Throughfall Deposition Chemistry in the Great Smoky Mountains National Park: Landscape and Seasonal Effects

Throughfall (TF) deposition of sulfur (SO42- as S) and inorganic nitrogen (N as NO3- + NH4+) are key drivers to stream acidification in base-poor regions of eastern United States (US). Significant declines in acid deposition over the past 20 years have occurred in this region, including Great Smoky Mountains National Park (GRSM). GRSM has long studied impacts of acid deposition on stream water quality since 1991; however, the last park-wide TF S and N deposition survey was completed in 2000. This study updates TF S and N depositions in GRSM where TF S deposition ranged between 1.6 and 5.5 kg ha(-1) year(-1) compared with 2000 data reported as between 6.5 and 33.6 kg ha(-1) year(-1), and TF N deposition ranged between 2.6 and 11.6 kg ha(-1) year(-1) compared with 2000 data as between 4.8 and 25.0 kg ha(-1) year(-1). Elevation largely influenced these ranges, where annual TF S and N depositions increased with increasing elevation. However, TF deposition for base cations (BC) slightly declined with increasing elevation. Annual precipitation volumes were greater at the higher elevation sites, and influenced TF ion depositions. However, TF depositions for N (NH4+) and BC (K+, Ca2+, and Mg2+) appeared to be regulated primarily by forest canopy exchange. BC canopy exchange and limited supply from bulk deposition at higher elevations may be a factor to continued stream acidification at higher elevations. It appears forest composition as classified as coniferous, deciduous, and mix did not influence TF ion depositions. Findings from this study provide necessary data for biogeochemical modeling efforts to predict the recovery period of stream water quality from acidification and have implications to the possible environmental effects of climate change.