A novel method combining FTIR-ATR spectroscopy and stable isotopes to investigate the kinetics of nitrogen transformations in soils

Understanding and quantifying N transformations in soil is critical for sustainable use of this important plant nutrient and for understanding the mechanisms through which polluting N species are discharged to the environment. Advanced methods such as the "isotope dilution technique", which uses stable N-isotopes to estimate gross mineralization and nitrification rates, answer this need. In this study the use of Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy for measuring isotopic N species concentrations directly in soil pastes was tested as a complementary technique to the commonly used isotope ratio mass spectrometry (IRMS). It is shown that, with proper chemometric tools (e.g., partial least squares [PLS]), FTIR-ATR enables simple tracking of changes in the concentrations of the isotopic species of nitrate and ammonium and allows estimation of the gross reaction rates of N transformations in soil. Soil incubations were performed by adding either ¹⁵NO₃^- or ¹⁵NH₄ ⁺ to the soils. The incubations with added ¹⁵NH ₄⁺ yielded a gross mineralization rate of 6.1 mg N kg ^-1 dry soil d^-1 compared with a net mineralization rate of 4.1 mg N kg^-1 dry soil d^-1 and a gross nitrification rate of 40.9 mg N kg^-1 dry soil d^-1 compared with a net nitrification rate of 29.5 to 25.3 mg N kg^-1 dry soil d^-1. The incubations with added ¹⁵NO₃^- yielded a gross nitrification rate of 18.6 mg N kg^-1 dry soil d^-1 compared with a net nitrification rate of 11.9 to 18.3 mg N kg^-1 dry soil d^-1. The combined use of FTIR-ATR and ¹⁵NO ₃^- or ¹⁵NH₄⁺ enrichment appears to provide an effective tool for almost real-time quantification of N-dynamics in soils with minimal interference.