Combining temperature ramp dry combustion and mid-infrared spectroscopy for enhanced soil organic carbon characterisation

Abstract

Soil organic carbon (SOC) is comprised of a complex mixture of plant and microbial-derived compounds with varying chemical compositions and stability. Understanding these components is crucial for modelling SOC turnover and stability. Conventional fractionation methods moslty use physical or chemical separations, but techniques such as mid-infrared (MIR) spectroscopy and thermal ramp dry combustion (TRDC) offer complementary, rapid approaches for SOC characterisation. This study integrates MIR spectroscopy and TRDC to characterise plant-derived compounds in soils. Using soils dosed with six plant-derived compounds (cellulose, lignin, starch, pectin, xylose, and biochar), we investigated their thermograms and MIR spectra. Each compound exhibited unique thermal decomposition peaks and MIR absorptions, reflecting their distinct chemical structures and oxidative stability. Correlation analyses revealed strong relationships between thermal peaks and MIR features. Models that combined MIR and TDRC spectra could estimate cellulose (${\rho }_c=0.92$), biochar (${\rho }_c=0.95$), pectin (${\rho }_c=0.77$) and starch (${\rho }_c=0.87$). Our findings show the potential for combining MIR spectroscopy and TRDC to characterise SOC chemical composition. By identifying distinct thermal and spectral features of plant-derived compounds, the combined approach could identify contributions of plant-derived compounds to labile and stable carbon pools, for enhancing our understanding of SOC decomposability. These methods may provide new insights into SOC turnover and stability mechanisms when applied to fractionated soils.