Fire can impact the soil either directly or indirectly. Soil impacts include physical, chemical or biological changes. Direct impacts include the combustion of organic matter. Indirect impacts result from changes in the ecosystem such as vegetation cover or changes in flora. Impacts from fire can be short-term, long-term or permanent depending on fire severity.
For example, soil fertility can increase after low intensity fires, which increases site productivity. Fires do this by restoring soil nutrients through the release of elements such as nitrogen, sulfur, phosphorus and carbon. However, high intensity fires can change the physical characteristics and make them more susceptible to nutrient loss due to erosion.
If fires are hot enough, they can kill microorganisms and partially sterilize the soil. Severe fires can increase soil density, reduce porosity and permanently alter soil texture. Due to decreased water infiltration rates and water storage capacity, damaged soils thereby accelerate run off and erosion.
This can lead to increased sedimentation , which can negatively impact water supply and reduce reservoir storage. Post-fire, soils can become water-repellent, which adds to increased runoff rates.
To assess fire severity and water repellency, residents can do an easy DIY water drop penetration test page 10 of this publication. For winegrowers, a study showed that burned soils leached more dissolved organic carbon DOC concentrations, but less sulfur. The results are available in this abstract and suggest that winegrowers may need to amend soils with organic carbon such as cover crops when replanting burned vineyards. Soils vary from near zero organic matter content to as much as half organic matter or more, in some cases.
In that case, soil generally smolders, which means it is burning but only very slowly, because not much air can get to the organic material in the soil. Mostly because it is full of water and non-burnable minerals. Dirt is made of many different things. It gets dry as the water in it evaporates. If it gets above about C it starts glowing red.
Reports on the effects of fire on soil N pools have been controversial, both because of the importance of N as it affects site productivity and because of its complicated response. 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 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.
Nutrient 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.
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 to detail the effects of fire on plants.
The 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. 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 is dependent to a large extent upon fire intensity. The responses of soil microbes to fires range from no detectable effect in low intensity fires to total sterilization of the surface layers of soil in very hot wildfires see Joergensen and Hodges ; and Renbuss et al.
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.
These authors found that the increased rates of microbial processes, such as denitrification and production of methane and carbon dioxide, persisted for one year following fire. Microinvertebrates and Fire There is a general reduction of microarthropods with increasing fire frequency. Similar studies have shown that reduction of litter mass with prescribed fire generally results in reductions of microarthropod numbers Dress and Boerner ; Brand The 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.
The consequences of these reductions for the decomposition of new leaf litter have not been thoroughly addressed. Macroinvertebrates and Fire The general pattern of macroinvertebrate responses to fire is often driven by changes in habitat structure, or by changes in the amount or the quality of food resources. Whenever fire affects vegetation, temperature or moisture, or the nutrient status of a soil, there is potential for impact on the soil invertebrate community.
Some arthropod groups increased in abundance but most decreased soon after fire. A study of litter dwelling and soil dwelling macroinvertebrates showed that the density of macroinvertebrates was significantly reduced one year after a prescribed fire Kalisz and Powell 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.
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