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

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 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.