|M.Sc Student||Kira Oz|
|Subject||New and Inovative Measuring Method Combining Ftir-Atr|
Spectroscopy and Stable Isotopes to Investigate
the Kinetics of Nitrogen
Transformations in Soils
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Raphael Linker|
|Professor Emeritus Abraham Shaviv|
|Full Thesis text|
Investigation of nitrogen (N) transformations in the soil is required to improve the quantification and understanding of the mechanisms through which polluting N-species are discharged to the environment. Methods using stable N-isotopes were developed in order to estimate gross mineralization and nitrification rates. The technique commonly used for measuring the isotopic abundance/enrichment, “isotope ratio mass spectrometry” - IRMS, is expensive and the sample preparation is both tedious and destructive and thus does not allow online and/or continuous measurements.
The general aim of this research was to develop a measuring method that would allow continuous estimation of the concentrations of isotopic species of nitrate and ammonium and enable improved estimation of the gross reaction rates of N transformations in the soil. The determination of N isotopic species in soils was based on combining attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR) with the "isotope dilution technique". The N-species of interest absorb in the mid-IR range (200-4000cm-1), and can be measured directly in soil pastes using the FTIR-ATR system. Both nitrate and ammonium have a band shift between the labeled and unlabeled species, allowing their quantification using chemometric tools. The data was analyzed using partial least squares regression, and neural network.
The calibrations were performed with several backgrounds (water, KCl 1N, soil paste) and different N-species content - 14NH4, 15NH4, 14NO2- , 14NO3- and 15NO3-. The calibrations yielded root mean square determination errors (RMSE) of 5-16 mgN/l (14NH4), 7-16 mgN/l (15NH4), 5-13 mgN/l (14NO3-), and 8-10 mgN/l (15NO3-) .
Following the calibration stage, two types of soil incubations were conducted. The incubations with added 14NH4 yielded RMSE of 21.0 ?gN?gsoil-1 (NH4), and 13.7 ?gN?gsoil-1 (NO3-) in Terra Rossa and 12.7 ?gN?gsoil-1 (NH4), and 10.4 ?gN?gsoil-1 (NO3-) in Hamra. The incubations with added 15NH4 yielded RMSE of 25.0 ?gN?gsoil-1 (total NH4), and 13.6 ?gN?gsoil-1 (total NO3-). The estimated gross mineralization rate, which was 1.24 ?gN?gsoil-1?day, was lower than expected due to over estimation of 15NH4 concentration thus affecting estimates of the abundance ratio of ammonium. The estimated gross nitrification rate of 19.5 ?gN?gsoil-1?day was also lower than expected due to the bias of the ammonium abundance ratio.
The 14NO3- and 15NO3- concentration predictions were satisfactory, and indicate the possibility of measuring nitrate concentrations while performing incubation experiments directly on the ATR crystal. The NH4 concentration predictions, which were satisfactory in the 14NH4 incubations, require further attention when 15NH4 is added to the soil.