Relationships between Biological and Thermal Indices of Soil Organic Matter Stability Differ with Soil Organic Carbon Level

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Thermal analysis techniques can provide an important addition to our understanding of soil organic matter (SOM) composition and stability. Several recent studies have linked thermal and biological stability of SOM; however, contrasting results have been reported. The objective of this study was to characterize the relationships between thermal and biological SOM stability for a wide range of mineral soils. Soils were collected from 28 sites from across the United States and analyzed by thermogravimetry (TG) and differential scanning calorimetry (DSC) coupled with CO2 evolved gas analysis (CO2–EGA). We compared thermal analysis results to mean soil respiration rates during incubation at 20°C for 365 d (R20). For soils with <30 g C kg−1 (low C), R20 was negatively correlated with the temperature at which half of the DSC energy is released and the temperature at which half of the CO2–EGA is evolved. Conversely, for soils with >30 g C kg−1 (high C), R20 was positively correlated with CO2–EGA and DSC energy released between 345 and 460°C and to SOM energy density (in J mg−1 C). Differences between low-C and high-C soils indicate the relative importance of mineral association of SOM in low-C soils and the abundance of intact plant debris that is relatively energy dense and thermally resistant but relatively easy to decompose in high-C soils. Above all, thermal analysis proved to be a useful technique for interpreting SOM stability, but sample C concentration must be considered because it affects the dominant SOM stabilization mechanisms and thermal analysis results.