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.