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.
Vol. 4, Iss. 12 Virtual Journal for Biomedical Optics
Du Changwen, Raphael Linker, Avi Shaviv, and Zhou Jianmin, "In Situ Evaluation of Net Nitrification Rate in Terra Rossa Soil Using a Fourier Transform Infrared Attenuated Total Reflection 15N Tracing Technique," Appl. Spectrosc. 63, 1168-1173 (2009)