Fires affect physical, chemical, and biological soil properties directly by transferring heat into soil and indirectly by changing vegetation and the dynamics of nutrients and organic matter. High soil temperatures can kill soil microbes and plant roots; destroy soil organic matter; and alter soil nutrient and water status. The degree of soil heating during fire depends on a variety of factors, including fuel characteristics, fire intensity and residence time, and properties of the soil and litter layer. Evidence indicates that low-intensity prescribed fires have little adverse effects on soil properties, and in fact, may even improve soil nutrient availability. High-intensity prescribed fires have a temporary negative effect on site nutrient status resulting from volatilization of nitrogen and sulfur, plus some cation loss due to ash convection. Severe fires may alter soil physical properties over a longer time period by consuming soil organic matter, and potentially may increase surface run-off and soil erosion.

It is important for managers to understand fire effects on soil in order to predict the susceptibility of their particular site to soil damage (altered structure, texture, nutrient status) and erosion. In the southern U.S. prescribed burns are typically low-intensity and soil temperatures are elevated for only brief durations, therefore soil damage rarely occurs. However, burning piled or windrowed debris, or burning under other conditions that create intense fires can potentially damage soil. This is particularly true if the soil and duff layers are dry. Coarse-textured soils on steep slopes are also more susceptible to erosion, particularly following intense site preparation burns or wildfires. Managers should be aware of guidelines that can mitigate potential effects of fire on soil damage, such as burning when duff and litter layers are moist.

The following sections summarize research on the effects of fire on soil and present management guidelines for minimizing negative effects.

• Overview
• Soil Heating
• Soil Physical Properties
• Soil Organic Matter
• Soil Nutrients
• Soil Biota
• Soil Erosion
• Management Guidelines

Fires affect physical, chemical, and biological soil properties primarily by transferring heat into soil (Neary et al. 1999). High soil temperatures kill soil microbes, kill or damage plant roots and seeds, destroy soil organic matter, and alter soil nutrient and water status (see figure: Temperature effects on soil). The degree of soil heating during fire depends on a variety of variables, including fuel characteristics, the intensity and residence time of the fire, properties of the soil (such as moisture content, soil texture, and organic matter content), and properties of the litter layer (such as moisture content, depth, and packing). Prescribed burns in the southern U.S. are typically low-intensity and soil temperatures are elevated for only brief durations, therefore soil damage rarely occurs. However, burning piled or windrowed debris, or burning under other conditions that create intense fires can potentially damage soil. This is particularly true if the soil and duff layers are dry.

Many of the changes in soil properties due to fire are linked to changes in soil organic matter. Soil organic matter contributes to soil structure by holding sand, silt, and clay particles into aggregates. Water and nutrient availability are also tightly linked to the total amount and quality of soil organic matter. Prescribed fires in the southern coastal plain typically decrease organic matter in the forest floor, while having either no effect or a weak positive effect on organic matter in the mineral soil (Table: Effects of Prescribed Burning on Soil Carbon and Nitrogen).

Fire may alter several physical soil properties, such as soil structure, texture, porosity, wetability, infiltration rates, and water holding capacity. In general, most fires do not cause enough soil heating to produce significant changes to soil physical properties (Hungerford et al. 1990). This is particularly true for low intensity prescribed fires. However, burning piled or windrowed debris, or burning when fuel and/or soil moisture conditions are extremely low, may elevate temperatures long enough to consume soil organic matter and alter the structure of soil clays (Ulery and Graham 1993).

Nutrient Pool Sizes in Forest Floor and Mineral Soils

Forest fires usually decrease the total amount of nutrients present on a site through some combination of oxidation, volatilization, ash transport, leaching, and erosion. However, though fire can diminish nutrient pool sizes, nutrient availability often increases due to (1) nutrients added to the soil as ash, (2) heating of soil organic matter, and (3) increased rates of biological mineralization due to increases in soil pH, temperature, and moisture (Wright and Bailey 1982, Pritchett and Fisher 1987). The nutrient pools that are most affected most by fire (e.g. fuels) are often insignificant when compared to other nutrient pools such as mineral soils.

As with soil physical properties, fire intensity is usually the most critical factor driving post-fire nutrient dynamics. High intensity fires usually decrease nutrient pools more than low intensity fires and can have many other post-fire impacts that lower site productivity. Different nutrients also differ in their sensitivity to fire (See figure: Temperature effects on soil). Due to its low temperature of volatilization, nitrogen loss is linked with the consumption of organic matter. Nitrogen in the organic soil horizon is particularly sensitive to fire and tends to diminish when organic soil horizons are consumed regardless of fire intensity, but mineral N concentrations tend to increase and become more available in the soil surface after burning (Wan et al. 2001). Volatilization of phosphorus and cations (K, Mn, Mg, and Ca) are usually minor due to the high volatilization temperatures of these minerals, however, their loss from severely burned sites may be caused by surface erosion, leaching, or transport of ash (Wright and Bailey 1982).

Fire also affects the total abundance and activity levels of soil biota such as microbes and soil invertebrates. Although relatively few studies have addressed this topic, available evidence again shows that the overall effect of fire on soil organisms is dependent to a large extent upon fire intensity. The responses of soil microbes to fires range from no detectable effect from low intensity prescribed fires to total sterilization of the surface layers of soil in very hot wildfires (Joergensen and Hodges 1970, Renbuss et al. 1973). Although there is a decrease in the abundance of microbes following fire, the remaining microbes can have levels of activity that are greater than that of the microbial community prior to the fire (Poth et al. 1995) with increased rates of denitrification and production of methane and carbon dioxide. Studies have also shown that the effects of fire on microinvertebrates, such as mites and springtails, can depend on fire frequency. For example, microinvertebrates may only be slightly affected by periodic fires, but drastically reduced by annual burns (Metz and Farrier 1971). The few studies looking at responses of soil macroinvertebrates to fire in the southeastern US suggest that the response is often driven by changes in habitat structure, or by changes in the amount or the quality of food resources. Interestingly, this can lead to an increase in some groups, such as in earthworm populations following burns in tallgrass prairie soils (e.g., James 1982).

A major concern of forest managers, particularly in the steep topography of the southern Appalachians, is how fires affect surface runoff and soil erosion. Danger of erosion depends, in part, on the amount of organic matter consumed on the soil surface. If mineral soil is exposed, rain impact may clog fine pores with soil particles, decreasing infiltration rates and aeration of the soil. Where fires do cause direct changes to the forest floor and soil surface, their indirect effects on soil hydrology and erosion will vary greatly depending on the topography, vegetation type, and climate. In the southeastern U.S., these conditions are such that fires rarely create serious erosion and surface-runoff problems, unlike areas in the western U.S. where rehabilitation and restoration efforts are often needed following wildfires. Nevertheless, it is still important for fire managers to assess the susceptibility of their particular site to soil damage and erosion. For example, sites on steep slopes within the Southern Appalachians are far more susceptible to erosion following burning than the flatter sites of the coastal plain. Managers should also be aware of guidelines that can mitigate potential effects of fire on soil damage and erosion, such as burning when duff and litter layers are moist.

The transfer of heat to soil is the main mechanism by which fires affect physical, chemical and biological soil properties (Neary et al. 1999). High soil temperatures can kill soil microbes, plant roots, and seeds; destroy soil organic matter; and alter soil nutrient and water status (see diagram: Temperature effects on soil) (Hungerford et al. 1991, Campbell et al.1995, DeBano et al 1998). 

Radiation and convection are responsible for most heat transfer from light fuels to soil. Conduction is the main heat-transfer mechanism in heavy fuels like duff, organic soils, and slash piles. Vaporization and condensation, which involve phase changes of water and organic compounds distilled by combustion, are also important mechanisms of heat transfer in soil. Water moves much faster through soil pores as a vapor and releases heat when it condenses.  

The degree of soil heating during any fire is highly variable and depends on:

• fuel characteristics: loading, size, arrangement, moisture content,
• fire behavior: rate of spread, flame length, intensity, duration, 
• litter layer: thickness, packing, moisture content, and 
• soil properties: texture, organic content, moisture content. 

Duff moisture and soil moisture are critical regulators of subsurface heating. When water is present in soil and duff, the temperature at any particular depth does not exceed water’s boiling point, 100^o C (212 ^o F), until the water has evaporated or moved into lower layers (Scotter 1970, DeBano et al. 1976).  If the surface organic layer is thick and moist, little soil heating will occur. However, if the litter layer is dry and consumed, the underlying soil can be heated substantially. For example, heat load into wet duff and mineral soil can be 20% of that penetrating dry duff and mineral soil (Frandsen and Ryan 1986). Peak soil temperatures can also be more than 538 ^o C (1000 ^o F) greater where duff and soils are dry.

The movement of heat downward through soil layers is not only dependent on the peak temperature reached, but also on the temperature duration. Heat penetrates deeper into soil the longer a heat source is present. Soil texture also affects heat transfer. The thermal diffusivity of quartz is about three times that of clay, therefore sandy soils heat more slowly than finer textured soils.

Heat flux to soils during fires is almost always less than the heat released aboveground.  As little as 8-10% (maximum of 25%) of heat can be transmitted downward to the soil (DeBano et al. 1977, Packham 1969, Raison et al. 1986). Resulting soil temperatures are almost always lower than above ground temperatures, but do reach levels that can alter soil biological, chemical, and physical properties (see diagram: Temperature effects on soil). The highest soil temperatures usually occur beneath heavy slash, particularly with the consumption of large piles of dry harvest residue or windthrow. Burning large slash piles produce long duration, high temperature heat pulses that penetrate deep into the soil, potentially altering both physical and biotic characteristics of the soil to significant depths. Prescribed burns in shrublands typically generate more soil heating than prescribed burns in either grasslands or forests. Due to the high water content of wetland soils, penetration of heat generated by a surface fire can be significantly less than in mineral soils. Organic matter has a lower thermal diffusivity than mineral soil, so penetration of heat is further reduced in organic wetland soils.  However, organic soils can become dry enough to burn, producing significant amounts of heat.

It is very difficult to characterize heat transfer to soil because of the variability of combustion and soil conductions (Neary et al. 1999); nonetheless, several mathematical models based on heat transfer theory have been developed to address this problem (Hungerford and Campbell 1991). Scotter (1970) developed one of the earlier models, however it was limited because it did not include moisture-aided heat flow, an important mechanism in heat movement through soil.  Aston and Gill (1976) later developed a model that describes the transfer of water, heat, and water vapor through soils. It has been shown to predict soil temperature profiles, moisture profiles, ground heat flux, and evaporation in Australian grasslands, but has not been tested for forests.  Other models that predict the downward heat pulse in soil beneath fires include Campbell et al (1995), Chinanzvavana et al. (1986) and Pafford and others (1985).  However, these models remain untested for systems in the southern U.S.

Fire may alter several physical soil properties, such as soil structure, texture, porosity, wetability, infiltration rates, and water holding capacity.  The extent of fire effects on these soil physical properties varies considerably depending on fire intensity, fire severity, and fire frequency.  In general, most fires do not cause enough soil heating to produce significant changes to soil physical properties (Hungerford et al. 1990).  This is particularly true for low intensity prescribed fires. Even where fires do cause direct changes to soil physical properties, their indirect effects on soil hydrology and erosion will vary greatly depending on the condition of the soil, forest floor, topography, and climate.  In the southeastern U.S., these conditions are such that fires do not create serious erosion and surface-runoff problems, unlike areas in the western U.S. where rehabilitation and restoration efforts are often needed following wildfires. Nevertheless, it is still important for fire managers to assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects. In the sections that follow, the potential effects of intense fires on soil physical properties is reviewed and the practical implications of these effects in the southeastern U.S. is discussed.  

Potential effects of fire on soil physical properties

Intense burns may have detrimental effects on soil physical properties by consuming soil organic matter (see diagram: Temperature effects on soil).  Soil organic matter holds sand, silt, and clay particles into aggregates, therefore a loss of soil organic matter results in a loss of soil structure. By altering soil structure, severe fires can increase soil bulk density (DeByle 1981), and reduce soil porosity (e.g., Wells et al. 1979), mostly through the loss of macropores (>0.6 mm diameter).  Soil porosity can also be reduced by the loss of soil invertebrates that channel in the soil (Kettredge 1938).  When fire exposes mineral soils, the impact of raindrops on bare soil can disperse soil aggregates and clog pores, further reducing soil porosity (Ralston and Hatchell 1971).

Intense fires (> 400 C) may also permanently alter soil texture by aggregating clay particles into stable sand-sized particles (Dyrness and Youngberg 1957, Ulery and Graham 1993), making the soil texture more coarse and erodible (Chandler et al. 1983).  In some cases, increasing the coarseness of clays can make soils more permeable to air and water.

Intense burns may also induce the formation of a water repellent soil layer by forcing hydrophobic substances in litter downward through the soil profile (DeBano 1969).  These hydrophobic organic compounds coat soil aggregates or minerals creating a discrete layer of water repellent soil parallel to the surface.  Water repellent soil layers are reportedly formed at temperatures of 176-288^o C and destroyed at >288^o C (Neary et al. 1999). Extensive water repellent layers can block water infiltration and contribute to runoff and erosion.  While formation of water-repellent layers is an important concern in western shrublands, particularly chaparral (DeBano et al. 1977), it has not been documented in the South.

Fire induced changes in soil structure and texture can potentially impair soil hydrology.  Decreased soil porosity and the formation of water repellent layers decrease water infiltration rates (DeBano 1971). Loss of soil organic matter and increased bulk density can decrease the water storage capacity of soils. In flat terrain, this contributes to soil desiccation, particularly in the surface soil layer (Dyrness and Youngberg 1957).  However, in steep terrain it can significantly accelerate runoff, ash transport, erosion, and mass wasting (Neary et al. 1999).  Just exposing soil surfaces can also cause soil erosion. Without the mitigating effects of vegetation on the impact of raindrops, bare soil surfaces can form a sealed surface layer resulting in much higher rates of surface runoff.  Surface erosion by wind or gravity can also increase when ground cover, surface litter, and/or duff protecting the mineral soil are removed.  For this reason, re-establishment of ground cover naturally or by seeding is the most effective erosion control following fire (Wells et al. 1979).

Recent evidence suggests that charcoal can also affect soil hydrology. Fine charcoal particles enhance the water-retentive properties of a soil and can make a sandy soil behave like a clay (Moore 1996).  While this effect could be ecologically significant in bottomland sites where it could contribute to poor drainage and waterlogged conditions, it has not been investigated or reported in the southern U.S.

By altering soil physical properties and soil hydrology, fire can also have indirect effects on plants. Plant uptake of nutrients and water is slowed in structurally degraded soils through the combined effects of lower soil moisture and lower soil porosity (Nye and Tinker 1977).  Root growth can also be impeded by increased bulk density and soil strength (Gerard et al. 1982).

The long-term effects of fire on soil physical properties range from a single season to many decades, depending on the fire severity, rate of recovery as influenced by natural conditions, post-fire use, and restoration and rehabilitation actions.  Persistent soil degradation following fire is more common in the cold and/or arid climates typical of the western U.S.. In the eastern U.S., recovery is usually more rapid. For example, Heyward (1937) found that excluding fire for as little as 10 years in longleaf pine forests resulted in more porous penetrable soil.

Effects of fire on soil physical properties in the southern U.S.

Single low-intensity prescribed burns in the southern U.S. typically do not cause dramatic changes to soil structure and texture.  Elevated soil temperatures during these fires are usually brief (Heyward 1938).  Clay minerals are not changed to a great extent during these fires because of their low content in surface soil horizons and the high temperatures required for aggregation into sand-sized particles (> 400 C; Ulery and Graham 1993).  Moreover, changes in soil pore space and infiltration rates are usually slight if organic soil layers are not completely consumed. 

While single prescribed burns may not have significant effects on soil, the high frequency of fires in the south can have cumulative effects on soil physical properties.  Soil organic matter is usually lower in soils that are repeatedly burned, and early researchers noted that burned soils were harder, denser, and less permeable than unburned soils (Garren 1943, Wahlenberg et al. 1939).  Decreased infiltration rates have also been reported at several burned sites in the south (Ozarks: Arend 1941, Mississippi: Meginnis 1935). 

By altering soil physical properties and removing protective surface covers, fire does lead to increased erosion rates in areas throughout the south.  However, in contrast to western states where fire-induced soil degradation and erosion is a serious issue, large soil losses after burns are rarely documented in the south.  Nevertheless, land managers should still assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects.

Soil organic matter (SOM), or humus, increases soil water-holding capacity and aggregation and typically contains 90% or more of soil nitrogen. Prescribed fire in pine forests of the southern coastal plain of the United States decreases organic matter (carbon, or C) and nitrogen (N) in the forest floor, while having either no effect or a weak positive effect on organic matter and N in the mineral soil (Table:Effects of Prescribed Burning on Soil Carbon and Nitrogen in Pine Forests of the Southern Coastal Plain.). (Little information exists on fire’s effect on C and N amounts on other ecosystems of the South.) Prescribed fire in the South has been reported to both increase and decrease the conversion of organically bound N to plant-available forms (ammonium and nitrate). Charcoal in burned soils is a relatively stable form of SOM, and may affect plant nutrient uptake and the competitive balance between plant species.

For more information on the above topics, click the links below:

• Fire Effects on Amounts of Soil Organic Matter (Carbon) and Nitrogen

• Fire Effects on the Biological and Chemical Properties of Soil Organic Matter

• Charcoal and Its Effect on Soil Properties

Prescribed fire

Prescribed fire in pine forests of the southern coastal plain of the United States has the clear effect of decreasing organic matter (carbon, or C) and nitrogen (N) in the forest floor, while having either no effect or a weak positive effect on organic matter and N in the mineral soil (Table:Effects of Prescribed Burning on Soil Carbon and Nitrogen in Pine Forests of the Southern Coastal Plain). The effect of prescribed fire on total (forest floor + mineral soil) C and N appears to be negative.  However, N fixation likely plays a role in preventing excessive N loss from coastal plain ecosystems (Stone 1971; Fisher and Binkley 2000).

There are pressing research needs involving organic matter in prescribed burn ecosystems in the Southern Coastal Plain as well as in other prescribed-burn southern ecosystems. Research in these areas would further understanding of fire’s effects on soil fertility and would also aid in the management of forests for maximum soil C storage.

• The effects of prescribed burning on soil organic matter and N in southern ecosystems outside of the Coastal Plain (e.g. mountains, piedmont) should be assessed as this topic has received little or no attention.  These areas differ in moisture, temperature, and topography from the Coastal Plain, and would likely exhibit different trends in organic matter and N.
• Carbon at depth (more than 20-50 cm below the mineral soil surface), as related to prescribed fire, has received little attention even though some fire-impacted southern soils are known to contain appreciable amounts of organic matter at depth.  For example, Stone et al. (1993) estimated that half of soil organic matter in Florida Spodosols was contained in the spodic horizons.  It is likely that fire was a factor in the genesis of Florida Spodosols and certain other soils of the South, as nearly all forest types in the southern costal plain were subject to frequent fires (Stanturf et al. 2002).
• The decreased probability of wildfire should be reflected in the soil C “accounting” of ecosystems managed by prescribed burning.  The “losses” of soil C due to prescribed burning are probably offset somewhat by decreased wildfire risk.

Wildfire

There is little information on wildfire effects on soil carbon and nitrogen in the South.  Results from one study are described below:

One year after a lightning-caused wildfire in Shenandoah National Park, Virginia, Groeschl et al. (1991) measured C and N amounts in low- and high-intensity areas of the wildfire area as well as in an adjacent unburned area.  Most of the soil C and N loss occurred in the forest floor,  with total losses (forest floor plus mineral soil to depth of 10 cm) of  26 and 68% (C, low- and high-intensity areas) and 10 and 65% (N, low- and high-intensity areas)  The losses in high-intensity areas are conservative estimates as erosion was observed in these areas.  In the low-intensity area of the wildfire, the relatively low N loss (10%) compared to C (26%) may reflect nitrogen fixation during the recovery period.

Biological properties

Fire appears to increase the amount and biodegradation rate of readily decomposable soil organic matter while simultaneously increasing the resistance of the stable portion of soil organic matter.  In laboratory mineralization assays of wildfire-impacted soils, Fernández et al. (1999) observed increases in the amounts and decomposition rate constants of labile (available) carbon (C) as well as decreases in the decomposition rates of the resistant portion of the C pool. The increased decomposition activity that immediately follows fire is likely a result of increased levels of readily decomposable C as well as increased pH, the conversion of nutrients to soluble forms, increased soil temperature, and increased water availability to microbes due to lessened plant water demands. The increased decomposition activity in recently burned soils is viewed as an important nutrient conservation mechanism, as it leads to microbial retention of nutrients that might otherwise be lost from the soil (Woodmansee and Wallach, 1981). The fire-related decreases in the decomposition rates of resistant carbon probably stem from the conversion of humus to “black carbon.”

Contrasting effects of prescribed fire on nitrogen mineralization (release of ammonium or nitrate from organic matter) have been reported. In a southeastern Missouri oak-hickory forest under long-term burning treatment, Vance and Henderson (1984) demonstrated that soils from burned areas were persistently lower in both extractable inorganic N (ammonium plus nitrate) and N mineralization than unburned areas.  They attributed the decreased nitrogen availability to poor substrate quality (perhaps aromatic nitrogen as noted below). Similarly, Bell and Binkley (1989) found increased N immobilization and decreased N mineralization in regularly burned loblolly forests of the South Carolina coastal plain. The high C:N ratios in the burned soils were proposed as an explanation, as high values of this ratio favor N immobilization. Notably, Bell and Binkley (1989) took their samples after a relatively severe burn. In contrast, Schoch and Binkley (1986) found increased N mineralization from the forest floor and increased N levels in foliage following a low-intensity prescribed burn in a previously unburned loblolly forest in the North Carolina piedmont. The apparent conflict between the results of Schoch and Binkley (1986) and other studies may be related to long-term burning effects, differences in C:N ratios, or differences in fire severity. It is of both economic and ecological interest to determine the conditions under which fire might expected to create a “pulse” of plant-available N. For more information, see Fire Effects on Soil Nutrients.

Chemical properties

Heating soil organic matter in the laboratory produces rapid losses of carbohydrates and proteins and eventually produces residues rich in aromatic compounds (Almendros et al. 1992, 2003). The aromatic compounds formed include aromatic forms of nitrogen (Almendros et al. 2003; Knicker et al. 1996), which may be responsible for the reduced N availability observed in some burned areas. Because N is the major limiting nutrient in most forests (Fisher and Binkley, 2000), the relationship between the chemical structure of N and N availability in burned systems is a pressing research need. Hydrophobic polymers are also formed upon heating, and these are probably responsible for the soil hydrophobicity that is observed after fire (Almendros et al., 1992).

The effects of fire on soil organic matter chemistry in soils subjected to repeated burning in the field have been less consistent than effects observed in the laboratory. Nevertheless, useful results have been obtained for specific ecosystems. For example, Guinto et al. (1999) observed an elevated ratio of alkyl carbon (i.e., lipids and waxes) to O-alkyl carbon (carbohydrates) in a burned sclerophyll forest of Australia. This ratio, which is a measure of the degree of humification, had a strong negative correlation with N mineralization in the field. Golchin et al. (1997) contrasted the soil organic matter from an annually burned grassland on volcanic soil with the organic matter of nearby formerly burned grasslands that were reverting to forests. The annually burned grassland had more aromatic and less alkyl carbon than the forested areas. These findings are consistent with other studies that have highlighted the role of fire-generated aromatic compounds in soil genesis (See Charcoal). For a detailed review of fires effect on soil organic matter chemistry, see Gonzalez-Perez et al. (2004)

Charcoal is a dark-colored, solid carbonaceous material that results from the combustion of organic material. Kuhlbusch (1998) has proposed some useful definitions of charcoal and related materials:

• Charcoal is the carbonaceous residue formed in-place during combustion. It retains the gross anatomical features of the original material.
• Soot is the solid-phase carbonaceous particulate material formed above a fire by gas-to-solid conversion.
• Black carbon is a component of charcoal and soot. BC is highly aromatic, resistant to oxidation at 340 ^oC under pure oxygen, and has a molar H/C ratio of 0.2 or less.

Given the widespread historic occurrence of fire throughout the South (Stanturf et al. 2002), the chemistry of organic matter in many southern soils is likely to have been strongly influenced by the formation of black carbon. Since there is little information available on the importance of charcoal in southern soils, the information presented here originates from other regions.

Soil black carbon content

The BC contents of eight Australian soils of variable char content ranged from near-zero to 30% (mass of BC per mass of total carbon), with results varying widely according to the analytical method employed. Most soils had BC contents between zero and 10% (Schmidt et al. 2001).

Role of charcoal in in soil formation

Charred organic matter is thought to be a source of stable aromatic carbon in many soils, according to a review by Schmidt and Noack (2000). The evidence cited by these authors includes the chemical similarity between humic acids extracted from certain soils and those extracted from charred material, a positive relationship between black carbon content and black color in German chernozemic soils, and the implication of fire as a factor in the genesis of deep black soils in the xeric moisture regime of Northern California. 

Charred organic matter may also play a role in translocating iron (Fe) and aluminum (Al) down the soil profile via organo-metal complexes, which is an important process in the genesis of Spodosols.  Fernández et al. (1997) found that humic extracts from wildfire-impacted soil had much higher levels of bound Fe and Al than those obtained from the soil of a nearby unburned area.

Decomposition of Black Carbon

The decomposition or chemical transformation of black carbon (BC) in ecosystems is likely, but slow.  Bird et al. (1999) studied BC levels in sandy savanna soils of Africa, and concluded that BC had an overall half-life of less than 100 years in surface soil.  Although it was difficult to distinguish between true decomposition and removal by transport, they noted that the charcoal in recently burned areas was hard, black, and of a vitreous luster whereas older charcoal was soft and brown. Zackrisson et al. (1996) noted that young charcoal was more effective than old in mitigating the inhibitory effects of phenolic crowberry (Empetrum spp.) extracts on the germination of European aspen (Populus tremula L.).

Effects of charcoal on plants

Charcoal, when applied to soil in amounts expected after wildfire, enhanced plant nitrogen uptake, altered the competitive balance between plant species (including ericaceous species), and stimulated moss and fern production in a Swedish boreal forest ecosystem. These effects were attributed to charcoal’s ability to bind and deactivate phenolic compounds in the soil (Wardle et al. 1998). There is little or no information on the whether such effects occur in southern ecosystems, which contain numerous ericaceous species.

This discussion of fire effects on soil nutrients will focus on the elements considered essential for plant growth and nutrition needed in relatively large quantities known as macronutrients (Table:Plant Essential Macronutrients). These are the nutrients most likely to impact site productivity and vegetation dynamics and therefore of most interest to forest managers and ecologists.

Nutrient Pool Sizes in Forest Floor and Mineral Soils

Forest fires whether planned or not usually decrease the total site nutrient pool (the total amount of nutrients present) through some combination of oxidation, volatilization, ash transport, leaching, and erosion. For example, a low intensity slash fire resulted in the following reductions in understory and forest floor fuel nutrient pools: 54-75% of N, 37–50% of P, 43–66% of K, 31–34% of Ca, 25–49% of Mg, 25–43% Mn (micronutrient), and 35–54% of B (micronutrient) (Raison et al., 1985). Volatilization and oxidation were the mechanisms responsible for the observed nutrient loss. While the reduction in site nutrient capital intuitively seems detrimental to forest productivity, and it can be, it is important to remember that the amount of nutrients found on site and nutrient availability are not always tightly linked. This is especially true when soil nutrients are considered. In fact, though fire can diminish nutrient pool sizes, nutrient availability often increases, and the pools that are affected most by fire (e.g. fuels) are often insignificant when compared to other nutrient pools such as mineral soils (See Figure). For example, soil fertility can increase after low intensity fires since fire chemically converts nutrients bound in dead plant tissues and the soil surface to more available forms or the fire indirectly increases mineralization rates through its impacts on soil microorganisms (Schoch and Binkley 1986). McKee (1982) found that in unburned Coastal Plain soils, calcium bound to the well developed O horizons in unburned stands might eventually become limiting.

Also important to consider is that some nutrients dynamics are more sensitive to fires than others. The concentration of potassium, calcium, and magnesium ions in the soil can increase or be unaffected by fires whereas nitrogen and sulphur often decrease (Hough 1981). Although the relationship between fire and soil nutrients is complex due to the interactions among many factors, fire intensity is usually the most critical factor driving post-fire nutrient dynamics, with greater nutrient losses occurring with higher fire intensity. Fire intensity has both direct and indirect impacts on many of the mechanisms that affect nutrient pools and cycling. Fire temperature directly determines both the amount and kinds of nutrients that will be volatilized (See diagram: Temperature effects on soil). Consider two elements, N and Ca: N begins volatilizing out of organic matter at only 200º C, whereas Ca must be heated to 1240º C for vaporization to occur (Neary et al. 1999). Nutrients are abundant in surficial organic soil layers, and the amount of these layers consumed is proportional to fire intensity. As an indirect effect, the physical transport of nutrients off site is related to fire intensity. Convective transport of ash varies from 1% in low intensity fires to 11% in high intensity fires (Neary et al. 1999). High intensity fires can also change the physical characteristics of the soil making it more susceptible to nutrient loss through erosion (McColl an Grigal 1977), though in much of the Coastal Plain, the modest topography makes erosion less likely than in Piedmont or mountainous terrain.

The impact of fire on site productivity is also related to intensity. While high intensity fires (which are also more likely to be high severity) tend to decrease site productivity, low intensity fires can increase site productivity (Carter and Foster 2003). In a study of low intensity prescribed fire in loblolly pine stands, Binkley et al. (1992) found that nearly all the fire effects were limited to the forest floor (O horizons) and that the effects were weak; when compared to an unburned stand, nutrient pools in frequently burned stands were unaffected (P, Mg, K, S), increased slightly (Ca), or decreased (N, S). Though the authors found that the N pool decreased in the O horizon, they observed that site productivity was unaffected, possibly due increased mineralization rates in other soil horizons. In a meta-analysis of fire effects on N (a statistical technique that allows an objective synthesis of previous studies) Wan et al. (2001) found that the N pool in fuels is decreased, soil total N pools were unaffected and ammonium and nitrate levels in the soil increased which increased N availability. Reports on the effects of fire on soil N pools have been controversial, both due to the importance of N as it affects site productivity and because of its complicated response.

Further complicating the picture are the interactions among time since fire, vegetation type, sampling techniques and fire intensity. Wan et al. (2001) found that the dynamics of N and fire vary considerably among ecosystems; higher N losses occurred in broad-leaved forests than in coniferous forests. They also found that the magnitude of the effects of fire depended both on how long ago the fire occurred and how the soil was sampled. Even so, the authors argue that the mineral soil N pool is so large (and relatively unaffected by fire) when compared to the organic soil and fuel N pools, that the N lost by consumption is insignificant.

Although the fire-soil nutrient relationship is complicated, some generalities do emerge. Fires typically result in the reduction of fuel and organic soil nutrient pool sizes, increase soil nutrient turnover rates, and redistribute nutrients through the soil profile (Fisher and Binkley 2000). Fire intensity will most likely determine post-fire soil nutrient dynamics. High intensity fires usually decrease nutrient pools more than low intensity fires and can have many other post-fire impacts that lower site productivity. Nutrients pools in the organic soil horizons are more likely to be impacted by fires than those in the mineral horizons. N and S in these pools are particularly sensitive to fires, and tend to diminish when organic soil horizons are consumed regardless of fire intensity, but mineral N concentrations tend to increase and become more available in the soil surface after burning (Wan et al. 2001). Pools of P, K, Mn, Mg, and Ca are generally not as likely to be impacted by low intensity fires, but can be lost after high intensity.

Plant Roots and Fire

There is a large amount of information available that details the effects of fire on plants in southern forests (see Fire Effects on Plants). These effects range from completely positive to completely negative. This range of effects depends largely on the community of plants present in a forested landscape (fire tolerant species, fire sensitive species, or a mixture), and on the intensity of the fire (low intensity prescribed fire, high intensity wildfire, or something in between). Fire almost always results in the death of some plants in a given system, and the extent to which plants are killed has a strong relationship to the effects of fire on roots. The killing of fire sensitive plants aboveground results in an input of dead roots belowground, and this input of new material has the potential to influence the decomposers (microbes) as well as the entire soil food-web at least in the short term.

Soil Microbes and Fire

The effects of fire on soil microbes in southern systems seems to be dependent to a large extent upon fire intensity. The responses of soil microbes to fires range from no detectable effect in the case of low intensity prescribed fires to total sterilization of the surface layers of soil in very hot wildfires (see Joergensen and Hodges 1970; and Renbuss et al. 1973). This early work focused primarily on the abundance of microorganisms and not their activity levels. This is interesting because workers have observed that although there is a decrease in abundance of microbes following fire, the remaining microbes can have levels of activity that are greater than that of the microbial community prior to the fire (Poth et al. 1995). These authors, working in tropical savanna systems in Brazil, found that the increased rates of microbial processes, such as denitrification and production of methane and carbon dioxide, persisted for one year following fire, but the nature and duration of microbial responses to fires in southeastern forests are not well known. (See diagram: Temperature effects on soil)

Microinvertebrates and Fire

In one of the few studies dealing with microinvertebrate responses to fire in southern forests, Metz and Farrier (1971) reported a general reduction of microarthropods with increasing prescribed fire frequency in loblolly pine stands on the coastal plain of South Carolina. In this study, the authors compared the abundance of microarthropods in plots that had either been burned every year, burned every 3 to 4 years, or left unburned for many years. The main results from this study were that abundances of mites and springtails were reduced by a small amount (~25%) by periodic prescribed fires, but that this reduction was dramatic (75-80% fewer) when prescribed fires were conducted annually. Similar studies in midwestern forests have shown similar results in that reduction of litter mass with prescribed fire generally results in reductions of microarthropod numbers (Dress and Boerner 2004; Brand 2002). The consequences of these reductions for the decomposition of new leaf litter have not been thoroughly addressed.

The response of microarthropods to fire has also been studied in many other systems including southern grasslands such as the tallgrass prairie systems in eastern Kansas and Oklahoma. These studies have generally found that microarthropods are decreased in abundance with frequent fire (Seasteadt et al. 1991). This negative effect of fire is mostly attributed to decreased habitat for mites and springtails, because many of these organisms live in decomposing leaf litter, and much of this litter is lost in fires. Other researchers in yet other ecosystems have suggested that changes in the size of the microarthropod population in soils of burned areas might serve as an indicator of fire intensity. One group working in southern California found that this type of microarthropod index worked well for estimating fire intensity when combined with other easily measured variables (Henig-Sever et al. 2001).

Macroinvertebrates and Fire

There have been few scientific studies of the responses of soil macroinvertebrates to fire in forested ecosystems of the southeastern U.S.. Of the few studies that have addressed these organisms responses, the general pattern observed is that the response is often driven by changes in habitat structure, or by changes in the amount or the quality of food resources. Thus, whenever fire affects vegetation, temperature or moisture, or the nutrient status of a soil, there is potential for impact on the soil invertebrate community. These impacts are not always predictable, as demonstrated by a study of ground and litter dwelling arthropods conducted by Hanula and Wade (2003). These authors found that the frequency of prescribed fires (plots burned annually, every two years, every four years, or unburned for 40 years) in longleaf pine flatwoods of northern Florida had dramatic effects on numerous organisms. Interestingly, most of the arthropod groups collected during the five-year study had negative responses to fire, but other groups were strongly favored by fire. For example, among 28 different spider groups that were collected, there were only four that responded positively to the frequent fires employed in the study.

Another study of litter dwelling and soil dwelling macroinvertebrates showed that the density of macroinvertebrates was significantly reduced one year after a prescribed fire in the upland forests of the Cumberland Plateau in Kentucky (Kalisz and Powell 2000).  Reduction in the number of beetle larvae accounted for a large proportion of the difference following fire, and the authors proposed that repeated fire in a single location could potentially have long-term negative effects on beetle populations and on the functions these beetles perform within the system.

Several studies have been conducted in grassland soils in Kansas that focused on the responses of soil macroinvertebrates to fire. Studies have repeatedly shown that earthworms are strongly affected by fire in tallgrass prairie soils, and the usual pattern observed is for fire to increase the abundance of earthworms in undisturbed areas (e.g., James 1982). However, in more disturbed areas (i.e. close to human habitations), fire also has the interesting effect of limiting the colonization of non-native earthworms into soils under frequently burned vegetation (Callaham et al. 2003). Results of this study suggested that the native earthworms in grassland soils are adapted to the warmer soil conditions frequently found under frequently burned vegetation, and that because fire improves the performance of grasses, the native earthworms may have strong habitat preferences for soils with abundant grass roots.

Future Directions for Research in the Southeast

Most of the information presented in this article comes from ecosystems that are not very well represented in the southeastern U.S.. We have discussed results from studies that were conducted in various systems in various geographic ranges, but there are relatively few studies available for ecosystems or states in the southeast. Clearly, there is a large gap in our knowledge of soil organism responses to fire in these systems. Basic studies of fire effects on soil invertebrates is needed. Some new studies have recently been initiated, and these should shed some light into the effects of this important land management practice on soil organisms in southern forests. Furthermore, there have been recent calls for increased effort toward identifying and understanding the linkages between aboveground and belowground organisms (Wardle et al. 2004). As such, future efforts should certainly include research on the effects of land management on aboveground vegetation and the subsequent effects on soil biota and the soil processes they influence.

Plants

Plants are the original source of energy in most soil systems, and this is certainly true for all forests of the southern US.  Leaf fall during autumn provides belowground systems with large inputs of energy and habitat for a vast array of soil organisms.  Furthermore, the roots of plants are an integral part of the biotic makeup of soils.  Whether live or dead, roots are important food resources in soils, and represent a basic source of energy for soil organisms from microbes all the way up the food chain to top predators in soil systems.

Microbes

Microbial organisms are an extremely important component of the biologically active portion of soil ecosystems.  The word "microbe" encompasses organisms of several different types that are not possible to observe without the help of a microscope.  The primary microbes in soil are bacteria and fungi.  Soil bacteria fall into several categories depending on their functions, but the primary role of bacteria in soil is recycling nutrients from dead plant material into forms that are available for use by live plants.  Soil fungi fall into two general categories: saprobic and mutualist.  The saprobic fungi are microbes that derive energy primarily from decaying wood, leaves and other organic material.  Mutualist fungi (sometimes called mycorrhizae) form associations with the roots of live plants that are mutually beneficial to the plant and to the fungus.  In these mutualisms, the plant provides the fungus with sugars (energy), and the fungus assists the plant in acquiring nutrients.  Although most bacteria and fungi could be considered beneficial to the functioning of soil systems in southern forests, it is important to mention that not all microbes are desirable members of the soil community and some can seriously impact the health of plants in the forest.  The activities of these undesirable, or pathogenic, microbes can lead to the mortality of valuable trees, and can cause significant economic loss in some forests.

Other microscopic organisms that can be grouped with the microbes are the protists including algae, amoebae, and ciliates.  Together, the actions and interactions of microbes and protists are largely responsible for the breakdown and recycling of dead plant material, and can strongly influence the fertility of a given site.

Soil Invertebrates

Soil invertebrates make up an extremely diverse assemblage of organisms ranging widely in size and function.  The most abundant soil organisms are also some of the smallest.  For example, soil nematode worms are sometimes found in densities greater than one million individuals per square meter of soil area, but they are tiny with body lengths generally between 0.1 and 1.2 mm.  Other types of very small but very abundant organisms include mites and springtails.  Both mites and springtails are somewhat larger than nematodes and can occur in densities in the tens of thousands of individuals per square meter. Because of their small size, these organisms are sometimes called microinvertebrates.  On the other end of the size spectrum are the largest soil invertebrates, sometimes called macroinvertebrates, a group that includes earthworms, millipedes, centipedes, and many insects.  For a list and some photos of some important soil organisms in southern forests, see Table:Soil Invertebrates in the Southeast.

The functions that soil invertebrates perform are mostly related to the breakdown and recycling of organic material in the litter layer and in the soil.  Many of the organisms mentioned above have been shown to influence the rate at which leaf litter or dead roots are processed into soil organic matter, and subsequently the availability of nutrients to plants.  However, many of the organisms living in soil, especially nematodes and beetle larvae, feed directly on the roots of living plants.  When these organisms reach high population densities, they can significantly damage the vegetation at a site.  This is usually only a problem in systems that are intensively managed for production of plants (e.g. in agricultural fields, nurseries, or tree plantations), and is generally negligible in diverse forest stands.

Habitats for Soil Organisms

Although soil may seem to be a fairly uniform and simple environment at first inspection, there are a large number of specific habitat types in a typical forest soil that are available for soil invertebrates to inhabit.  From the soil surface where mites, springtails and other insects inhabit freshly fallen leaf litter, down through layers of decomposing leaves and further down through mineral horizons of soil where nematodes feed on the deepest roots of trees, the entire soil volume is occupied by living creatures.  Not only is the soil a varied habitat vertically, but even more diverse habitat types exist horizontally across a forest landscape.  For example, the soil environment (moisture and temperature) on a ridge top is very different from that found in a valley bottom.  At finer scales, there can be significant differences in the chemical, physical and biological characteristics of soil underlying individual trees of different species.  So, from soil surface to deep depths, and across the whole forest landscape, the soil is a very complex mosaic of habitat types supporting a highly diverse community of organisms.

Fire can lead to increased soil temperatures in the months following a burn by removing the forest floor, blackening soil surfaces, and opening the canopy. At the same time, removal of above-ground biomass may increase soil moisture by reducing evapotranspiration. For example, Swift et al. (1993) found that using the fell-and-burn technique in the Southern Appalachians increased soil temperatures 2 to 5°C at 10 cm depth during the first 16 months after treatment.  In the same study, they also observed increased soil moisture.

Because soil temperature and moisture regulate many processes (including microbial activity, organic matter decomposition, and herbaceous and woody plant growth), these responses to burning will tend to accelerate rates of microbial transformations and decomposition of residual forest floor and extend the active growth season for some sprouts and seedlings.

Fire can indirectly lead to soil erosion by removing protective surface covers and altering soil physical properties. The degree of erosion depends on several factors, including:

• soil texture- coarse textured soils erode more readily
• slope- the steeper the slope, the more likelihood of erosion
• vegetation- quick recovery of vegetation can slow erosion
• fire severity- more severe burns increase likelihood of erosion
• litter and duff- residual layers of litter and duff can buffer erosion
• precipitation- high rates of post-burn rainfall increase chances of erosion 

Unlike areas in the Western U.S., where the combination of impaired hydrology, steep topography, and slow vegetative growth make fire-induced soil erosion a serious issue, large soil losses following prescribed fire are uncommon in the southern U.S.  While research consistently indicates that fires do increase erosion rates, the actual rates of soil loss are generally lower than erosion caused by other common forest operations (road building, harvesting, thinning, mechanical site preparation; Yoho 1980).  For example, studies conducted throughout the Coastal Plain and Piedmont reported higher erosion rates from burned areas than nearby unburned areas (Table:Fire Effects on Erosion in the S.E.). However, only one site (North Carolina: Copley et al 1944) showed erosion greatly exceeded natural background levels for the duration of the study (or 1.8 inches per 1,000 years, the estimated erosion rate in central US before large-scale human intervention) (Ralston and Hatcher 1971).  Several additional studies have also reported minimal or no soil erosion following low intensity prescribed fires in the Piedmont and Coastal Plain (Goebel and others 1967, Brender and Cooper 1968, Cushwa and others 1971, Douglass and Van Lear 1983). 

Southern Appalachians, Swift et al. (1993) reported that soil erosion was spotty and related to points of local soil disturbance; no soil left their study sites. The Forest Service in its site-preparation burning program on the Sumter National Forest in the mountains of South Carolina uses summer burns in heavy fuels with little visible evidence of soil erosion on slopes of up to 45 percent (Van Lear and Danielovich 1988). By comparison, soil losses as high as 27.6 tons/acre/year have been reported following intense wildfires in ponderosa pine forests (Campbell et al. 1977). 

Despite the lack of evidence for large soil losses following fires in the South, land managers should still assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects.  Coarse-textured soils on steep slopes may be particularly susceptible to erosion, particularly following intense site preparation burns or wildfires. For more information on this topic, see:

• Management guidelines for reducing fire effects on soil and water, and
• Tools for predicting risk of soil erosion or soil damage following prescribed burns.

Although large soil losses following fires is not as common in the South as in western areas, land managers should still assess the susceptibility of their particular site to soil damage and erosion and follow guidelines below for mitigating these potential effects. These guidelines primarily list steps that can be taken before before planned burns. For information on steps that can be taken after wildfires to minimize soil erosion, see Burned Area Emergency Rehabilitation.

Guidelines for Controlling Soil Heating and Loss of Organic Soil

Fire may cause erosion by removing protective coverings of vegetation, surface litter, duff, and/or root mats. Ensuring that some ground cover remains following fire is one of the most important precautions managers can take to reduce the susceptibility of a burned area to erosion. Fires also contribute to erosion by heat-induced changes to soil physical properties; therefore controlling the amount of soil heating can also mitigate fires’ effects on erosion. The amount of soil heating and/or loss of organic matter caused by prescribed fires can be reduced by controlling the following factors:

• Burning while the moisture content of large diameter fuels, lower duff, and soil is high limits the amount of heat penetration into lower soil surfaces. Moisture not only protects the duff layer adjacent to the soil, but also prevents the fire from consuming soil humus and prevents heat damage to soil.
• Pile burns or windrow burns cause more intense soil heating in localized areas than broadcast burns. Also hand piles tend to burn more quickly than machine piles.
• Burning slash while it is still green with cause less soil heating than burning cured slash.
• Utilizing or yarding more material in areas with heavy dead and down fuel loads can decrease the potential amount of soil heating.
• Rapid ignition techniques (aerial drip torch, for example) can sometimes be used to shorten the duration of the burn and the amount of soil heating.

For guidelines on how to minimize damage to organic soils during burns, see Organic Soils: Management Concerns for Prescribed Burners.

Guidelines for Protecting Riparian Areas during and after Fires

The following precautions should be taken when burning riparian areas near waterways to help to reduce sedimentation and contamination:

• Buffer strips along riparian areas should be left to stabilize slopes and decrease the potential for stream sedimentation. Width of buffer strips should follow applicable agency policy.
• Riparian areas can be burned in spring when conditions are favorable for rapid vegetation recovery.
• Fire retardant chemicals should not be used in or near waterways. Fire retardant has the greatest impact on small or slow moving bodies of water.
• Restrict the location of firelines to gentle slopes along the contour. Avoid firelines that lead directly into watercourses on erosive soils and or steep slopes. If this is unavoidable, install water bars and leadoff ditches at regular intervals.
• Rehabilitate any firelines that were constructed as soon as possible. Replacement of soil and plant material removed during construction is an effective method of fireline rehabilitation.
• Leaving large woody debris and duff can help to protect soil from erosion, and aid nutrient cycling and mycorrihzal function following burns.
• Take extreme precaution when logging slash is burned on steep slopes. Surface runoff and soil erosion may occur before vegetation colonizes sites.

While serious erosion and surface-runoff problems are not as common in the south as they are in the western US, it is still important for managers to assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects. Coarse-textured soils on steep slopes may be particularly susceptible to erosion, particularly following intense site preparation burns or wildfires.

The WEPP model (Water Erosion Prediction Project) can be used to assess the effects of forest management activities on erosion. WEPP is a process based, event-by-event hillslope and watershed erosion model developed as a next generation model to the widely-used Universal soil loss equation. Although it is most often used to predict soil erosion on agricultural and range lands, it has recently been upgraded to predict the impact of forest management activities on soil erosion. The USFS is currently conducting studies that will help to validate the model for burning in southeastern forests.

• Table:Hazard Ratings for Soil Loss Following Prescribed Burns predicts the risk of soil loss based on topography, soil type, and fuel loading.
• Table:Potential Risk for Adverse Soil Effects in the Southern Region predicts risk of soil damage from underburns and clearcuts for different soil conditions in the Coastal Plain, Piedmont, and Appalachians.
• Table:Soil Rating Criteria for Potential Damage to Soil by Fire predicts risk of soil damage from fires based on soil surface thickness, soil texture, slope, and aspect.

Techniques for Monitoring Erosion

1986). al. et (Lyon photogrammetry and 1986), (Lance markers radioactive 1976), (Wright troughs erosion catchments 1983), Warrington (Blaney bridges as such purposes, research appropriate more measuring techniques Other 1979). (McRae pins management used be can inexpensive easy relatively methods these of Some fires. following monitoring variety a>There are a variety of methods available for monitoring soil erosion following fires. Some of these methods are relatively easy and inexpensive and can be used for management purposes, such as erosion pins (McRae et al. 1979). Other techniques for measuring erosion are more appropriate for research purposes, such as soil erosion bridges (Blaney and Warrington 1983), soil catchments or erosion troughs (Wright et al. 1976), radioactive markers (Lance et al. 1986), and photogrammetry (Lyon et al. 1986).

Click on the appropriate table (A, B, or C) to see risk ratings for soils based on surface layer thickness and slope.

Table adopted from NRCS (Natural Resources Conservation Service, USDA) National Forestry Manual 1998.