A Novel Method Combining FTIR-ATR Spectroscopy and Stable Isotopes to Investigate the Kinetics of Nitrogen Transformations in Soils

Abstract

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 15NH4+ 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 15NO3- or 15NH4+ enrichment appears to provide an effective tool for almost real-time quantification of N-dynamics in soils with minimal interference.