Recognizing both denitrification and nitrogen consumption improvise performance of stream diel N2 flux models

Authors: NIFONG, RACHEL - Oak Ridge Institute For Science And Education (ORISE); Taylor, Jason; Moore, Matthew; FARRIS, JERRY - Arkansas State University

Submitted to: Limnology and Oceanography Methods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2020
Publication Date: 5/5/2020
Citation: Nifong, R.L., Taylor, J.M., Moore, M.T., Farris, J.L. 2020. Recognizing both denitrification and nitrogen consumption improvise performance of stream diel N2 flux models. Limnology and Oceanography Methods. 16:168-182. https://doi.org/10.1002/lom3.10361.
DOI: https://doi.org/10.1002/lom3.10361

Interpretive Summary: Nitrogen significantly enhances agricultural production to meet the food and fiber needs of a growing human population. Excess nitrogen often travels from agricultural fields into water bodies via agricultural ditches with unintended negative outcomes. Efforts to mitigate inputs of excess nitrogen into water bodies include the promotion of denitrification which transforms excess nitrogen into nonreactive nitrogen gas. As nitrogen gas makes up the majority of Earth’s atmosphere, denitrification is difficult to measure. Recent advances in modeling have enabled researchers to make daily estimates of denitrification in freshwater environments. An ARS postdoctoral researcher worked with two ARS scientists and an academic partner to conduct a series of measurements using this new method in various ditch environments to estimate daily denitrification fluxes. We found that the current model did not replicate observed patterns in shallow ditches due to the strong influence of hourly variation in light and temperature. The research team expanded the current model to take advantage of more data and also investigate potential improvement within the model by incorporating factors that representing other naturally occurring processes known to influence nitrogen gas flux. While more studies are needed to understand when and where nitrogen gas consumption processes take place in agricultural ditches, a new model developed with this research represents a significant step forward in improving our ability to accurately estimate nitrogen gas fluxes from agricultural ditches. Researchers can use this model to assess how different best management practices enhance denitrification and mitigate nitrogen pollution in agricultural ditch networks.

Technical Abstract: There is keen interest to enhance denitrification within intervening aquatic habitats between agricultural areas and downstream aquatic ecosystems to reduce nitrogen (N) loading impacts. We conducted a series of measurements to examine whole system in situ diel denitrification estimates in experimental ditch and stream environments using a Bayesian one-station diel N2 flux model. Model estimates revealed complex patterns that indicate fluxes may be controlled by the balance of both N2 production via denitrification and consumption driven by physical or biological processes associated with strong diel patterns in environmental conditions. We investigated potential improvements in model fits to observed data associated with the addition of a N2 consumption term to represent biological (N2 fixation) or physical (bubble formation and N2 scavenging) mechanisms associated with daytime photosynthesis. We also expanded the current one-station diel flux model to a two-station model to estimate denitrification in discrete reaches. Our modified diel N2 flux models improved model fit significantly across three metrics (Nash-Sutcliffe Efficiency (NSE), root mean square ratio (RSR), and percent bias (PBIAS)) increasing their utility in shallow, open canopy, lotic systems. While more studies are needed to understand specific mechanisms associated with N2 consumption processes in small agricultural drainages as well as environmental conditions affecting their relative importance, these results improve estimates of N2 flux where dynamic conditions and heterogeneity of habitats create severe diel patterns in factors controlling dissolved gas concentrations and prohibit accurate estimates of N2 flux using existing models.