A numerical approach for understanding the main parameters and processes influencing the soil organic matter decay in wildfire events

High temperatures reached at topsoil during wildfires can induce changes in physical, chemical, and biological soil properties. In the end, these changes are related to the loss of soil organic matter (SOM) and control the post-fire soil management decision. Therefore, the objectives of this study were: (1) to develop a numerical model to predict the SOM decay during wildfires, and (2) to study which are the main parameters that control the soil response. The model couples the energy balance for soil heating, and the species conservation for water and SOM using high temperature-induced vaporization and combustion kinetics. Fluid flow was neglected; however, radiative energy conducted through pores was included as a function of the volumetric pore radius. The soil thermal evolution showed values of r²>0.92 when the radiative term in the thermal conductivity was neglected, and r²>0.98 when the volumetric pore radius was adjusted. The results showed that the main parameters that control the soil response were soil texture, soil water content, volumetric pore radius, and oxygen availability. Also, soil response depends on the surface temperature and exposure time. Soil water content enhances the thermal properties and determines the amount of heat consumed during vaporization because of the high enthalpy of this endothermic reaction. On the other hand, neglecting oxygen flux leads to restricted oxidation, limiting the SOM decay. In terms of texture, silty soils showed the lower soil response, clay and loamy soils an intermediate response, and sandy soils had a higher response. Also, the volumetric pore radius enhances the soil thermal conductivity at high temperatures, leading to higher temperatures near the soil surface. These results suggest that the normalized SOM decay does not depend on the initial SOM content.