Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Using Stable Isotopes of Nitrogen and Oxygen as Environmental Indicators of Nitrogen Deposition in the Sonoran Desert

Auteur : Bell, Michael David

Université de soutenance : University of California – Riverside

Grade : Doctor of Philosophy (PhD) in Plant Biology 2012

Résumé
Undisturbed wildland ecosystems are impacted by anthropogenic nitrogen (N) emissions being deposited significant distances from their sources. The main sources of atmospheric N inputs in the United States include industrial and automotive exhaust and emissions from agricultural wastes and fertilizers. Emission sources can be differentiated by analyzing the δ18O and δ15N of emitted compounds. Through a combination of field and laboratory studies, this dissertation aims to identify the various emission sources impacting the western Sonoran Desert and to determine how they are altering plant available nitrogen in the region. The first objective of this research was to measure if fractionation of HNO3 δ18O and δ15N occurs to filters of commonly used ambient HNO3 collectors. These collectors were then placed along a N deposition gradient to measure the isoscapes of HNO3 δ15N and δ18O and extrapolate how each emission source contributes to regional anthropogenic N. Lastly, soil and plant tissue were collected at each site to evaluate whether atmospheric patterns were conservedthrough the ecosystem. Results suggest that ambient HNO3 does not fractionate isotopically when binding to the passive sampler filters when exposed in a continuous stirred tank reactor. The HNO3 δ18O and δ15N were within 0.5‰ of the source HNO3 when exposed to controlled high (20 μg/m3 ) and low (10 μg/m3 ) concentrations for four weeks. When exposed under field conditions, the samplers verified that anthropogenic nitrogen impacting the Coachella Valley came from two sources based on the changes to HNO3 δ 18O and δ15N across the area ; vehicle emissions from the Los Angeles air basin and agricultural emissions from around the Salton Sea. There was also a distinct separation in values for sites within Joshua Tree National Park suggesting that the Little San Bernardino Mountains act as effective barrier from air pollution moving in the park. Finally, surface soil NO3- was the most effective indicator of anthropogenic additions ; with NO3- concentrations and δ18O linearly tracking atmospheric HNO3 concentrations. The leaf tissue δ15N of the regionally dominant shrub, Larrea tridentata, also decreased at sites with increasing anthropogenic inputs. Both the atmospheric samplers and soil surface N analyses will provide land managers with effective tools to quickly identify regions of high anthropogenic inputs in a desert environment.

Mots clés : Biogeochemistry ; Plant biology ; Ecology ; 15N ; 18O ; Bioindicators ; Desert ; Nitrogen Deposition ; Soil Nitrogen

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