Fire Effects on Soil

Fires affect physical, chemical, and biological soil properties directly by transferring heat into soil and indirectly by changing vegetation and the dynamics of nutrients and organic matter. High soil temperatures can kill soil microbes and plant roots; destroy soil organic matter; and alter soil nutrient and water status. The degree of soil heating during fire depends on a variety of factors, including fuel characteristics, fire intensity and residence time, and properties of the soil and litter layer. Evidence indicates that low-intensity prescribed fires have little adverse effects on soil properties, and in fact, may even improve soil nutrient availability. High-intensity prescribed fires have a temporary negative effect on site nutrient status resulting from volatilization of nitrogen and sulfur, plus some cation loss due to ash convection. Severe fires may alter soil physical properties over a longer time period by consuming soil organic matter, and potentially may increase surface run-off and soil erosion.

It is important for managers to understand fire effects on soil in order to predict the susceptibility of their particular site to soil damage (altered structure, texture, nutrient status) and erosion. In the southern U.S. prescribed burns are typically low-intensity and soil temperatures are elevated for only brief durations, therefore soil damage rarely occurs. However, burning piled or windrowed debris, or burning under other conditions that create intense fires can potentially damage soil. This is particularly true if the soil and duff layers are dry. Coarse-textured soils on steep slopes are also more susceptible to erosion, particularly following intense site preparation burns or wildfires. Managers should be aware of guidelines that can mitigate potential effects of fire on soil damage, such as burning when duff and litter layers are moist.

The following sections summarize research on the effects of fire on soil and present management guidelines for minimizing negative effects.

• Overview
• Soil Heating
• Soil Physical Properties
• Soil Organic Matter
• Soil Nutrients
• Soil Biota
• Soil Erosion
• Management Guidelines