Testing assumptions for nitrogen transformation in a low-gradient agricultural stream

William I. Ford, James F. Fox, Erik Pollock, Harold Rowe, Suvankar Chakraborty

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


Common assumptions of the nitrogen cycle in agricultural streams point to biologic transformation of nitrate being tightly linked to benthic carbon turnover within fine sediments of the streambed. While the nitrogen and carbon linkage has been supported in agricultural streams using multi-week stream injection studies, few studies have tested these assumptions using multi-year time-series of carbon and nutrient species. We made elemental and isotopic measurements from 8. years of weekly suspended sediment samples on the main-stem, 14. months of dissolved nitrate samples on the main-stem and tributaries, and point observations of sediment samples from benthic algae and stream banks in an agricultural stream to test the assumptions. Results from Empirical Mode Decomposition of carbon and nitrogen time series suggest agreement with the prevailing assumption and coupling of benthic carbon dynamics with nitrate from late spring through fall during the 8. year sampling period for the temperate stream. During late spring, summer and fall, autotrophic growth and organic matter decomposition assist with controlling temporary sequestration of nitrate and denitrification in stream sediments, respectively. Contrary to conventional wisdom, our data results suggest decoupling of carbon and nitrogen dynamics from winter through mid-spring for much of the 8. year sampling period. During the winter and spring, nitrate loadings from upland fertilizer application and delivery of upland sediments by storm events are shown to instantaneously increase sediment nitrogen and instantaneously decrease transported nitrate. The result is attributed to abiotic transfer of transient nitrate storage during the winter and early spring due to variably charged sesquioxides within streambed sediments. The results provide the first study, to our knowledge, of the potential importance of nitrate sorption in the stream nitrogen cycle and potential implications are that sorption could retard nitrate loadings from downstream transport and increase the potential for denitrification beyond which would be expected with purely biologic based assumptions.

Original languageEnglish
Pages (from-to)908-922
Number of pages15
JournalJournal of Hydrology
StatePublished - Aug 1 2015

Bibliographical note

Funding Information:
We thank Jason Backus with the Kentucky Geological Survey Laboratory for analysis of the concentration data and numerous undergraduate and graduate research associates that assisted with data collection and analysis. We thank the University Of Kentucky Department Of Civil Engineering for partial funding of the graduate student while at UK. We gratefully acknowledge financial support of this research under National Science Foundation Award #0918856 and Kentucky Science & Engineering Foundation Award #2687-RDE-015.

Publisher Copyright:
© 2015 Published by Elsevier B.V.


  • Agricultural watersheds
  • Autotrophic production
  • Empirical mode decomposition
  • Stream nitrogen cycle
  • Surficial fine-grained laminae

ASJC Scopus subject areas

  • Water Science and Technology


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