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We report on the contamination of commercial 15-nitrogen ( 15 N) N 2  gas stocks with  15 N-enriched ammonium, nitrate and/or nitrite, and nitrous oxide.  15 N 2  gas is used to estimate N 2  fixation rates from incubations of environmental samples by monitoring the incorporation of isotopically labeled  15 N 2  into organic matter. However, the microbial assimilation of bioavailable  15 N-labeled N 2  gas contaminants, nitrate, nitrite, and ammonium, is liable to lead to the inflation or false detection of N 2  fixation rates.  15 N 2  gas procured from three major suppliers was analyzed for the presence of these  15 N-contaminants. Substantial concentrations of  15 N-contaminants were detected in four Sigma-Aldrich  15 N 2  lecture bottles from two discrete batch syntheses. Per mole of  15 N 2  gas, 34 to 1900 µmoles of  15 N-ammonium, 1.8 to 420 µmoles of  15 N-nitrate/nitrite, and ≥21 µmoles of  15 N-nitrous oxide were detected. One  15 N 2  lecture bottle from Campro Scientific contained ≥11 µmoles of  15 N-nitrous oxide per mole of  15 N 2  gas, and no detected  15 N-nitrate/nitrite at the given experimental  15 N 2  tracer dilutions. Two Cambridge Isotopes lecture bottles from discrete batch syntheses contained ≥0.81 µmoles  15 N-nitrous oxide per mole  15 N 2 , and trace concentrations of  15 N-ammonium and  15 N-nitrate/nitrite.  15 N 2  gas equilibrated cultures of the green algae  Dunaliella tertiolecta  confirmed that the  15 N-contaminants are assimilable. A finite-differencing model parameterized using oceanic field conditions typical of N 2  fixation assays suggests that the degree of detected  15 N-ammonium contamination could yield inferred N 2  fixation rates ranging from undetectable, <0.01 nmoles N L −1  d −1 , to 530 nmoles N L −1  d −1 , contingent on experimental conditions. These rates are comparable to, or greater than, N 2  fixation rates commonly detected in field assays. These results indicate that past reports of N 2  fixation should be interpreted with caution, and demonstrate that the purity of commercial  15 N 2  gas must be ensured prior to use in future N 2  fixation rate determinations.

The Contamination of Commercial 15N2 Gas Stocks with 15N–Labeled Nitrate and Ammonium and Consequences for Nitrogen Fixation Measurements