A Novel Method Combining FTIR‐ATR Spectroscopy and Stable Isotopes to Investigate the Kinetics of Nitrogen Transformations in Soils. Issue 1 (30th January 2014) - Kira, Oz
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 15 NO3 – or 15 NH4 + to the soils. The incubations with added 15 NH4 + 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 15 NO3 − 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 NO3 − or 15 NH4 + enrichment appears to provide an effective tool for almost real‐time quantification of N‐dynamics in soils with minimal interference.