1. Fire Effects on Soil
September 20, 2006

2. Functions of Soil within Ecosystems
Provides a medium for plant growth and supplies nutrients
Regulates the hydrologic system, receiving, holding, and releasing water
Recycles nutrients and organic wastes
Habitat for living organisms and supports the detritus food chain
How does fire alter these functions?

3. Changes in Soil Properties
Changes in soil properties due to heat and temperature
Physical properties
Chemical properties
Biological properties
Variable effects depending on fire regime
Frequency, timing, and intensity

4. Fire effects on soil properties is dependent on sensitivity to change
Sensitivity is temperature dependent
Relatively insensitive- exceed 460˚C
Moderately sensitive ˚C
Sensitive- less than 100˚C

5. Relatively Insensitive
Soil Property Threshold
temperature (°C)
 Manganese
 Calcium
 Magnesium
 Clay alteration
 Phosphorus 774
 Potassium 774 

6. Moderately Sensitive Soil Property Threshold temperature (°C)
Sulfur
Soil structure
Soil wettability 250
Nitrogen 200
Organic matter 100

7. Sensitive Soil Property Threshold temperature (°C) Bacteria 60-120
Fungi
Plant roots
Seeds
Small Mammals

8. Maximum ground surface temperatures within different ecosystems

9. Sensitivity and Soil Depth
Most significant effects at the soil surface
Peak temperature decreases rapidly with depth
Soil temperature with depth is dependent on fire intensity and fuel load, including litter layer

10. Physical Effects: Removal of Plant Material
Increases soil temperatures
Loss of shading from vegetation
Removal of insulating litter
Altered albedo of soil surface
Litter removal increases surface runoff and erosion potential

11. Physical Effects: soil organic matter
Def. – All organic carbon compounds in the soil, including plant and animal residues at various stages of decomposition
OM has a dominant role in surface horizons
Aggregation of mineral particles by OM forms the structure of surface horizons
Sum of exchangeable cations found on soil colloids, negative charge

12. Physical Effects: soil organic matter
Combustible, volatile during fire
Moderately sensitive, little lost at <100 ˚ C
Complete combustion between 220 and 440 ˚ C
Long term effect depends on replenishment rate

13. Destruction of organic matter leads to changes in many physical soil properties:
Destroys soil structure
Reduces porosity
Increases bulk density
Mass of dry soil per unit of bulk volume
Reduces infiltration
Increases runoff and erosion

14. Physical Effects : Water repellency
Humic acids, hydrophobic substances (HS), present at soil surface
HS increase with fire
Leach downward with temperature gradient
100 – 300˚C fire
Bond with cooler soil
Forms water repellent zone
Most notable in Chaparral

15. Chemical Effects: Soil Nutrients
Fire converts nutrients from biological (organic matter) to mineral form, sensitivity varies
Nutrient Transformations include:
Particulate loss in smoke, i.e. phosphorus
Direct gaseous volatilization into the atmosphere
i.e. Volatilized nitrogen, phosphorus, sulfur
Deposition in ash on the soil surface
Phosphorus, potassium, calcium, magnesium, and nitrogen
Long term retention depends on the ability of plants to utilize available nutrients and erosive forces

16. Chemical Effects: pH Fire raises pH
Cations released during combustion are deposited on soil surface, i.e. calcium, magnesium
Usually only altered in upper soil horizons temporarily

17. Chemical Effects: Nitrogen
Short term: Plant available N increases with fire (nitrate and ammonium)
N locked up in plant tissues
Most lost during fire, some deposited on soil
Long term: Total N decreases with fire
Organic N volatilizes at low temps (200 ˚C ), complete loss at 500 ˚C
Implications for site productivity, i.e. tallgrass

18. Biological effects - Direct
Biological properties are the most sensitive
Fire most lethal near surface
Microbes
Rhizosphere
Mychorrizae
Recolonization from deeper soil and off site and with plant regeneration
Short-lived

19. Biological Effects: Indirect
Chemical changes, i.e. pH, effect microbes
Vegetation mortality influences:
Mycorrhizae
Organisms associated with rhizosphere
Combustion of woody debris, litter, and soil organic matter
Longer term impact on soil temperature and organisms in the soil surface