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 15N 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 14NO3–N and 15NO3–N concentrations. There is a significant shift of the ν3 absorption band of nitrate according to N species, namely from the 1275 to 1460 cm−1 region for 14NO3 to the 1240–1425 cm−1 region for 15NO3. 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−1 for aqueous solutions and 5.9–7.8 mg N kg−1 (dry soil) for pastes of a Terra rossa soil. These PLSR models were used to monitor the changes of 15NO3–N and 14NO3–N content in the same Terra rossa soil during an incubation experiment in which [15NH4]2SO4 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 15NH4–N, and the net mineralization of soil organic N.

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