PARENT SESSION
Contributed Oral Session 32: Soil Nutrient Dynamics and Acid Deposition
Monday, August 8, 1:30 PM - 5:00 PM, Meeting Room 521 A, Level 5, Palais des congrès de Montréal

Calculating pre-measurement atmospheric deposition, stream chemistry, and soil chemistry for an alpine watershed in Rocky Mountain National Park, Colorado.

Hartman, Melannie^*,1, Baron, Jill^{1, 2}, Ojima, Dennis¹, ¹ Natural Resource Ecology Laboratory, Fort Collins, CO, USA² U.S. Geological Survey, Fort Collins, CO, USA

ABSTRACT- Wet and dry deposition of N and S species have been measured for 25 years or less in the United States, making it impossible to know with certainty atmospheric deposition amounts from earlier in the 20th century. Likewise, measurements of ecosystem status, such as surface water chemistry and soil base saturation, for the first half of the 20th century are largely unavailable. Pre-measurement deposition amounts and ecosystem characteristics have interest for scientists attempting to understand nutrient cycling processes and long-term ecological response to anthropogenic inputs. Pre-measurement values are therefore of interest to policy makers who strive to protect ecosystems from undesirable environmental change. We used DayCent-Chem, a daily timestep ecosystem-hydrogeochemical model, to investigate the changes in soil and stream water chemistry that may have occurred in an alpine ecosystem over the past century. We used daily historic VEMAP climate to derive climate inputs to the model for 1900 to when climate observations began in the 1980s. We also developed hindcasts of NO₃^-, NH₄⁺, and SO₄^2- concentrations in precipitation. We ran the model backwards in time, from the known ecosystem state in 1999 to 1900, to estimate stream water pH, stream acid neutralizing capacity (ANC), and soil base saturation that may have existed in the past. The model predicted that annual mean ANC stream concentrations were approximately 20 eq L^-1 higher, that pH was about 0.3 units greater, and that and soil base saturation was about 9% higher in 1900 compared to the late 1990s. Going back in time, simulated stream [NO₃^-] decreased exponentially with precipitation [NO₃^-].

Key words: nitrogen deposition, alpine, biogeochemistry, simulation modeling