HR: 14:20h
AN: H52C-03    [PDF]
TI: Nitrogen isotopes as indicators of streamflow generation processes in a headwater forested catchment: Focusing on atmospheric NO$_{3}^{-}$ contribution using $\delta^{18}$O signature
AU: * Ohte, N
EM: nobu@bluemoon.kais.kyoto-u.ac.jp
AF: Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Kyoto, 606-8502 Japan
AU: Sebestyen, S D
EM: sdsebest@syr.edu
AF: College of Environmental Science and Forestry, Sate University of New York, 1 Forestry Drive, Syracus, NY 13210 United States
AU: Doctor, D H
EM: dhdoctor@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wankel, S D
EM: sdwankel@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Shanley, J B
EM: jshanley@usgs.gov
AF: US Geological Survey, 87 State St. Room 324, Montpelier, VT 05602 United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Boyer, E W
EM: ewboyer@syr.edu
AF: College of Environmental Science and Forestry, Sate University of New York, 1 Forestry Drive, Syracus, NY 13210 United States
AB: To quantify the contributions of atmospheric nitrogen deposition and mechanisms of nitrate discharge to stream, nitrogen chemistry and isotopes ($\delta$$^{15}$N and $\delta^{18}$O of NO$_{3}^{-}$) of streamwater were studied as part of an ongoing study of nutrient dynamics at the Sleepers River Research Watershed in Vermont, USA. We employed novel analytical procedures for high throughput of NO$_{3}^{-}$ isotopic measurements. The denitrifier method for measurement of $\delta^{15}$N and $\delta^{18}$O of NO$_{3}^{-}$ requires a smaller volume of water samples than previously applied methods, thus it enables fine resolution analysis of isotopes for stream, well, and soil water samples. Samples were collected throughout the spring 2003 snowmelt. Snowmelt runoff was initiated in the middle of March and peaked at the end of the month. Then, the runoff rate decreased gradually through April and May, and responded to several storm events. The highest concentration of NO$_{3}^{-}$ in the stream was observed at the beginning of snowmelt (the end of March), and thereafter it declined continuously. The temporal course of NO$_{3}^{-}$ discharge process during snowmelt period was divided into four phases based on changes in the relationship between runoff rate and NO$_{3}^{-}$ concentration. During the earliest phase (very low runoff rate and highest NO$_{3}^{-}$ concentration) isotope signatures, especially $\delta^{18}$O of NO$_{3}^{-}$, indicated higher contribution of the atmospherically derived NO$_{3}^{-}$, meaning that the direct discharge from snow pack was the dominant source of NO$_{3}^{-}$ to the stream. This also suggested that streamwater consisted only of a small volume of groundwater discharge and melt water of the in-stream snow pack and/or stream-covering snow pack. The $\delta^{15}$N and $\delta^{18}$O isotope compositions of NO$_{3}^{-}$ during the middle phase of snowmelt indicated that the contribution of the NO$_{3}^{-}$ generated by nitrifiers in soil increased gradually accompanied with increase of groundwater level. These detailed descriptions in the changes of NO$_{3}^{-}$ discharge during snowmelt events were enabled by the dual-isotope analysis of NO$_{3}^{-}$. The fine resolution isotope analysis of NO$_{3}^{-}$ in our experiment can provide advantages for elucidating the discharge mechanisms of nitrogen in forested watersheds with high atmospheric nitrogen depositions.
DE: 1800 HYDROLOGY
DE: 1860 Runoff and streamflow
DE: 1863 Snow and ice (1827)
DE: 4845 Nutrients and nutrient cycling
DE: 4870 Stable isotopes
SC: Hydrology [H]
MN: 2003 Fall Meeting