Hydro-ecological linkages in urbanizing watersheds: An empirical assessment of in-stream nitrate loss and evidence of saturation kinetics

• Published in: Journal of Geophysical Research Atmospheres Vol. 114; no. G4
• Main Authors: Claessens, Luc, Tague, Christina L., Band, Lawrence E., Groffman, Peter M., Kenworthy, Stephen T.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 03.12.2009; American Geophysical Union
• Subjects: Biogeochemistry; Drought; Earth sciences; Earth, ocean, space; Exact sciences and technology; Freshwater; Geobiology; High flow; Hydrology; Kinetics; Low flow; Nitrogen; Nitrogen removal; removal; Saturation; Streams; Surface water; Water quality; Watersheds; United States; Maryland; USA, Maryland; models; Travel time; mass balance; geologic sections; nitrogen; streams; concentration; transport; drainage divide; nitrates; North America; forests; saturation; drainage basins; Flow regime; drought; First order; discharge; kinetics; Rate constant; export; flow
• ISSN: 0148-0227; 2169-8953; 2156-2202; 2156-2202; 2169-8961
• DOI: 10.1029/2009JG001017

We examined the effect of discharge on in‐stream NO3− loss for a suburban stream in Maryland and addressed two questions: (1) How important is in‐stream nitrogen removal in this biogeoclimatic setting? (2) Is there evidence of saturation kinetics? We conducted the study in a 2.4 km forested stream section, over a period of drought and wet conditions (2001–2004). We calculated NO3− loss from monthly and seasonal mass balances and constructed NO3− loss duration curves. NO3− loss was fairly constant across flow regimes, averaging 0.8 g N ha−1 d−1 per unit watershed area. Proportional NO3− loss (relative to transport) reached 96% at low flows and converged to 0% at high flows. Duration curve analysis showed that proportional NO3− loss was generally low, averaging 6% loss, and only 5% of the time exceeded 22% loss. We extrapolated NO3− removal to the 3.8 km2 watershed. In‐stream NO3− removal corresponded to only 2% of watershed nitrogen inputs and 3% of total losses. Our results suggest that for urbanizing watersheds subject to high nitrogen inputs, in‐stream processing has only a limited potential for reducing watershed nitrogen export. We examined whether NO3− loss patterns showed evidence of saturation kinetics. Estimates of water travel time were used to calculate first‐order NO3− loss metrics. NO3− loss first‐order rate constant (Kc) was negatively related with NO3− concentration. This evidence of saturation kinetics suggests that the common use of first‐order process models is not valid in streams subject to high nitrogen inputs, requiring the need for higher‐order process models.