Journal
ENVIRONMENTAL SCIENCE & TECHNOLOGY
Volume 55, Issue 12, Pages 7890-7899Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.est.0c07102
Keywords
N inventory; N concentration; N species; water quality; nutrient; conterminous US; spatial variation; random forest; machine learning
Categories
Funding
- USEPA
- Oak Ridge Institute for Science and Education Postdoctoral Research Fellowship Program
- National Academies of Science Research Associateship Program through USEPA
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The study found that nitrogen input rates significantly influenced stream nitrogen concentrations, with model results for high nitrogen input watersheds indicating potential contributions from contaminated groundwater to high stream nitrogen concentrations as well as the mitigating role of wetlands.
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.
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