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

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

Limnol. Oceanogr., 49(3), 2004, 809-820 | DOI: 10.4319/lo.2004.49.3.0809

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 x 10-5 m-1 (uncertainty range of 3 x 10-5 to 21 x 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 NO3- 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.

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