Comparison of denitrification rates in lake sediments obtained by the N2 flux method, the 15N isotope pairing technique and the mass balance approach

Coupled nitrification-denitrification rates in sediments of the shallow lake Nuldernauw, The Netherlands, were measured using the N2 flux method and the 15N isotope pairing technique. It was clarified by different checks that the coupled nitrification-denitrification can be measured with the N2 flux method after a pre-incubation time of about 10 days. These checks included tests on contamination by atmospheric nitrogen, the incubation of irradiated sediments with and without addition of HgCl2 and addition of nitrate. The results of the two methods were different. In general the coupled nitrification-denitrification rates measured with the 15N isotope pairing technique were lower than measured with the N2 flux method. With the 15N isotope pairing technique the denitrification rates of muddy sediments seemed to depend on the sampling date: spring sediments gave much higher denitrification rates than sediments collected in winter. In comparison with the denitrification calculated from a mass balance of the lake the 15N method appears to underestimate the denitrification, whereas results of the N2 flux method were in agreement with mass balance data. The following explanation for this difference was suggested. The 15N isotope pairing technique is based on the idea that the different nitrate species (14NO−3 and 15NO−3) are uniformly mixed and that the sediment is divided in aerobic and anaerobic layers. When however micro sites exist, probably uniform mixing of the nitrate species can not be assumed because of the very tight coupling between nitrification and denitrification in these sites. The coupled nitrification-denitrification will be underestimated.