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

Area of Science:

• Environmental Science
• Ecology
• Hydrology

Background:

• Stream nitrogen (N) concentrations are influenced by complex environmental and anthropogenic factors.
• Understanding these drivers is crucial for managing aquatic ecosystem health and water quality.
• Previous studies have highlighted the role of nitrogen inputs but lacked comprehensive national-scale analyses.

Purpose of the Study:

• To identify and quantify the environmental and anthropogenic drivers of stream nitrogen concentrations nationwide.
• To investigate the relationship between nitrogen inputs, sources, and the forms of nitrogen in streams.
• To develop predictive models for stream nitrogen concentrations based on watershed characteristics.

Main Methods:

• Utilized summer low-flow data from 4997 streams across the conterminous US (2000-2014).
• Integrated watershed information with US EPA's National Rivers and Streams Assessment data.
• Employed random forest models incorporating nitrogen inputs, climatic, and landscape factors.

Main Results:

• Watershed nitrogen inputs explained 51% of the variation in stream total nitrogen (TN).
• Nitrate (NO3) concentrations increased with nitrogen inputs, suggesting nitrogen saturation.
• Models explained 70% (TN), 58% (dissolved inorganic N), and 60% (total organic N) of spatial variation.

Conclusions:

• Nitrogen inputs are a primary driver of stream nitrogen concentrations and forms.
• Nitrogen saturation enhances nitrate export to aquatic ecosystems.
• High nitrogen input watersheds may receive contributions from groundwater and benefit from wetland mitigation.