fire effects on soil
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Fire Effects on Soil
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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?
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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
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Fire effects on soil properties is dependent on sensitivity to change
• Sensitivity is temperature dependent
• Relatively insensitive- exceed 460˚C
• Moderately sensitive- 100-400˚C
• Sensitive- less than 100˚C
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Relatively InsensitiveSoil Property Threshold
temperature (°C) Manganese 1962
Calcium 1484
Magnesium 1107
Clay alteration 460-980
Phosphorus 774
Potassium 774
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Moderately Sensitive
Soil Property Threshold temperature (°C)
Sulfur 375 Soil structure 300 Soil wettability 250 Nitrogen 200 Organic matter 100
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Sensitive
Soil Property Threshold temperature
(°C) Bacteria 60-120 Fungi 60-80 Plant roots 48-54 Seeds 70-90 Small Mammals 49-63
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Maximum ground surface temperatures within different
ecosystems
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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
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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
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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
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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
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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
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Physical Effects : Water repellency
• Humic acids, hydrophobic substances (HS), present at soil surface
• HS volatized with fire• Leach downward with
temperature gradient– Intense: 175-200˚C fire– Little: < 175˚C – Destruction: 280-400˚C
• Bond with cooler soil• Forms water repellent zone• Most notable in Chaparral
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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
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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
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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
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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
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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
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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
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TTYP
• Explain the patterns in runoff rates and sediment yield between the different treatments.
• What are some factors that may contribute to these differences?
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Runoff Rate (mm / h)
Johansen et al. 2001
• % ground cover - 20% (#4) vs. 31% (#3)• Steep rising limbs = low infiltration• Raindrop splash - compaction
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Sediment Yield between burned and unburned plots
Johansen et al. 2001
• Sediment yields in burned plots increased by a factor of 25