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

[摘要] Assessing nutrient critical load exceedances requirescomplete and accurate atmospheric deposition budgets for reactive nitrogen(N r ). The exceedance is the total amount of N r deposited to theecosystem in excess of the critical load, which is the amount of N r input below which harmful effects do not occur. Total deposition includesall forms of N r (i.e., organic and inorganic) deposited to theecosystem by wet and dry pathways. Here we present results from the SouthernAppalachian Nitrogen Deposition Study (SANDS), in which a combination ofmeasurements and field-scale modeling was used to develop a complete annualN r deposition budget for a deciduous forest at the Coweeta HydrologicLaboratory. Wet deposition of ammonium, nitrate, nitrite, and bulk organic Nwere measured directly. The dry deposited N r fraction was estimatedusing a bidirectional resistance-based model driven with speciatedmeasurements of N r air concentrations (e.g., ammonia, ammonium aerosol,nitric acid, nitrate aerosol, bulk organic N in aerosol, total alkylnitrates, and total peroxy nitrates), micrometeorology, canopy structure,and biogeochemistry. Total annual deposition was ∼  6.7 kg N ha −1  yr −1 , which is on the upper end of N r critical load estimatesrecently developed for similar ecosystems in the nearby Great Smoky MountainsNational Park. Of the total (wet  +  dry) budget, 51.1 % was contributedby reduced forms of N r (NH x   =  ammonia  +  ammonium), withoxidized and organic forms contributing ∼  41.3 % and 7.6 %,respectively. Our results indicate that reductions in NH x depositionwould be needed to achieve the lowest estimates ( ∼  3.0 kg N ha −1  yr −1 ) of N r critical loads in southern Appalachian forests.

[学科分类] 大气科学