To investigate how hydrological and biogeochemical processes control the forms and concentrations of stream nutrients in upland forests affected by elevated atmospheric nitrogen deposition, I traced sources of water, nitrogen, and dissolved organic carbon (DOC) at the Sleepers River Research Watershed (northeastern Vermont, USA). To augment long-term weekly hydrochemical data that have been collected at the site since 1991, I collected high-frequency stream water samples over multiple storm flow events from 2002 to 2005. During autumn, baseflow nitrate concentrations decreased by an order of magnitude during leaf fall and dissolved organic nitrogen briefly became the dominant form of stream nitrogen. Quantifying terrestrial to aquatic linkages revealed how coupled hydrological and biogeochemical processes affected stream nutrient variation during autumn including the "nitrate crash" and rebound. A mass balance showed that a stream reach during baseflow was a net sink for nitrate while a net source of DOC and DON. Nitrate concentrations rebounded from this "nitrate crash" during storm flow events. Isotopic signatures and end-member mixing analysis revealed when nitrate and dissolved organic nitrogen (DON) were flushed to the stream from terrestrial source areas and that up to 30% of the stream nitrate was directly contributed from atmospheric sources. Results from snowmelt studies showed when solutes entered the stream and that variable source areas were linked to the stream by preferential shallow subsurface and overland flowpaths. The highest stream nutrient concentrations occurred when nitrate originated from atmospheric sources as well as nitrified sources and terrestrial organic matter was the dominant source of DOC and DON. In the third component of my work, I examined the long-term record of stream hydrochemistry to explore the relationship between catchment wetness and stream nutrient loadings and to assess how stream nutrient loadings may respond to climate change. Model results suggest that leaching of nitrate and DOC will seasonally shift due to anthropogenic climate forcing and affect the timing and magnitude of annual stream loadings in the northeast USA during the next century. Net annual stream runoff (+8%) and DOC flux (+9%) increases were primarily affected by increased winter precipitation. In contrast, decreased annual flux of stream nitrate (-2%) reflected a greater effect of growing season controls on stream nitrate that resulted as days shifted to the longer growing seasons. Overall, these studies identified hydrological transport processes, source variation, and biogeochemical transformations as key processes that influenced stream nutrient variation.
|Advisor:||Boyer, Elizabeth W., Briggs, Russell D.|
|Commitee:||Germain, Rene, Mitchell, Myron J., Shanley, James B.|
|School:||State University of New York College of Environmental Science and Forestry|
|Department:||Forest Resources Management|
|School Location:||United States -- New York|
|Source:||DAI-B 69/03, Dissertation Abstracts International|
|Keywords:||Atmospheric nitrogen deposition, Climate change, DOM composition, Dissolved organic carbon, Hydrological connectivity, Nitrate isotopes, Nutrient retention, Sleepers River Research Watershed, Snowmelt, Vermont|
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