Stable isotope profiles of nitrogen gas indicate denitrification in oxygen‐stratified humic lakes

Mid‐summer N 2 profiles were analyzed from nine oxygen‐stratified, humic‐acid‐rich lakes using a continuous flow isotope ratio mass spectrometer and a Gasbench II device. Sample preparation steps were performed under water to avoid air contamination. The instrument precision for the δ 15 N measurement was high (0.03‰), but for the whole sampling and analysis procedure the mean deviation between replicate samples was 0.13‰ for the δ 15 N measurements and 5.5% for the N 2 gas concentration analysis. The results show that the Gasbench peripheral was suitable for measurement of the 15 N natural abundance of dissolved nitrogen gas, with denitrification indicated by the oversaturation and slightly (<1‰) depleted δ 15 N values of the dissolved N 2 gas in the suboxic zones of some of the study lakes. Calculated values for the denitrified (excess) N 2 varied between –5.3 and 0.7‰. The denitrification potential was determined using the 15 N tracer method, with results showing nitrate‐inducible denitrification and no signs of anaerobic ammonium oxidation (anammox). Copyright © 2011 John Wiley & Sons, Ltd.