Seasonality of Recharge Drives Spatial and Temporal Nitrate Removal in a Karst Conduit as Evidenced by Nitrogen Isotope Modeling

Admin Husic, James Fox, Nabil Al Aamery, William Ford, Erik Pollock, Jason Backus

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Nitrogen removal rates can vary with time, space, and external environmental drivers, but are underreported for karst environments. We carried out a multi-year study of a karst conduit where we: (a) measured inputs and outputs of sediment nitrogen (SN and δ15NSed) and nitrate (NO3 and δ15NNO3); (b) developed, calibrated, and applied a numerical model of nitrogen physics and biogeochemistry; and (c) forecasted the impacts of climate and land use changes on nitrate removal and export. Data results from conduit inputs (SN = 0.43% ± 0.07%, δ15NSed = 5.07‰ ± 1.01‰) and outputs (SN = 0.36% ± 0.09%, δ15NSed = 6.45‰ ± 0.71‰) indicate net-mineralization of SN and increase of δ15NSed (p < 10−2). However, δ15NSed increase cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically heavier mineral nitrogen (δ15NNO3 = 11.25‰ ± 6.96‰). Modeled SN and δ15NSed sub-routines provided a boundary condition for DIN simulation and improved NO3 model performance (from NSE = 0.06 to NSE = 0.68). Modeled spatial zones of removal occur in close proximity to conduit entrances, where deposition of labile organic matter promotes a three-fold increase in denitrification (∼60 mg N m−2 d−1). Modeled temporal periods of removal occur during the dry-season where longer residence times cause up to 90% removal of NO3 inputs. Projected effects of environmental drivers suggest an increase in denitrification (+14.1%); however, this removal is largely offset by greater nitrate soil leaching (+28.1%) from wetter regional climate. Results suggest that conduits underlying mature karst terrain experience spatiotemporal removal gradients, which are modulated by solute and sediment delivery.

Original languageEnglish
Article numbere2021JG006454
JournalJournal of Geophysical Research: Biogeosciences
Volume126
Issue number10
DOIs
StatePublished - Oct 2021

Bibliographical note

Funding Information:
The authors thank an associate editor and two anonymous reviewers for their critical review of this manuscript, which greatly improved its quality. The authors also would like to acknowledge primary funding from Kentucky Senate Bill 201. Steve Workman, James Currens, and Chuck Taylor from the Kentucky Geological Survey are greatly acknowledged for their contributions and extensive laboratory and field work performed in the Cane Run Watershed. Lastly, three of the authors were partially supported by the National Science Foundation (Award 1632888).

Funding Information:
The authors thank an associate editor and two anonymous reviewers for their critical review of this manuscript, which greatly improved its quality. The authors also would like to acknowledge primary funding from Kentucky Senate Bill 201. Steve Workman, James Currens, and Chuck Taylor from the Kentucky Geological Survey are greatly acknowledged for their contributions and extensive laboratory and field work performed in the Cane Run Watershed. Lastly, three of the authors were partially supported by the National Science Foundation (Award 1632888).

Publisher Copyright:
© 2021. American Geophysical Union. All Rights Reserved.

Keywords

  • environmental drivers
  • karst
  • nitrate
  • numerical modeling
  • stable isotopes

ASJC Scopus subject areas

  • Soil Science
  • Forestry
  • Water Science and Technology
  • Paleontology
  • Atmospheric Science
  • Aquatic Science
  • Ecology

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