of uncertainty. More complete and accurate deposition budgets are needed to reduce uncertainty in critical load exceedances.

Description:

Assessment of critical load exceedances for nutrients 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. Total deposition budgets are often incomplete due to lack of consideration of organic forms of Nr and uncertain due to reliance on models for estimating the dry deposited fraction. Here we present results from the Southern Appalachian Nitrogen Deposition Study (SANDS) in which a combination of measurements and field-scale modeling is used to develop a complete annual (2015) Nr deposition budget for a mixed deciduous forest at the Coweeta Hydrologic Laboratory in southern Appalachia. 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. Wet deposition contributed 58% of total annual deposition. Approximately 53% of the total (wet + dry) budget was contributed by reduced forms of Nr (NHx = ammonia + ammonium), with oxidized and organic forms of Nr contributing 40% and 7%, respectively. The dry deposited fraction comprised 60% NHx, with oxidized and organic Nr contributing 35% and 5%, respectively. Total annual deposition was 7.0 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. Our results indicate that reductions in NHx deposition would be needed to achieve the lowest estimates (approx. 3.0 kg N ha-1 yr-1) of Nr critical loads in southern Appalachian forests.

URLs/Downloads:

Record Details: