Stream denitrification and total nitrate uptake rates measured using a field 15 N 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 15 N 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% K 15 NO 3 and a conservative solute tracer. Two 15 N tracer addition experiments were performed on consecutive days, the first under ambient NO 3 − concentrations (23 µg N L −1 ) and the second with a NO 3 − addition of approximately 500 µg N L −1 . We fit first‐order NO 3 − uptake and two‐box denitrification models to the longitudinal measurements of tracer 15 N in dissolved NO 3 − , N 2 , and N 2 O in stream water to determine rates. Total NO 3 − 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 NO 3 − and with NO 3 − 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 NO 3 − and with NO 3 − addition, respectively. Denitrification resulted almost exclusively in N 2 production (>99%) and comprised about 16% (±10%) of total NO 3 − uptake rate under ambient NO 3 − concentrations and about 1% (±1%) of total NO 3 uptake rate with NO 3 − addition. Denitrification rate expressed on a mass flux basis was about 12 µmol m −2 h −1 under ambient NO 3 − concentrations, a value within the range reported for other streams with low NO 3 − concentrations.