Linking thermal analysis with size fractionation for low-cost quantification of soil organic carbon fractions

Soil organic carbon (SOC) is highly complex, and its heterogeneous composition still eludes complete characterisation. While a range of physical and chemical methods have been developed to differentiate distinct SOC pools, most are expensive and labour-intensive, limiting large-scale application. Thermal analysis offers a potentially time- and cost-efficient alternative for quantifying SOC with differing stability. This study compared two thermal methods, oxidation-only and oxidation-pyrolysis-oxidation using step-ramping protocols, to identify the most appropriate approach and assess its consistency with an established framework quantifying three biologically significant SOC fractions derived from size fractionation followed by solid-state 13 C NMR characterisation. A total of 111 soil samples collected across diverse pedo-climatic conditions in Australia’s agricultural regions were size fractionated and characterised using both thermal and 13 C NMR approaches. The thermal oxidation method quantified thermal SOC fractions more consistently than the oxidation-pyrolysis-oxidation method, which produced inconsistent and inaccurate outputs. Compared with 13 C NMR-derived fractions, thermal oxidation yielded slightly higher particulate and slowly decomposable humus-like organic carbon fractions, particularly at higher concentrations, while resistant organic carbon was significantly lower. This pattern suggests a compensatory redistribution among SOC fractions, reflecting that the two methods target different regions of the resistant organic carbon continuum. Permanova analysis confirmed that SOC fraction estimates derived from thermal oxidation and 13 C NMR were significantly different. Despite differences between the two methods, the results highlight the potential of thermal analysis to support process-based SOC modelling, national greenhouse gas inventories, soil monitoring frameworks, soil health assessment, carbon accounting, and natural capital evaluation.