After a bushfire, the soil undergoes significant physical, chemical, and biological changes, especially in the topsoil layer. The kind of soil that remains is typically characterized by the following features:
- Loss of Organic Matter: The fire consumes much of the soil organic matter (SOM), particularly the litter and humus layers, which affects soil nutrient cycling and reduces cation exchange capacity and aggregate stability
- Increased pH: Ash deposition from burnt vegetation releases alkaline elements, causing a temporary increase in soil pH. This increase is usually ephemeral as new humus forms and ash is leached or eroded over time
- Increased Nutrient Availability but Potential Loss: Combustion releases nutrients like phosphorus and ammonium nitrogen, increasing their availability shortly after the fire. However, nitrogen and other nutrients can also be volatilized or lost through erosion and leaching
- Physical Changes: The soil often becomes more water-repellent (hydrophobic) due to the presence of ash and burned organic compounds, which decreases water permeability. Bulk density may increase, and water holding capacity and aggregate stability tend to decrease
- Reduced Microbial Activity: Microbial biomass and activity decline immediately after fire due to heat and changes in chemical properties, although some enzymatic activities may increase
- Increased Soil Erosion and Landslide Risk: With vegetation loss, soil is more susceptible to erosion and slope instability, especially during rainfall, due to reduced root reinforcement and altered soil cohesion
Overall, the soil after a bushfire is typically a mineral soil with reduced organic matter, temporarily elevated pH, increased nutrient availability but also increased vulnerability to erosion and hydrophobicity. Recovery of soil quality and structure can take several years and depends on factors like fire severity, ecosystem type, and post-fire management
. In summary, the soil remaining after a bushfire is often nutrient-enriched ash-covered mineral soil with reduced organic content, increased pH, hydrophobic properties, and altered physical structure that can lead to erosion and slope instability until vegetation and soil biota recover.
