Ph.D Thesis

Ph.D StudentEshed Revital
SubjectReal-Time Monitoring of N-Species Isotopes by FTIR
Spectroscopy - A Novel Tool to Investigate
Short-Term Isotope Ddynamics and N2O
Formation in Soil
DepartmentDepartment of Civil and Environmental Engineering
Supervisors PROFESSOR EMERITUS Abraham Shaviv


Transformations of N-species in agricultural systems may adversely affect the environment via emissions of N2O, which enhance the greenhouse effect and indirectly damage the ozone layer, or nitrate leaching to water resources. Soil microbial processes, commonly occurring simultaneously, are considered responsible for up to 70% of N2O emissions into the atmosphere. A better understanding and quantification of the pathways leading to N2O formation is essential to the development of mitigation approaches. To this date, this knowledge is not yet sufficient. Distinction between the different pathways of N2O emissions from soil is very important, but can be very difficult. Effective tracing of N-transformations is possible using stable N isotopes. Enrichment of species of interest with labeled 15N allows follow-up of its gross oxidation or/and reduction rates and its products. Quantification of the various isotopic specie is commonly done using Isotope Ratio Mass Spectrometry, which allows determination of the isotope ratios in reactants and product species, but cannot be used online. In addition, it requires time-consuming preparation steps, which may increase the risk of isotope exchange within samples before analysis.

The present research focuses on the alternative measurement of isotopic N-species, using Fourier Transform Infra-Red (FTIR) spectroscopy, which enables online or very close to direct measurements of changes in the isotopic composition of species. Practically, offering a non-destructive technique that requires minimal samples preparation and allows measurement of concentrations of reactants, products and intermediates. Spectra in mid-IR range, combined with advanced methods for signal processing, were used to quantify isotopic compositions, as isotopic enrichment induces relatively small shifts in absorption bands. Since the FTIR spectrum is sensitive to the molecular structure, it was used to distinguish between different isotopomers and isotopologues of N2O.

Current work novelty is the use of an innovative system for investigating the N-dynamics via Long Path Attenuated Reflectance FTIR (LP-ATR-FTIR). The system is based on two types of measuring cells, combined to allow simultaneous tracking of changes in N-species, both of mineral N in solution/soil phase (by ATR component) and of gaseous N2O (via LP). The ability to measure different isotopic N2O emissions at real time and mineral N species in “almost real time”, offers a more precise approach for tracing changes in concomitantly occurring reactions.

The study examined impacts of environmental conditions and farming management, such as the thickness of the soil layer, aeration conditions and fertilization or manuring, on N-transformations and gross vs. net formation rates. Focus was made on identifying and quantifying N2O emissions and their sources (nitrification, de-nitrification and possibly nitrifier de-nitrification) and at the same time determine gross rates of mineralization and nitrification. Efforts were also made to determine the composition and function of microbial populations affecting the main changes under different conditions. Our results emphasize the advantage of using independent methods and multi-phase investigation in order to provide deeper understanding and quantification of concomitantly occurring pathways contributing to the formation of the undesired N species and may allow development of improved mitigation management approaches.