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 f ourier 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 15 nO 3 – or 15 nH 4 + to the soils. The incubations with added 15 nH 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 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 15 nO 3 − 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 15 nO 3 − or 15 nH 4 + enrichment appears to provide an effective tool for almost real-time quantification of n-dynamics in soils with minimal interference.