Low threshold for nitrogen concentration saturation in headwaters increases regional and coastal delivery

Abstract

River corridors store, convey, and process nutrients from terrestrial and upstream sources, regulating delivery from headwaters to estuaries. A consequence of chronic excess nitrogen loading, as supported by theory and field studies in specific watersheds, is saturation of the biogeochemically-mediated nitrogen removal processes that weakens the capacity of the river corridor to remove nitrogen. Regional nitrogen models typically assume that removal capacity exhibits first-order behavior, scaling positively and linearly with increasing concentration, which may bias the estimation of where and at what rate nitrogen is removed by river corridors. Here we estimate the nitrogen concentration saturation threshold and its effects on annual nitrogen export from the Northeastern United States, revealing an average 42% concentration-induced reduction in headwater removal capacity. The weakened capacity caused an average 10% increase in the predicted delivery of riverine nitrogen from urban and agricultural watersheds compared to estimates using first-order process assumptions. Our results suggest that nitrogen removal may fall below a first-order rate process as riverine concentration increases above a threshold of 0.5 mg N l −1 . Threshold behavior indicates that even modest mitigation of nitrogen concentration in river corridors above the threshold can cause a self-reinforcing boost to nitrogen removal.

Sinuosity‐Driven Hyporheic Exchange: Hydrodynamics and Biogeochemical Potentials

“…As mentioned before, these boundary conditions covary over space and time (Creed et al., 2015), constraining exchange dynamics, residence times, and substrate supply (e.g., Dwivedi et al., 2018; Gomez‐Velez et al., 2017; Song et al., 2020; Wu et al., 2021). Future work should consider the temporal dynamics and correlations of discharge and concentrations (e.g., Dwivedi et al., 2018; Gomez‐Velez et al., 2017), the influence of heterogeneity in sediments (e.g., Sawyer, 2015), and the potential implications of including our conceptualization into reach‐scale water quality models, where the effects of concentrations on reaction rates play a significant role (e.g., Schmadel et al., 2020).…”

Small Reservoirs as Nitrogen Transformers: Accounting for Seasonal Variability in Inorganic and Organic Nitrogen Processing

“…Under these circumstances, the lower end of the reservoirs may become supply limited because any upstream DIN inputs are retained rapidly in the upper end of the reservoir (Wollheim et al., 2018). As discharge slowly increases, high retention proportions at the reservoir scale are maintained because previously N limited sections now receive DIN (Schmadel et al., 2020; Wollheim et al., 2018). With further increases in discharge, DIN supplies to reservoirs increase at a faster rate than DIN demand, eventually saturating the system (Bernot & Dodds, 2005; Wollheim et al., 2018).…”

River Basin Simulations Reveal Wide-Ranging Wetland-Mediated Nitrate Reductions

“…The nitrate removal efficiency of the wetlands within any HRU h was represented using a nitrogen removal constant (NRC h ) (Table ), a model input parameter that describes the wetland nitrogen removal rate (m year –1 ). The NRC h parameter has been labeled a “settling velocity” but describes the aggregate removal of N via natural processes (i.e., denitrification and settling) , and is effectively equivalent to a nitrogen uptake velocity (m year –1 ), which has been used to empirically describe and model aggregate nitrogen removal in streams, rivers, lakes, and wetlands. − Higher NRC h values depict higher rates of denitrification from the wetlands in HRU h . Removal rates as high as 40 m year –1 have been used in applications of the SWAT modeling framework, which we assumed to be the maximum nitrate removal efficiency .…”