Increases in atmospheric nitrogen (N) emissions from agricultural and combustion sources with subsequent deposition can impact carbon (C) and N cycling in sensitive subalpine ecosystems. Chronic N deposition can decrease vegetation biodiversity, alter biogeochemical cycling, and impair montane watersheds. Study of ecosystem C and N sources and accumulation during primary succession indicated that deposition may be an important N source for unvegetated areas on the Pumice Plain of Mount St. Helens. This finding spurred further research into N deposition impacts on subalpine ecosystems in the Cascade Range.
Snowpack storage and processing of N deposition has important implications for informing N emission sources and ecosystem N cycling. We measured rates, N forms, and sources of N deposition in subalpine snowpack at Mount Rainier National Park. Measured ambient deposition rates could exceed critical loads for lichen biodiversity and N enrichment of montane watersheds. Ammonium deposition was the dominant form measured in snowpack. However, snowpack microbial nitrification may be converting this ammonium into nitrate, facilitating leaching of N deposition to watersheds.
We examined the influence of snow regime on subalpine ecosystem C and N cycling at Mount Rainier under ambient conditions and in climate change scenarios. Timing of snow release influenced ecosystem C and N storage and loss. Climate change may reduce snow accumulation by up to 80% at Mount Rainier by 2050. Snowpack loss may enhance ecosystem C and N accumulation during the growing season and increase winter N leaching.
We examined the N deposition fate in the subalpine ecosystem as well as N deposition critical loads under ambient conditions and in climate change scenarios. Soil was the major deposition fate while leaching was the dominant N loss. Increasing N deposition by 75% above ambient rates exceeded N loss critical loads, causing detrimental impacts in subalpine meadows and watersheds. Chronic elevated N deposition was found to increase vegetation biomass and N storage. Climate change increased inorganic N leaching to watersheds and enhanced vegetation N uptake independently of the N deposition rate. Thus, critical loads should be considered dynamic thresholds that shift with ecological conditions even with stable or decreasing N deposition rates.
|Advisor:||Evans, R. Dave|
|Commitee:||Adam, Jennifer C., Bishop, John G., Lamb, Brian K.|
|School:||Washington State University|
|School Location:||United States -- Washington|
|Source:||DAI-B 78/12(E), Dissertation Abstracts International|
|Keywords:||Climate change, Nitrogen deposition, Primary succession, Stable isotopes|
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