Fire Effects on Soil

What are the 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?

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

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

Relatively Insensitive Soil Property Threshold temperature (°C) Manganese1962 Calcium1484 Magnesium1107 Clay alteration Phosphorus774 Potassium774

Moderately Sensitive Soil Property Threshold temperature (°C) Sulfur 375 Soil structure300 Soil wettability250 Nitrogen200 Organic matter100

Sensitive Soil Property Threshold temperature (°C) Bacteria Fungi60-80 Plant roots48-54 Seeds70-90 Small Mammals49-63

Maximum ground surface temperatures within different ecosystems

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

Physical Effects: Removal of Plant Material by Fire 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

Physical Effects: soil organic matter Def. – All organic carbon compounds in the soil, including plant and animal residues at various stages of decomposition Aggregation of mineral particles by OM forms the structure of surface horizons

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

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

Physical Effects : Water repellency Humic acids, hydrophobic substances (HS), present at soil surface HS volatized with fire Leach downward with temperature gradient –Intense: ˚C fire –Little: < 175˚C –Destruction: ˚C Bond with cooler soil Forms water repellent zone Most notable in Chaparral

Chemical Effects: Soil Nutrients Fire converts nutrients from biological (organic matter) to mineral form, sensitivity varies Nutrient Transformations include: –Particulate loss in smoke (P) –Direct gaseous volatilization into the atmosphere (N, P, S) Deposition in ash on the soil surface –P, K, Ca, Mg, N Long term retention depends on the ability of plants to utilize available nutrients and erosive forces

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

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

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

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

Post-fire runoff and erosion from rainfall simulation: Southwestern Ponderosa Pine Los Alamos National Lab, New Mexico: intercanopy spaces of ponerosa pine forest after the Cerro Grande fire (May 2000) 4 plots (3 X 10 m): 2 in severely burned area, two control plots Rainfall simulations: representing a 100 y rainfall event (total rainfall: 120 mm) –1 h rain – DRY RUN –24 h interval no rain –0.5 h rain – WET RUN –0.5 h interval no rain –0.5 h rain – VERY WET RUN Flume at end of plots collected runoff and sediment Johansen et al. 2001

TTYP Explain the patterns in runoff rates and sediment yield between the different treatments. What are some factors that may contribute to these differences?

Runoff Rate (mm / h) Johansen et al % ground cover - 20% (#4) vs. 31% (#3) Steep rising limbs = low infiltration Raindrop splash - compaction

Sediment Yield between burned and unburned plots Johansen et al Sediment yields in burned plots increased by a factor of 25