Hysteresis Patterns of Watershed Nitrogen Retention and Loss Over the Past 50 years in United States Hydrological Basins. Issue 4 (18th April 2021)

• Record Type: Journal Article
• Authors: Newcomer, Michelle E.; Bouskill, Nicholas J.; Wainwright, Haruko; Maavara, Taylor; Arora, Bhavna; Siirila‐Woodburn, Erica R.; Dwivedi, Dipankar; Williams, Kenneth H.; Steefel, Carl; Hubbard, Susan S.
• Is Part Of: Global biogeochemical cycles. Volume 35:Issue 4(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-04-18
• Subjects: catchment scale nitrogen retention -- ecosystem variability -- nitrogen dynamics -- watershed exports -- Watersheds -- watershed N hysteresis; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2020GB006777
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Patterns of watershed nitrogen (N) retention and loss are shaped by how watershed biogeochemical processes retain, biogeochemically transform, and lose incoming atmospheric deposition of N. Loss patterns represented by concentration, discharge, and their associated stream exports are important indicators of integrated watershed N retention behaviors. We examined continental United States (CONUS) scale N deposition (e.g., wet and dry atmospheric deposition), vegetation trends, and stream trends as potential indicators of watershed N‐saturation and retention conditions, and how watershed N retention and losses vary over space and time. By synthesizing changes and modalities in watershed nitrogen loss patterns based on stream data from 2200 U.S. watersheds over a 50 years record, our work revealed two patterns of watershed N‐retention and loss. One was a hysteresis pattern that reflects the integrated influence of hydrology, atmospheric inputs, land‐use, stream temperature, elevation, and vegetation. The other pattern was a one‐way transition to a new state. We found that regions with increasing atmospheric deposition and increasing vegetation health/biomass patterns have the highest N‐retention capacity, become increasingly N‐saturated over time, and are associated with the strongest declines in stream N exports—a pattern, that is, consistent across all land cover categories. We provide a conceptual model, validated at an unprecedented scale across the CONUS that links instream nitrogen signals to upstream mechanistic landscape processes. Our work can aid in the future interpretation of in‐stream concentrations of DOC and DIN as indicators of watershed N‐retention status and integrators of watershed hydrobiogeochemical processes. Plain Language Summary: Watershed conditions around the world are changing in response to human activities. Indicators of watershed conditions can be streamflow measurements, river chemistry, and landscape characteristics, such as vegetation productivity. In‐stream nitrogen (N) concentrations or exports (flow delivering N downstream) is a potential indicator of watershed conditions because of its significant potential to exacerbate hypoxic conditions along coastal zones. Our work provides an updated conceptual model for understanding watershed N retention conditions in response to atmospheric deposition patterns and watershed mechanisms. In particular, we utilize the wealth of publically available continental US scale stream data from the US Geological Survey to demonstrate how watersheds can respond, recover, or transition to a new steady‐state following atmospheric N‐deposition. Key Points: Declines in nitrogen (N) exports in 34% of CONUS stream gages coincide with increasing N deposition and increasing vegetation productivity Watershed N retention displays both unique hysteresis (recovery) patterns and one‐way transitions to new states Atmospheric deposition and vegetation explain >45% of the variability in watershed N retention trends, land use trends explain <10%