Nitrogen inputs drive nitrogen concentrations in U.S. streams and rivers during summer low flow conditions

Abstract

Ecological and human health impairments related to excess nitrogen (N) in streams and rivers remain widespread in the United States (U.S.) despite recent efforts to reduce N pollution. Many studies have quantified the relationship between N loads to streams in terms of N mass and N inputs to watersheds; however, N concentrations, rather than loads, are more closely related to impacts on human health and aquatic life. Additionally, concentrations, rather than loads, trigger regulatory responses. In this study, we examined how N concentrations are related to N inputs to watersheds (atmospheric deposition, synthetic fertilizer, manure applied to agricultural land, cultivated biological N fixation, and point sources), land cover characteristics, and stream network characteristics, including stream size and the extent of lakes and reservoirs. N concentration data were collected across the conterminous U.S. during the U.S. Environmental Protection Agency's 2008-09 National Rivers and Streams Assessment (n = 1966). Median watershed N inputs were 15.7 kg N ha yr. Atmospheric deposition accounted for over half the N inputs in 49% of watersheds, but watersheds with the highest N input rates were dominated by agriculture-related sources. Total N input to watersheds explained 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 that terrestrial factors that can be managed, such as agricultural N use practices and wetland or riparian areas, control the spatial variability in stream N concentrations across the conterminous U.S.

Where Have All the Nutrients Gone? Long‐Term Decoupling of Inputs and Outputs in the Willamette River Watershed, Oregon, United States

“…Other regional and national studies relating terrestrial nutrient balances and water quality have identified hydrology as an important driver, including at the national scale for TP (Metson et al, 2017), along the West Coast of the U.S. for TN (Schaefer et al, 2009; Sobota et al, 2009), and for the Northeastern U.S. over time for both TN and TP (Hale et al, 2015). At the national scale, interannual variability in precipitation drives nutrient export over time, but inputs drive patterns in space (Bellmore et al, 2018; Sinha & Michalak, 2016).…”

Transport of N and P in U.S. streams and rivers differs with land use and between dissolved and particulate forms

“…Compared to marine and lake ecosystems, streams and rivers may respond differently to N and P enrichment because of stronger terrestrial linkages (e.g., geology, land use, dependence on terrestrial organic matter; Tank et al 2010), stronger influence of hydrology (Green and Finlay 2010, Leong et al 2014), and associated biogeochemical processes that are unique to the unidirectional flow of water (nutrient spiraling; Newbold et al 1983). Therefore, stream and river biogeochemical processes likely interact in complex ways with point and non‐point nutrient inputs from human activities to affect dissolved and particulate nutrient transport to lakes and coastal zones (Alexander et al 2008, Dodds and Smith 2016, Bellmore et al 2018).…”

Non-Parametric Analysis of Nitrogen Trends in the form of Nitrate and Nitrite in Rivers and Streams of the Contiguous United States for 1990-2019

“…While acknowledging these excellent research projects, a national scale assessment with monthly, yearly, and decadal timescales seems justified. Although there are a few national-scale studies, their scope, objective, and approach are different from this project [16][17][18].…”