J.K. Bohlke Karen L. Casciotti Tyler B. Coplen 2004 <p><span>The bacterial denitrification method for isotopic analysis of nitrate using N</span>₂<span>O generated from </span><i>Pseudomonas aureofaciens</i><span> may overestimate </span><i>δ</i>¹⁵<span>N values by as much as 1–2‰ for samples containing atmospheric nitrate because of mass-independent </span>¹⁷<span>O variations in such samples. By analyzing such samples for </span><i>δ</i>¹⁵<span>N and </span><i>δ</i>¹⁸<span>O using the denitrifier </span><i>Pseudomonas chlororaphis</i><span>, one obtains nearly correct </span><i>δ</i>¹⁵<span>N values because oxygen in N</span>₂<span>O generated by </span><i>P. chlororaphis</i><span> is primarily derived from H</span>₂<span>O. The difference between the apparent </span><i>δ</i>¹⁵<span>N value determined with </span><i>P. aureofaciens</i><span> and that determined with </span><i>P. chlororaphis</i><span>, assuming mass-dependent oxygen isotopic fractionation, reflects the amount of mass-independent </span>¹⁷<span>O in a nitrate sample. By interspersing nitrate isotopic reference materials having substantially different </span><i>δ</i>¹⁸<span>O values with samples, one can normalize oxygen isotope ratios and determine the fractions of oxygen in N</span>₂<span>O derived from the nitrate and from water with each denitrifier. This information can be used to improve </span><i>δ</i>¹⁵<span>N values of nitrates having excess </span>¹⁷<span>O. The same analyses also yield estimates of the magnitude of </span>¹⁷<span>O excess in the nitrate (expressed as </span><i>Δ</i>¹⁷<span>O) that may be useful in some environmental studies. The 1-</span><i>σ</i><span> uncertainties of </span><i>δ</i>¹⁵<span>N, </span><i>δ</i>¹⁸<span>O and </span><i>Δ</i>¹⁷<span>O measurements are ±0.2, ±0.3 and ±5‰, respectively. </span></p> application/pdf 10.1002/rcm.1318 en Elsevier Using dual-bacterial denitrification to improve δ15N determinations of nitrates containing mass-independent 17O article