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. 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 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 by N input intensity. Model results for high N input watersheds indicated potential contributions from contaminated groundwater to high stream N concentrations, but also the mitigating role of wetlands.

Impact/Purpose

Understanding the controls on nutrient concentrations is important for managing aquatic ecosystems. States often develop criteria for nutrient concentrations to help mitigate and manage the negative outcomes caused by excess nutrients in streams. This work focuses on the biologically important summer N concentrations of rivers and streams across the conterminous US. Our coupled analysis of nutrient inputs and stream N concentrations across many watersheds can aid in assessing water quality issues in rivers and streams at regional or national scales. Nearly 5000 US watersheds are examined in this work, capturing the wide variability in stream N concentrations and the associated watershed characteristics across the country. Through this work, we evaluate to what extent variations in N inputs are reflected in stream concentrations of total N and N species in the conterminous US, examine the relationships between various landscape and climate drivers and stream nutrient levels, and identify the context dependency of these relationships in response to intensifying amounts of N inputs. The modeling framework we developed in this study is capable of generating new predictions of water quality in the past or into the future, and helping identify problematic hotspots and monitoring priorities for land managers across the country. It has large implications for the past, current, and future management of surface water resources.

Citation

Lin, J. Context is everything: Interacting inputs and landscape characteristics control stream nitrogen. Monthly Ag Issues Forum Call, NA, April 22, 2021.