The Effects of Fire on Soil Ecology

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The Effects of Fire on Soil Ecology

• Adapted from a lecture by Nick Brazee and Amy
  Ramsey

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Lecture topics

• Introduction
• Fire in the NW
• Effects of Fire
• Soil Temperature
• Bacteria and fungi
• Mycorrhizae
• Invertebrates
• Decomposition
• Nitrogen
• Conclusions

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Decreasing moisture
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Low-severity prescribed fire
High-severity wildfire
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Soil Temperature

• During Wildfire
• Soil is heated through conduction from downed
  logs, litter, and duff
• Volatilization of nutrients, destruction of
  microbes, plant roots, removal of organic layer
• Soil Sterilization (high intensity fires)
• Post Wildfire
• Blackened residual organic matter, loss of
  overstory vegetation
• Warmer daytime temps, and cooler nighttime temps

Agee 1993
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Soil Temperature

• Ground temps
• Average 200-300OC
• Rarely gt750OC
• Hot Spots 1500OC
• Soil Moisture increases heat conductance

Agee 1973
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Soil Temperature

• 40-70OC protein degradation and plant tissue
  death
• 48-54OC roots killed
• 70-90OC seed mortality
• gt 50OC soil microbes begin to experience mortality

Swezy and Agee 1991
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Soil Temperature

• Soil Thermometer
• gt700OC Inorganic P, Na
• 350-600OC Organic P, S, and K volatize
• 200OC Nitrogen volatizes
• 175OC Hydrocarbons
• 100OC Carbon volatizes

Agee 1993
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Effects on Soil Microorganisms

• Depends on
• Intensity of fire
• Maximum temperatures
• Soil-water content
• Duration of heating
• Depth of heating

• Site conditions
• Fuels
• Aspect
• Elevation
• Etc.
• Pre- and post-fire weather
• Ecosystem type
• Vegetation

Hungerford et al. 1995, Neary et al. 1999
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Effects on Soil Microorganisms

• Hot fires more significant and long-lasting than
  cooler fires
• Low-intensity, rapidly-moving fires have no major
  effects
• Long duration, high-intensity fires have greatest
  effects
• Burning moist soils more severe effects than dry
  soils
• Microorganism impacts linked with effects and
  responses of vegetation greatest in organic
  horizon and top 1-2 cm

Agee 1993, Neary et al. 1999, Warcup 1981, Wells
et al. 1979
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Effects on Bacteria

• More resistant to heat than fungi
• Lethal temperatures
• 210C in dry soil
• 110C in wet soil
• Nitrosomonas (involved in nitrification)
• 140C in dry soil
• 75C in wet soil
• Immediate post-fire decline, followed by an
  increase
• Higher ratio of bacteria to fungi following fire

Photo from BLM, Dunn and DeBano 1977, Perry and
Rose 1983, Warcup 1981, Wells et al. 1979
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Effects on Fungi

• Lethal temperatures
• 155C in dry soil
• 100C in wet soil
• Saprophytic Fungi
• Successional patterns
• Mycorrhizas
• Highly dependent on fire intensity

Dunn and DeBano 1977, Holm 1995, Moser 1949,
Peterson 1970 and 1971, Rahko 1997
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Dahlberg 2002
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Effects on Mycorrhizas

• Generally fire decreases numbers and species
• Low intensity fires may have no effect
• Ectomycorrhizas
• May increase initially after high-intensity fire
  due to increased nutrient release
• Arbuscular mycorrhizas
• Greater numbers reduced in dry vs. wet soils
• Significant time to recover to pre-fire levels

Agee 1993, Herr et al. 1994, Klopatek et al.
1988, 1990, 1994, Schoenberger and Perry 1982,
Wright and Tarrant 1957
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Effects on Invertebrates

• Numbers and species change
• Increase, decrease, stay the same
• Habitat
• Litter invertebrates greater mortality
• Deeper in soil less mortality
• Fire-favoring characteristics
• Mobility
• Thick cuticle

http//www.zephyrus.co.uk/earwig.jpg,
http//www.biologie.fu-berlin.de/agachazi/ springs
chwanz.jpg, Collett 2003, Wikars and Schimmel 2001
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(beetles)
(beetles)
Wikars and Schimmel 2001
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Decomposition

• Fire is an agent of decomposition with biggest
  impact on the O horizon
• Loss of forest and
• shrub canopy (high intensity)
• Increased soil temperature and pH
• Increased nutrient cycling, and parent material
  weathering

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Decomposition, continued

• Fire can remove litter and duff that took decades
  to build up on the forest floor
• CN can be changed in minutes to hours
• Decomposition of residual OM may be retarded, but
  eventually returns to pre-fire levels
• Pulse of inorganic nutrients, readily available
  for plant uptake

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CASE STUDY Prescribed fire MISSION CREEK SITE
FIRE AND FIRE SURROGATES STUDY
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Ponderosa pine ecosystem
PRETREATMENT - 2001
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DURING FIRE SPRING 2004
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POST FIRE - 2004
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SOIL ENZYMES Beta glucosidase degrades
cellulose and other carbohydrates Acid
phosphatase mineralization of organic P
N-acetyl-glucosaminidase (chitinase) NAG
degrades chitin Phenol oxidase degrades
lignin
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Pre (2001) and post treatment (2004) enzyme
activities in control sites at Mission Creek
(nmol g-1 hr-1)   B glucosidase Acid
phosphatase NAG Phenol oxidase Pre post pr
e post pre post pre post   Mean333 264 154 199 124
128 34 13 SD 218 106 87 144 106 98 118 32
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N-cycling

• Nitrogen
• Easily volatized (200OC), and lost to ash
  convection, erosion, and leaching
• Increased soil temperature and pH increased
  N-mineralization (organic - NH4)
• Available NH4 and higher soil pH increased
  nitrification (NH4 NO3-)
• Increased water flow through soil and lack of
  uptake allows nitrate to leach easily from the
  soil

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N-cycling Entiat wildfire, E. Washington

• 39 N-loss compared to unburned areas,
• 907 kg ha-1 of N lost
• Average annual inputs from precipitation, 1.0 kg
  ha-1 yr-1

Grier 1975
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Conclusions

• Fire intensity determines levels of soil heating,
  and whether nutrients are volatized
• Fire increases decomposition by burning OM, but
  can reduce numbers of soil organisms. They
  eventually recover depending on fire intensity.
  Some soil organisms are favored by fire.
• Pulse of N after the fire, first NH4 then after
  time NO3- (leaches easily)
• Prescribed fire influences soil enzymes some
  increase activity, others decrease.
• Fire adapted ecosystems are resilient to the
  stresses that fire induces