Nitrification and mineralization of organic nitrogen (N) are important N transformation processes in soil, and mass spectrometry is a suitable technique for tracing changes of <sup>15</sup>N isotopic species of mineral N and estimating the rates of these processes. However, mass spectrometric methods for tracing N dynamics are costly, time consuming, and require long and laborious preparation procedures. This study investigates mid-infrared attenuated total reflection (ATR) spectroscopy as an alternative method for detecting changes in <sup>14</sup>NO<sub>3</sub>–N and <sup>15</sup>NO<sub>3</sub>–N concentrations. There is a significant shift of the ν<sub>3</sub> absorption band of nitrate according to N species, namely from the 1275 to 1460 cm<sup>−1</sup> region for <sup>14</sup>NO<sub>3</sub><sup>−</sup> to the 1240–1425 cm<sup>−1</sup> region for <sup>15</sup>NO<sub>3</sub>. This shift makes it possible to quantify the N isotopes using multivariate calibration methods. Partial least squares regression (PLSR) models with five factors yielded a determination error of 6.7–9.2 mg N L<sup>−1</sup> for aqueous solutions and 5.9–7.8 mg N kg<sup>−1</sup> (dry soil) for pastes of a <i>Terra rossa</i> soil. These PLSR models were used to monitor the changes of <sup>15</sup>NO<sub>3</sub>–N and <sup>14</sup>NO<sub>3</sub>–N content in the same <i>Terra rossa</i> soil during an incubation experiment in which [<sup>15</sup>NH<sub>4</sub>]<sub>2</sub>SO<sub>4</sub> was applied to the soil, allowing the estimation of the contributions of applied N and mineralized N to the net nitrification rate, the potential losses of the applied <sup>15</sup>NH<sub>4</sub>–N, and the net mineralization of soil organic N.

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