Using dual‐bacterial denitrification to improve δ 15 N determinations of nitrates containing mass‐independent 17 O

The bacterial denitrification method for isotopic analysis of nitrate using N 2 O generated from Pseudomonas aureofaciens may overestimate δ 15 N values by as much as 1–2‰ for samples containing atmospheric nitrate because of mass‐independent 17 O variations in such samples. By analyzing such samples for δ 15 N and δ 18 O using the denitrifier Pseudomonas chlororaphis , one obtains nearly correct δ 15 N values because oxygen in N 2 O generated by P. chlororaphis is primarily derived from H 2 O. The difference between the apparent δ 15 N value determined with P. aureofaciens and that determined with P. chlororaphis , assuming mass‐dependent oxygen isotopic fractionation, reflects the amount of mass‐independent 17 O in a nitrate sample. By interspersing nitrate isotopic reference materials having substantially different δ 18 O values with samples, one can normalize oxygen isotope ratios and determine the fractions of oxygen in N 2 O derived from the nitrate and from water with each denitrifier. This information can be used to improve δ 15 N values of nitrates having excess 17 O. The same analyses also yield estimates of the magnitude of 17 O excess in the nitrate (expressed as Δ 17 O) that may be useful in some environmental studies. The 1‐ σ uncertainties of δ 15 N, δ 18 O and Δ 17 O measurements are ±0.2, ±0.3 and ±5‰, respectively. Copyright © 2004 John Wiley & Sons, Ltd.