Citation:

Bellmore, R., J. Compton, M. Weber, R. Hill, D. Thornbrugh, AND Reneej Brooks. Effects of Nitrogen Inputs and Watershed Characteristics on Summer Stream Nitrogen Concentrations: A National-Scale Analysis. American Geophysical Union, San Francisco, CA, December 13 - 18, 2015.

Impact/Purpose:

ORISE post-doc Rebecca Bellmore, along with colleagues from EPA and ORISE, used data from EPA's 2008-2009 National Rivers and Streams Assessment to better understand the effects of natural and human factors on stream nitrogen (N) concentrations across the US. Human activities have doubled nitrogen release to the environment across the globe, and have increased N inputs by nearly five-fold since 1900 in the US. This N release to waters can lead to increases in nitrate concentrations in streamwater that are harmful to human health and ecosystems. In this study, Dr. Bellmore and colleagues with EPA and ORISE combined data for agricultural inputs such as fertilizers, legumes and manure, wastewater inputs and atmospheric inputs from cars and factories with chemistry data collected for approximately 1100 streams in the US. The researchers fond that atmospheric inputs dominated across many of the sites, but that where N concentrations were high, agricultural inputs were the largest source. Stream N concentrations were strongly related to the total amount of N going into the watershed, indicating that the human-derived N inputs are impacting stream N concentrations across the US. They also found that local stream management and land-use factors of forest and wetland cover may moderate the effects of increasing N inputs. These findings help the researchers better understand where different N sources are contributing to N-related impacts in streams across the US, which can help with management and protection of our water resources.

Description:

Nitrogen (N) inputs to the landscape have been linked previously to N loads exported from watersheds at the national scale; however, stream N concentration is arguably more relevant than N load for drinking water quality, freshwater biological responses and establishment of nutrient criteria. In this study, we combine national-scale anthropogenic N input data, including synthetic fertilizer, crop biological N fixation, manure applied to farmland, atmospheric N deposition, and point source inputs, with data from the 2008-09 National Rivers and Streams Assessment to quantify the relationship between N inputs and in-stream concentrations of total N (TN), dissolved inorganic N (DIN), and total organic N (TON) (calculated as TN – DIN). In conjunction with simple linear regression, we use multiple regression to understand how watershed and stream reach attributes modify the effect of N inputs on N concentrations. Median TN was 0.50 mg N L-1 with a maximum of 25.8 mg N L-1. Total N inputs to the watershed ranged from less than 1 to 196 kg N ha-1 y-1, with a median of 14.4 kg N ha-1 y-1. Atmospheric N deposition was the single largest anthropogenic N source in the majority of sites, but, agricultural sources generally dominate total N inputs in sites with elevated N concentrations. The sum of all N inputs were positively correlated with concentrations of all forms of N [r2 = 0.44, 0.43, and 0.18 for TN, DIN, and TON, respectively (all log-transformed), n = 1112], indicating that watershed N inputs are strongly related to stream N concentrations during the summer, despite this being a biologically active and N-retentive period. Additionally, model results suggest that watershed characteristics like wetland area, riparian disturbance and forest cover moderate the effects of watershed N loading on in-stream N concentrations, and different forms of N are likely to respond differently to increasing agricultural and atmospheric N inputs depending on local watershed characteristics.

Record Details: