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Assessing the Nitrogen Saturation Status of Appalachian Forests using Stable Isotopes of Nitrate

Rose, Lucy (2014) Assessing the Nitrogen Saturation Status of Appalachian Forests using Stable Isotopes of Nitrate. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Chronic elevated nitrogen (N) deposition can lead to ecosystem N saturation, which is theorized to occur when N supply exceeds biological demand and excess N leaches to receiving waters. This research examined the post-depositional fate and transport of atmospheric nitrate in Appalachian forests across spatial and temporal scales by characterizing the nitrate stable isotopic composition (δ15N, δ18O, and Δ17O) of precipitation, soil water, and streams. Data indicate that elevated N deposition does not saturate biological demand; rather, N processing becomes more extensive as N availability increases. Along a regional N deposition gradient (North Carolina to New Hampshire), mean proportions of atmospheric nitrate in streams were inversely related to long-term annual average nitrate deposition. Stream nitrate concentrations were also negatively correlated to the proportion of atmospheric nitrate in streams (R2=0.23; p<0.05). Similar relationships occurred along an N saturation gradient in four watersheds at Fernow Experimental Forest (West Virginia). The most N-saturated watershed had the highest stream nitrate concentrations (mean=3.7 mg L-1) but the lowest proportions of atmospheric nitrate in the stream (mean=5%). Conversely, the stream in the N-limited watershed had the lowest nitrate concentrations (mean=0.0 mg L-1) and the highest proportions of atmospheric nitrate (mean=42% among samples with sufficient nitrate for isotope analysis). High spatial variability of nitrate sources in one watershed at Fernow (WS4) suggests a decoupling of source dynamics across spatial scales. Proportions of atmospheric nitrate in soil solution ranged from zero to 96% across WS4, but consistently low proportions of atmospheric nitrate in the stream suggest that watershed areas with high proportions of atmospheric nitrate may not contribute significantly to the stream. Storm event water and nitrate isotope data support this idea, indicating transient hydrologic flowpaths from hillslopes to the stream during storms. Although these transient flowpaths resulted in a wide range of mean event water contributions to stormflow (6% to 34%) during events, the maximum proportion of atmospheric nitrate in stormflow was only 8%. These trends in nitrate source contributions to streams along gradients of space, time, N deposition, and N saturation suggest that the widely-accepted mechanisms of nitrogen saturation require reevaluation.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Rose, Lucylab125@pitt.eduLAB125
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairElliott, Emilyeelliott@pitt.eduEELLIOTT
Committee MemberAdams, Mary
Committee MemberBain, Danieldbain@pitt.eduDBAIN
Committee MemberStewart, Brianbstewart@pitt.eduBSTEWART
Committee MemberWerne, Josefjwerne@pitt.eduJWERNE
Date: 24 September 2014
Date Type: Publication
Defense Date: 16 July 2014
Approval Date: 24 September 2014
Submission Date: 19 August 2014
Access Restriction: 5 year -- Restrict access to University of Pittsburgh for a period of 5 years.
Number of Pages: 259
Institution: University of Pittsburgh
Schools and Programs: Dietrich School of Arts and Sciences > Geology and Planetary Science
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: nitrogen deposition, stable isotopes, atmospheric nitrate, Fernow Experimental Forest
Date Deposited: 24 Sep 2014 18:32
Last Modified: 24 Sep 2019 05:15


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