Linking thermal stability and organic chemistry with surface soil organic matter stability-A study across ecozones

Understanding soil organic carbon (SOC) stability is crucial given its influence on nutrient cycling and C storage. The biological and chemical properties of SOC offer valuable insights into its persistence and C retention capacity, and understanding these properties can help evaluate sustainable land management practices. In this study, we link thermal stability and chemical properties of SOC to its biodegradability using 108 soil samples collected from diverse ecological zones in Canada, New Zealand, and Scotland. We used Rock-Eval (RE) pyrolysis for thermal analysis to assess thermal stability (T50), conducted a 98-day incubation study to evaluate the biological stability of SOC, and utilized X-ray absorption near-edge structure (XANES) spectroscopy to determine the chemical characteristics of SOM. Our findings show a strong negative linear correlation between thermal stability, T50, and mineralized C in topsoil, which can be explained from an energetic perspective. The SOC characterized by stronger bonds, including organo-mineral associations or organic-organic bonds, requires more energy for breakdown. Higher thermal energy requirements reflect stronger soil organic matter (SOM) bonds, consequently leading to lower mineralization rates. Moreover, we observed a strong correlation between the Hydrogen Index (HI) derived from RE pyrolysis and mineralized C, affirming the validity of HI as a promising metric for assessing the labile pool of SOC.

Chemical functional groups identified using XANES spectroscopy, particularly alkyl-C and the alkyl/O-alkyl-C ratio, which signify the degree of decomposition, exhibited strong positive correlations with T50, highlighting their role in enhancing SOM thermal stability. In contrast, ketones and aromatic groups showed a strong negative correlation with T50. This inverse relationship could be attributed to ketones representing labile byproducts of microbial decomposition, which are less thermally stable. Similarly, the aromatic groups in this study, likely derived from lignin and tannins, may indicate early-stage decomposition products rather than highly condensed, recalcitrant aromatic compounds typically associated with stable SOM. This suggests that these functional groups are more indicative of labile SOM fractions in the studied soils. This research established a strong connection between thermal stability and the chemical and biological stability of surface SOM. It demonstrates the efficacy of RE thermal analysis as a potent tool across various landscape and ecological zones.