Temperature resolved alteration of soil organic matter composition during laboratory heating as revealed by C and N XANES spectroscopy and Py-FIMS

Abstract

Although soil organic matter (SOM) is an important part of global cycles, its alteration on a molecular scale during wildfires is not sufficiently known. Therefore, we investigated stepwise heated soil samples under controlled conditions up to 700 °C in the laboratory on molecular-scale using complementary synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy at the carbon (C) and nitrogen (N) K-edges and temperature-resolved in-source pyrolysis-field ionisation mass spectrometry (Py-FIMS). The XANES spectra and Py-FIMS indicated a relative enrichment of unsaturated, substituted aromatic C and non-peptide N (nitriles or N in aromatic systems) in the samples from 300 °C to 500 °C. Furthermore, Py-FIMS revealed an alteration of macromolecular plant materials, e.g. suberins, up to 200 °C. This was followed by decomposition of suberins, free fatty acids and sterols above 300 °C and depolymerisation of lignin dimers starting at 400 °C. Finally, carbonisation was the dominant process at 500 °C.