Linking thermal stability and organic chemistry with soil organic matter stability: a study across ecozones

Understanding the stability of soil organic matter (SOM) is central to predicting soil carbon persistence and informing sustainable land management. We combined thermal, chemical, and biological approaches to evaluate SOM stability in 108 soils spanning diverse ecozones in Canada, New Zealand, Scotland, and the Subarctic. Thermal stability was measured using Rock–Eval (RE) pyrolysis, chemical composition was characterized with X-ray absorption near-edge structure (XANES) spectroscopy, and biological stability was assessed through 98-day C mineralization assays. Across soils, thermal stability (T50) was strongly and negatively correlated with mineralized C, indicating that stronger bond structures reduce biodegradability. Importantly, our results demonstrate that T50 not only reflects SOM quality but also serves as a proxy for the extent of organo-mineral associations that contribute to stabilization. The Hydrogen Index (HI) showed a positive relationship with mineralized C, confirming its role as a reliable indicator of labile SOM. XANES results further revealed that alkyl-C and the alkyl/O-alkyl-C ratio were positively related to thermal stability, whereas ketones and aromatic groups correlated negatively with T50, suggesting they are labile byproducts of microbial decomposition or contributions from lignin and tannins, rather than highly recalcitrant aromatic compounds typically associated with stable SOM. Together, these findings highlight that SOM persistence is shaped by both intrinsic chemical composition and extrinsic mineral protection. Rock–Eval pyrolysis and XANES spectroscopy thus provide complementary insights into SOM stability and, when combined with biological assays, offer generalizable tools for evaluating soil carbon resilience across ecosystems.