The Adirondack Manipulation and Modeling Project (AMMP): design and preliminary results

The Adirondack Manipulation and Modeling Project (AMMP) was initiated in 1990 to investigate the effects of changing chemical inputs in the Adirondack Mountains of New York (USA). The four study sites are located across a west to east gradient of declining atmospheric deposition, increasing acid neutralizing capacity (ANC) and decreasing NO₃^- concentration in surface waters in the Adirondacks. We hypothesize that the two sites in the western Adirondacks (Woods Lake (WL) and Pancake-Hall Creek (PHC)) exhibit lower critical loads of N, S and H⁺ than the central and eastern sites (Huntington Forest (HF) and Pack Forest (PF), respectively). To test this hypothesis, chemical treatments of plots were initiated in 1990 including (NH₄)₂SO₄ (1000 and 2000 eq ha^-1 year^-1), H₂SO₄ (1000 eq ha^-1 year^-1), HNO₃ (1000 eq ha^-1 year^-1), Ca+Mg+SO₄^2-1 (1000 eq ha^-1 year^-1). Three of the sites (WL, PHC and HF) are nothern hardwood ecosystems underlain by Spodosols. The PF site is a Pinus resinosa plantation on a glacial outwash plain (Typic Udipsamment). Ions in total deposition, throughfall and soil leachates were monitored continuously. Changes in the solid phase chemistry of the mineral soil were assessed annually using the buried soil-bag approach. Vegetation responses to chemical treatments were analyzed by ascertaining changes in growth, composition and nutrient concentrations of both the overstory and understory. Elemental cycling models used in the AMMP include NuCM and VEGIE-CHESS. Preliminary results of bulk precipitation and throughfall from 1991-1992 suggest that the western sites may have greater inputs of SO₄^2-1 and have lower capacity to retain NO₃^- passing through the foliage. Soil solutions (1991-1992) in reference plots of the western sites (WL and PHC) had higher concentrations of NO₃^- in soil solution than the central and eastern Adirondack sites (HF and PF). For all sites except PF, ammonium sulfate additions resulted in increased concentrations of SO₄^2- and NO₃^- in soil solutions. Results of model simulations with NuCM for HF are presented which suggest that the model adequately simulated the pattern of increasing SO₄^2- concentrations in response to (NH₄)₂SO₄ treatment, but that the increase in NO₃^- was underestimated.