Modeling the timeline for lake and stream acidification from excess nitrogen deposition for Rocky Mountain National Park.
Hartman, Melannie*,1, Baron, Jill1, Ojima, Dennis1, 1 Natural Resource Ecology Laboratory, Fort Collins, CO, USA
ABSTRACT- Nitrogen wet deposition of 3-5 kg N/ ha to the east side of the Colorado Front Range in Rocky Mountain National Park, in the form of nitrate and ammonium, is among the highest measured in the State. Soils control the potential for lake and stream acidification from excess nitrogen, through loss of soil base cations. Base cations leach from soils with acid anions such as sulfate and nitrate. But because nitrogen is a critical plant nutrient, any realistic projection of nitrogen-caused acidification must include understanding ecosystem nutrient cycling. We coupled two widely accepted and tested models, one of ecosystem biogeochemistry (the daily version of CENTURY) and the other of soil and water chemical equilibrium (PHREEQC). The model was calibrated for simulations of forest, tundra, and rock surface with a rich 20-year data set available from Loch Vale Watershed (LVWS). The objectives were to model how and when acidification will occur under current and potential future nitrogen deposition amounts. When we increased annual nitrogen deposition by 2.5% per year for 15 years, beginning with current deposition rates the first year, the model predicted decreasing stream pH and ANC, decreasing soil base saturation, and increasing base cation, nitrate, and aluminum mobilization. Fifty percent of the additional nitrogen added leached into streamflow. There was little increase in total soil organic matter, soil organic matter N:C ratios, and NPP, indicating that LVWS has limited capacity to biologically assimilate excess nitrogen.
Key words: deposition, biogeochemistry, nitrogen, acidification
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