Fire Effects on Soils- Overview
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).
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.
Encyclopedia ID: p681
