Interpreting ramped combustion thermograms using 13C NMR spectroscopy to characterize soil organic matter composition

Abstract

While many advanced analytical methods have been applied to soil organic matter (SOM), its highly complex and heterogeneous chemical composition still eludes complete characterization. Analytical thermal analysis has been proposed as a relatively rapid, inexpensive method for SOM characterization that requires no pre-treatment, but is challenging due to a lack of direct information about chemical composition. The goal of this study was to inform the interpretation of coupled differential scanning calorimetry and evolved gas analyses (DSC, CO2-EGA) using spectral correlations with solid phase 13C NMR data. We used a subset of soils collected as part of the Australian National Soil Carbon Research Program (SCaRP), which were physically fractionated and characterized using conventional analytical methods. Correlating the well-understood NMR spectra with the less-understood DSC and CO2-EGA thermograms provided some indications of which chemical compounds combust at which temperatures. Overall, the EGA data generated stronger correlations compared to correlations with DSC data, which was attributable to greater variability in DSC data due thermal reactions associated with minerals. Direct comparison of NMR and thermal data for the mineral associated organic matter in the fine (<50 µm) fraction was not possible due to the need to demineralize samples prior to NMR analyses. Thermal analyses showed substantial differences in samples pre- and post-HF pretreatment, and the NMR data for HF treated samples showed scattered and weak correlation patterns with DSC and EGA data for untreated samples. While precise chemical compositions cannot be gleaned directly from thermal analyses results, thermal approaches provide an avenue of investigation into SOM reactivity based on bioenergetics that may be a quantitative representation of SOM persistence.