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 N-15 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% (KNO3)-N-15 and a conservative solute tracer. Two 15N tracer addition experiments were performed on consecutive days, the first under ambient NO concentrations (23 mug N L-1) and the second with a NO3- addition of approximately 500 mug N L-1. We fit first-order NO3- uptake and two-box denitrification models to the longitudinal measurements of tracer N-15 in dissolved NO3-, N-2, and N2O in stream water to determine rates. Total NO; uptake rates were 0.028 m(-1) (0.32 mug N m(-2) s(-1)) and 0.01 m(-1) (1.6 mug 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 N-2 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 mumol m(-2) h(-1) under ambient NO3- concentrations, a value within the range reported for other streams with low NO3- concentrations.