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 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.