Effects of Wildfire on Soil Properties

Fire intensity

Rate of thermal energy production from fire

Short-term impacts on soil properties

Immediate changes in soil temperature, moisture, and nutrient availability

Long-term impacts on soil properties

Loss of organic matter and altered soil structure, nutrient cycling, and microbial community composition

Organic matter combustion

Major combustion of organic matter starts at 200-250 C and completes around 460 C

Mechanisms behind soil C increase

Incorporation of unburnt residues, transformation to recalcitrant forms, N-fixing species introduction, and reduced mineralization rates

Seedling survival

Exposed mineral soil post-fire provides a preferred seedbed and moist mineral soils have the best survival rates

Structural stability: low to moderate fires

Hydrophobic films increase stability of soil aggregates

Structural stability: high-temperature fires

Organic cements in aggregates are destroyed, but new cementing oxides can increase stability of remaining aggregates

Bulk density increase

Caused by collapse of organo-mineral aggregates and clogging of soil pores with ash and clay, leading to reduced water holding capacity and increased runoff and erosion risk

Formation of water-repellent layer

Depth mainly influenced by heating, soil moisture, and particle size; typically does not exceed 6-8 cm in depth; irregular fire patterns create patches of water-repellent and permeable soil; water-repellent layers can weaken within 3 months but may persist for over a year

Particle-size distribution

Not directly affected by fire, but selective erosion on steep surfaces can remove fine particles, causing soil coarsening

Rain splash detachment

Newly burned stands show much higher detachment and soil loss rates than older, recovered stands

Post-fire erosion

Loss of vegetation and organic matter horizon increases raindrop impact and surface runoff

Fire removes nutrients via five processes

Oxidation of compounds to a gaseous form (combustion); vaporization (volatilization) of compounds that are solid at normal temperatures; convection of ash particles in fire‐generated winds; leaching of ions in solution out of the soil following fire; accelerated erosion following fire

Increase in soil pH

Soil heating breaks down organic acids, raising pH levels; release of bases (K, Ca, Mg) from ash neutralizes soil acidity, boosting base saturation

Fire and ammonium (NH4+)

Direct product of combustion; adsorbed onto soil surfaces and held if not immediately taken up; can transform into nitrate (NO3-) if not fixed in clay minerals

Fire and nitrate (NO3-)

Formed from ammonium through nitrification weeks/months post-fire; leaches downward if not promptly taken up by plants; nitrate levels increase over time; nitrogen levels typically return to pre-fire conditions within a few years

Prompt plant recolonization

Reduces nitrate leaching and stabilizes nitrogen availability

Fire and phosphorus

Fire converts organic phosphorus to orthophosphate (usable by biota); in acidic soils, orthophosphate binds to Al, Fe, and Mn oxides

Hematite formation

Created through the thermal transformation of ferrihydrite at 250-300 C; high crystallinity and low capacity to form MAOM