Simultaneous measurement of benthic denitrification, with the isotope pairing technique and the N2 flux method in a continuous flow-through system

Denitrification rates in a freshwater sediment were measured simultaneously with the N2 flux method and the 15N isotope pairing technique, using a continuous flow-through system designed for direct application of both techniques. With the N2 flux method, denitrification was 151±4 μmol N m−2 h−1 and with the isotope pairing technique denitrification was 149±4 μmol N m−2 h−1. These identical results strongly suggested that both methods measured the true denitrification activity in the system. Using the conventional batch modes, the N2 flux method has shown much larger denitrification rates than 15N isotope pairing. This discrepancy could be explained by the much longer incubation time and smaller water column, required by the N2 flux method. Model simulations of a N2 flux batch incubation showed how NO3− and NH4+ accumulated in the water column and in turn increased nitrification and denitrification activity much above the initial level, as incubation proceeded.