Stream denitrification and total nitrate uptake rates measured using a field 15N tracer addition approach

Patrick J. Mulholland, H. Maurice Valett, Jackson R. Webster, Steven A. Thomas, Lee W. Cooper, Stephen K. Hamilton, Bruce J. Peterson

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161 Scopus citations

Abstract

We measured denitrification and total nitrate uptake rates in a small stream (East Fork of Walker Branch in eastern Tennessee) using a new field 15N tracer addition and modeling approach that quantifies these rates for entire stream reaches. The field experiment consisted of an 8-h addition of 99 atom% K15NO3 and a conservative solute tracer. Two 15N tracer addition experiments were performed on consecutive days, the first under ambient NO3- concentrations (23 μg N L-1) and the second with a NO3- addition of approximately 500 μg N L-1. We fit first-order NO3 - uptake and two-box denitrification models to the longitudinal measurements of tracer 15N in dissolved NO3-, N2, and N2O in stream water to determine rates. Total NO3- uptake rates were 0.028 m-1 (0.32 μg N m-2 s-1) and 0.01 m-1 (1.6 μg N m -2 s-1) under ambient NO3- and with NO3- addition, respectively. Denitrification rates were 0.0046 m-1 (uncertainty range of 0.002 to 0.008 m-1) and 9 × 10-5 m-1 (uncertainty range of 3 × 10 -5 to 21 × 10-5 m-1) under ambient NO3- and with NO3- addition, respectively. Denitrification resulted almost exclusively in N2 production (>99%) and comprised about 16% (±10%) of total NO 3- uptake rate under ambient NO3- concentrations and about 1% (±1%) of total NO3 uptake rate with NO3- addition. Denitrification rate expressed on a mass flux basis was about 12 μmol m-2 h-1 under ambient NO3- concentrations, a value within the range reported for other streams with low NO3- concentrations.

Original languageEnglish
Pages (from-to)809-820
Number of pages12
JournalLimnology and Oceanography
Volume49
Issue number3
DOIs
StatePublished - May 2004

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