Impacts of changing climate and atmospheric deposition on N and S drainage losses from a forested watershed of the Adirondack Mountains, New York State

Biogeochemical responses to changing climate and atmospheric deposition were investigated using nitrogen (N) and sulfur (S) mass balances, including dry deposition and organic solutes in the Arbutus Lake watershed in the Adirondack Mountains, New York State. Long-term monitoring of wet-only precipitation (NADP/NTN, 1983-2001) and dry deposition (AIRMoN, 1990-2001) at sites adjacent to the watershed showed that concentrations of SO42- in precipitation, SO42- in particles,and SO2 vapor all declined substantially (P<0.005) in contrast to no marked temporal changes observed for most N constituents (NH4+ in precipitation, HNO3 vapor, and particulate NO3-), except for NO3- in precipitation, which showed a small decrease in the late 1990s. From 1983 to 2001, concentrations of SO42- in the lake outlet significantly decreased (-2.1 mueq L-1 yr(-1), P<0.0001), whereas NO3- and dissolved organic N (DON) concentrations showed no consistent temporal trends. With the inclusion of dry deposition and DON fluxes into the mass balance, the retained portion of atmospheric N inputs within the main subcatchment increased from 37% to 60%. Sulfur outputs greatly exceeded inputs even with the inclusion of dry S deposition, while organic S flux represented another source of S output, implying substantial internal S sources. A significant relationship between the annual mean concentrations of SO42- in lake discharge and wet deposition over the last two decades (r=0.64, P<0.01) suggested a considerable influence of declining S deposition on surface water SO42- concentrations, despite substantial internal S sources. By contrast, interannual variations in both NO3- concentrations and fluxes in lake discharge were significantly related to year-to-year changes in air temperature and runoff. Snowmelt responses to winter temperature fluctuations were crucial in explaining large portions of interannual variations in watershed NO3- export during the months preceding spring snowmelt (especially, January-March). Distinctive response patterns of monthly mean concentrations of NO3- and DON in the major lake inlet to seasonal changes in air temperature also suggested climatic regulation of seasonal patterns in watershed release of both N forms. The sensitive response of N drainage losses to climatic variability might explain the synchronous patterns of decadal variations in watershed NO3- export across the northeastern USA.