Nitrogen inputs drive nitrogen concentrations in U.S. streams and rivers during summer low flow conditions
Citation:
Bellmore, R., J. Compton, Reneej Brooks, EricW Fox, R. Hill, D. Sobota, D. Thornbrugh, AND M. Weber. Nitrogen inputs drive nitrogen concentrations in U.S. streams and rivers during summer low flow conditions. SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 639:1349-1359, (2018). https://doi.org/10.1016/j.scitotenv.2018.05.008
Impact/Purpose:
This paper presents results of the first study examining the relative effects of N inputs to watersheds, land cover characteristics, and stream network characteristics on stream N concentrations across the conterminous United States. NRC post-doc Rebecca Bellmore based at NHEERL's Western Ecology Division, along with colleagues from EPA, Oregon DEQ 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 conterminous US. Human activities have more than doubled nitrogen release to the environment across the globe. This N release to waters can lead to increases in nitrate concentrations in stream water that are harmful to human health and ecosystems. In this study, Dr. Bellmore and colleagues combined data for agricultural inputs such as fertilizers, legumes and manure, wastewater inputs and atmospheric inputs from cars and factories with river and stream N concentration data collected for nearly two thousand streams in the US. Based on EPA's national survey of nearly two thousand streams, watershed N input best predicted stream N concentration of all landscape, stream and climate variables. The team found that atmospheric inputs dominated across many of the sites, but where N concentrations were high, agricultural inputs were the largest source. Stream N concentrations were strongly related to the total input of N to the watershed, indicating that the human-derived N inputs are impacting stream N concentrations across the US. They also found that local riparian management and land-use factors of forest and wetland cover can moderate the effects of increasing N inputs. These findings help scientists and managers better understand where different N sources are contributing to N-related impacts in streams across the US in order to inform management and protection of US water resources. Terrestrial factors that can be influenced by land management, such as agricultural N use and wetland or riparian ecosystem conservation and restoration, largely control the spatial variability in stream N concentrations across the conterminous U.S. This work is supported by SSWR 4.03 Task C "Monitoring and multi-media modeling approaches for verifying reductions".
Description:
Ecological and human health impairments related to excess nitrogen (N) remain widespread in U.S. streams and rivers despite recent efforts to reduce N loading. Many studies have quantified the relationship between N inputs to watersheds and stream N export in terms of mass loads. However, N concentrations and related water quality measures – rather than loads – often trigger management responses. In this study, we examined how N inputs to watersheds (atmospheric deposition, synthetic fertilizer, manure applied to agricultural land, crop biological N fixation, and point sources), land cover characteristics, and stream network characteristics, including stream size and the extent of lakes and reservoirs, related to N concentrations measured in the conterminous U.S. during the U.S. Environmental Protection Agency’s 2008-2009 National Rivers and Streams Assessment (n=1966). Median watershed N inputs were 15.7 kg N ha-1 yr-1, with 23% of surveyed watersheds receiving N inputs ≥40 kg N ha-1 yr-1. Total N input to a watershed was the best predictor of stream N concentration, explaining 42% and 38% of the variability in total N and dissolved inorganic N concentrations, respectively. Land cover characteristics were also important predictors, with wetland cover muting the effect of agricultural N inputs on N concentrations and riparian disturbance exacerbating it. In contrast, stream variables showed little correlation with N concentrations. This suggests terrestrial factors that can be influenced by land management, such as improving agricultural N use efficiency and restoration of wetland or riparian areas, control the spatial variability in stream N concentrations across the conterminous U.S.
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DOI: Nitrogen inputs drive nitrogen concentrations in U.S. streams and rivers during summer low flow conditions