Dissertation/Thesis Abstract

Biogeochemical Responses to Atmospheric Nitrogen Deposition in Subalpine Ecosystems of the Cascades
by Poinsatte, Justin Paul, Ph.D., Washington State University, 2017, 183; 10260924
Abstract (Summary)

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.

Indexing (document details)
Advisor: Evans, R. Dave
Commitee: Adam, Jennifer C., Bishop, John G., Lamb, Brian K.
School: Washington State University
Department: Biology
School Location: United States -- Washington
Source: DAI-B 78/12(E), Dissertation Abstracts International
Subjects: Ecology, Biogeochemistry
Keywords: Climate change, Nitrogen deposition, Primary succession, Stable isotopes
Publication Number: 10260924
ISBN: 978-0-355-09408-4
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