Soil Heating During Fires

The transfer of heat to soil is the main mechanism by which fires affect physical, chemical and biological soil properties (Neary et al. 1999). High soil temperatures can kill soil microbes, plant roots, and seeds; destroy soil organic matter; and alter soil nutrient and water status (see diagram: Temperature effects on soil) (Hungerford et al. 1991, Campbell et al.1995, DeBano et al 1998). 

Radiation and convection are responsible for most heat transfer from light fuels to soil. Conduction is the main heat-transfer mechanism in heavy fuels like duff, organic soils, and slash piles. Vaporization and condensation, which involve phase changes of water and organic compounds distilled by combustion, are also important mechanisms of heat transfer in soil. Water moves much faster through soil pores as a vapor and releases heat when it condenses.  

The degree of soil heating during any fire is highly variable and depends on:

• fuel characteristics: loading, size, arrangement, moisture content,
• fire behavior: rate of spread, flame length, intensity, duration, 
• litter layer: thickness, packing, moisture content, and 
• soil properties: texture, organic content, moisture content. 

Duff moisture and soil moisture are critical regulators of subsurface heating. When water is present in soil and duff, the temperature at any particular depth does not exceed water’s boiling point, 100^o C (212 ^o F), until the water has evaporated or moved into lower layers (Scotter 1970, DeBano et al. 1976).  If the surface organic layer is thick and moist, little soil heating will occur. However, if the litter layer is dry and consumed, the underlying soil can be heated substantially. For example, heat load into wet duff and mineral soil can be 20% of that penetrating dry duff and mineral soil (Frandsen and Ryan 1986). Peak soil temperatures can also be more than 538 ^o C (1000 ^o F) greater where duff and soils are dry.

The movement of heat downward through soil layers is not only dependent on the peak temperature reached, but also on the temperature duration. Heat penetrates deeper into soil the longer a heat source is present. Soil texture also affects heat transfer. The thermal diffusivity of quartz is about three times that of clay, therefore sandy soils heat more slowly than finer textured soils.

Heat flux to soils during fires is almost always less than the heat released aboveground.  As little as 8-10% (maximum of 25%) of heat can be transmitted downward to the soil (DeBano et al. 1977, Packham 1969, Raison et al. 1986). Resulting soil temperatures are almost always lower than above ground temperatures, but do reach levels that can alter soil biological, chemical, and physical properties (see diagram: Temperature effects on soil). The highest soil temperatures usually occur beneath heavy slash, particularly with the consumption of large piles of dry harvest residue or windthrow. Burning large slash piles produce long duration, high temperature heat pulses that penetrate deep into the soil, potentially altering both physical and biotic characteristics of the soil to significant depths. Prescribed burns in shrublands typically generate more soil heating than prescribed burns in either grasslands or forests. Due to the high water content of wetland soils, penetration of heat generated by a surface fire can be significantly less than in mineral soils. Organic matter has a lower thermal diffusivity than mineral soil, so penetration of heat is further reduced in organic wetland soils.  However, organic soils can become dry enough to burn, producing significant amounts of heat.

It is very difficult to characterize heat transfer to soil because of the variability of combustion and soil conductions (Neary et al. 1999); nonetheless, several mathematical models based on heat transfer theory have been developed to address this problem (Hungerford and Campbell 1991). Scotter (1970) developed one of the earlier models, however it was limited because it did not include moisture-aided heat flow, an important mechanism in heat movement through soil.  Aston and Gill (1976) later developed a model that describes the transfer of water, heat, and water vapor through soils. It has been shown to predict soil temperature profiles, moisture profiles, ground heat flux, and evaporation in Australian grasslands, but has not been tested for forests.  Other models that predict the downward heat pulse in soil beneath fires include Campbell et al (1995), Chinanzvavana et al. (1986) and Pafford and others (1985).  However, these models remain untested for systems in the southern U.S.