Citation:

Walker, JohnT, X. Chen, D. Schwede, Z. Wu, J. Bash, R. Daly, A. Djurkovic, C. Oishi, E. Edgerton, J. Knoepp, M. Puchalski, AND C. Miniat. Nitrogen deposition budget for a southern Appalachian deciduous forest. National Atmospheric Deposition Program 2019 Fall Science Symposium, Boulder, Colorado, November 04 - 07, 2019.

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:

This presentation summarizes results from the Southern Appalachian Nitrogen Deposition Study (SANDS). Measurements and field-scale modeling are used to develop a complete annual (2015) reactive nitrogen (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. Total annual deposition was  8.5 kg N ha-1 yr-1, which is greater than Nr critical load estimates recently developed for similar ecosystems in nearby Great Smoky Mountains National Park. Wet deposition contributed slightly more (54%) than dry deposition at the annual scale. Approximately 52% of the total (wet + dry) budget was contributed by reduced forms of Nr (NHx = ammonia + ammonium), with oxidized and organic forms of Nr contributing  38% and 10%, respectively. The dry deposited fraction comprised  60% NHx, with oxidized and organic Nr contributing  33% and 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.

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