Seasonal watershed-scale influences on nitrogen concentrations across the Upper Mississippi River basin

ABSTRACT

In the Upper Mississippi River basin, after spring fertilizer application, total nitrogen concentration [TN] in streams drops by ~73% from June to September, consistent with effects of seasonal nutrient loading. We hypothesized that this seasonal variability might be affected by land cover (e.g. wetland, cultivated area extent). To test this hypothesis, we adopted a linear mixed-effects modeling approach including periodic functions. However, inclusion of wetland area was not indicated, suggesting that increased model complexity associated with inclusion of contemporary wetland extent was unjustified. While consideration of cultivated extent in relation to mean annual [TN] was shown to improve performance, no evidence was observed that cultivated extent could enhance explanation of intra-annual [TN]. Improved understanding of cause and effect will require improved spatiotemporal data on nitrogen sources and additional critical field experimentation, which in turn is expected to set the stage for advancement in physically- or process-based modeling of nutrient concentrations.

KEYWORDS:

• watershed hydrology
• nitrogen
• seasonality
• nutrient loading
• nutrient dynamics
• contaminant hydrology

Editor A. Fiori Associate editor M. Hutchins

Editor A. Fiori Associate editor M. Hutchins

Acknowledgements

We appreciate nutrient concentration data and assistance from Dale Robertson and David Saad (USGS) and spatial data assistance from Ellen D’Amico and Amy Prues. We also thank Grey Evenson and Brent Johnson for helpful suggestions on the paper. This project was supported in part by an appointment to the Science Education Programs at the US Environmental Protection Agency, administered by ORAU through the US Department of Energy Oak Ridge Institute for Science and Education. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Statements in this publication reflect the authors’ professional views and opinions and should not be construed to represent any determination or policy of the US Environmental Protection Agency. An anonymous reviewer’s comments along with those of the associate editor greatly improved the manuscript.

Author contribution

Conceptualization, MLW and OM; data curation, MLW; formal analysis, MLW; investigation, MLW; methodology, MLW and OM; software, MLW and OM; supervision, OM; writing – original draft, MLW; writing – review and editing, MLW and OM.

Disclosure statement

M Wine declares the Oak Ridge Institute for Science and Education at US EPA funding as a potential conflict of interest.

Data availability

Precipitation and temperature data were obtained from the PRISM Climate Group (https://prism.oregonstate.edu/). Solute concentration data are available from the SPARROW (Spatially Referenced Regression on Watershed Attributes) Group.

Supplementary material

Supplemental data for this article can be accessed here