In situ Evaluation of Net Nitrification Rate in Terra Rossa Soil Using a Fourier Transform Infrared Attenuated Total Reflection 15 N Tracing Technique

Abstract

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 v₃ 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.