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