Abstract

Soil microbiology is crucial for soil system functioning. Fire can affect soil microbes directly through heating and indirectly by modifying soil properties. Microbes will also be affected by post-fire environmental factors and the reestablishment of vegetation. The most important factor affecting soil microbes seems to be the burn severity, which is controlled by such factors as fire intensity, duration, and soil properties which normally causes a decrease in the numbers of microbes. The temperatures reached in the topsoil are often sufficient to affect soil microorganisms and other soil properties related to the post-fire microbial recolonization. In extreme cases, the topsoil can undergo complete sterilization. Fungi seem to be more sensitive to heating than bacteria and actinomycetes, and a higher impact under wet soil conditions has been reported. In the case of fungi that form arbuscular mycorrhizas, almost all the studies show a negative influence resulting in a reduced number of propagules. An important factor is the presence of fungal resistant structures, such as sclerotia, from which new mycelia originate to colonize new plants. The activity of soil microorganisms also decreases due to changes in the quality of organic matter. In the shortterm, mainly due to the increase in soluble carbon and nutrients in affected soils, an increase in heterotrophic bacteria population basal respiration is commonly observed. After depletion of the easily mineralized organic compounds, this initial increase in microbial basal respiration is generally followed by a decrease as the remaining carbon and nitrogen forms are more recalcitrant to microbial attack. Some other changes in soil properties such as increase in pH (due to ash deposition) have been argued as the cause of the post-fire increase in the bacteria/fungi ratio. As pioneer species, a rapid recolonization of soil by photoautotroph microorganism (such as algae) has been reported after moderate and high intensity fires. The diversity of microorganisms can be modified by fire in several ways as a consequence of differences in heat sensitivity, survival strategies, colonization mechanisms, and sensitivity to soil and microclimate changes. Destruction and creation of new ecological niches and changes in biomass and composition of aboveground vegetal species can also impact microogranism diversity.