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Title: Dual nitrate isotopes clarify the role of biological processing and hydrologic flow paths on nitrogen cycling in subtropical low-gradient watersheds

Nitrogen (N) is an important nutrient as it often limits productivity but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low-relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low-relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flow paths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in two of the three watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flow paths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification and denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly becausemore » a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, and mixing) affected nitrate concentrations. In conclusion, together, these results highlight the hydrological and biological controls on N cycling in low-gradient watersheds and variability in N delivery flow paths among adjacent watersheds with similar physical characteristics.« less
 [1] ;  [2] ;  [3] ;  [4] ;  [2] ;  [2]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Climate Change Science Institute and Environmental Sciences Division
  2. Univ. of Georgia, Athens, GA (United States). Warnell School of Forestry and Natural Resources
  3. Univ. of Saskatchewan, Saskatoon, SK (Canada). Global Institute for Water Security and School of Environment and Sustainability; Univ. of Aberdeen, Aberdeen (United Kingdom). School of Geosciences
  4. Univ. of Saskatchewan, Saskatoon, SK (Canada). Global Institute for Water Security and School of Environment and Sustainability; Luxembourg Institute of Science and Technology, Belvaux (Luxembourg). Environmental Research and Innovation Department
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Biogeosciences
Additional Journal Information:
Journal Volume: 121; Journal Issue: 2; Journal ID: ISSN 2169-8953
American Geophysical Union
Research Org:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
Country of Publication:
United States
59 BASIC BIOLOGICAL SCIENCES; subtropics; low-relief watersheds; dual stable isotopes of nitrate; streams; groundwater
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1402317