Context is Everything: Interacting Inputs and Landscape Characteristics Control Stream Nitrogen

To understand the environmental and anthropogenic drivers of stream nitrogen (N) concentrations across the conterminous US, we combined summer low-flow data from 4997 streams with watershed information across three survey periods (2000–2014) of the US EPA’s National Rivers and Streams Assessment. Wa...

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Bibliographic Details
Published inEnvironmental science & technology Vol. 55; no. 12; pp. 7890 - 7899
Main Authors Jiajia Lin, Compton, Jana E, Hill, Ryan A, Herlihy, Alan T, Sabo, Robert D, Brooks, J. Renée, Weber, Marc, Pickard, Brian, Paulsen, Steve G, Stoddard, John L
Format Journal Article
LanguageEnglish
Published United States American Chemical Society 15.06.2021
Subjects
Anthropogenic factors
Aquatic ecosystems
Contaminants in Aquatic and Terrestrial Environments
Ecosystem
Environmental Monitoring
Groundwater
Groundwater pollution
Human influences
Landscape
Low flow
Nitrogen
Nitrogen - analysis
Rivers
Seasons
Spatial variations
Streams
Watersheds
United States > US
N concentration
spatial variation
N species
nutrient
water quality
N inventory
random forest
machine learning
conterminous US
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Summary:To understand the environmental and anthropogenic drivers of stream nitrogen (N) concentrations across the conterminous US, we combined summer low-flow data from 4997 streams with watershed information across three survey periods (2000–2014) of the US EPA’s National Rivers and Streams Assessment. Watershed N inputs explained 51% of the variation in log-transformed stream total N (TN) concentrations. Both N source and input rates influenced stream NO3/TN ratios and N concentrations. Streams dominated by oxidized N forms (NO3/TN ratio > 0.50) were more strongly responsive to the N input rate compared to streams dominated by other N forms. NO3 proportional contribution increased with N inputs, supporting N saturation-enhanced NO3 export to aquatic ecosystems. By combining information about N inputs with climatic and landscape factors, random forest models of stream N concentrations explained 70, 58, and 60% of the spatial variation in stream concentrations of TN, dissolved inorganic N, and total organic N, respectively. The strength and direction of relationships between watershed drivers and stream N concentrations and forms varied with N input intensity. Model results for high N input watersheds not only indicated potential contributions from contaminated groundwater to high stream N concentrations but also the mitigating role of wetlands.
Bibliography:USDOE
ISSN:0013-936X
1520-5851
DOI:10.1021/acs.est.0c07102