Protective-mechanism-exclusive thermal stability modes of soil organic matter: Novel implication for wildfire effect on soil organic carbon

Stability of soil organic matter (SOM) is considered to be governed by different protection mechanisms including physical protection (PP), biochemical protection (BCP) and physical plus biochemical protection (PBCP). The thermostability of SOM protected by different mechanisms is unknown, despite its importance for understanding the stability of soil organic carbon (SOC) under frequently occurred wildfires. In this study, we reported for the first time that pyrolysis of SOM under different protection mechanisms in three types of soil (shrub soil, cultivated soil, and meadow soil) followed their own distinct modes regardless of soil type. Specifically, SOM-PP, SOM-PBCP and SOM-BCP from each type of soil were pyrolyzed in the double-step-shape, the mono-step-shape and the linear modes, respectively, when they were heated from room temperature to 800 °C by thermogravimeter. There were more thermolabile organic fractions (pyrolysis temperature < 200 °C) enriched in SOM-PP, while more thermostable organic fractions enriched in SOM-BCP and SOM-PBCP. These findings are of great importance for deeper insight into stability responses of SOM with different occurrence of wildfire.