| Summary: | The increasing use of the stable isotope <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> for the quantification of ecological processes requires analytical approaches able to distinguish between labelled and unlabeled N forms. We present a method coupling anoxic sediment slurries and membrane inlet mass spectrometry to quantify dissolved <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and <sup>14</sup>N-NO<sub>3</sub><sup>−</sup>. The approach is based on the microbial reduction of <sup>14</sup>N-NO<sub>3</sub><sup>−</sup> and <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> mixed pool, the determination of the produced <sup>29</sup>N<sub>2</sub> and <sup>30</sup>N<sub>2</sub><sub>,</sub> and the calculation of the original <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> and <sup>14</sup>N-NO<sub>3</sub><sup>−</sup> concentrations. The reduction is carried out in 12 mL exetainers containing 2 mL of sediment and 10 mL of water, under anoxia. To validate this approach, we prepared multiple standard solutions containing <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> alone or in combinations with <sup>14</sup>N-NO<sub>3</sub><sup>−</sup>, with final concentrations varying from 0.5 to 3000 µM. We recovered nearly 90% of the initial <sup>14</sup>N-NO<sub>3</sub><sup>−</sup> or <sup>15</sup>N-NO<sub>3</sub><sup>−</sup>, over a wide range of concentrations and isotope ratios in the standards. We applied this method to a <sup>15</sup>N-NO<sub>3</sub><sup>−</sup> dilution experiment targeting the measurement of nitrification in sediments with and without the burrower <i>Sparganophilus tamesis</i>. The oligochaete did not stimulate nitrification, likely due to limited ventilation and unfavorable conditions for nitrifiers growth. The proposed method is reliable, fast, and could be applied to multiple ecological studies.
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