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What Fire Does to Soil Microbes

By Steven Shafer, Ph.D., Soil Health Institute interim chief scientific officer and retired soil microbiologist

Posted Oct. 11, 2018

Fire affects many important ecosystem processes. Much of what we understand about the impact of fire on terrestrial ecosystems comes from many decades of research on the effects of forest and prairie fires on plant communities and succession, nutrient cycling, erosion, and soil properties.

Soil itself is a complex ecosystem that supports all living things above ground. Soils also host an incredible diversity of bacteria, fungi and other microbes that are affected by various factors such as soil nutrients, seasonal changes, drought, pH, chemical applications, plant species and farming practices. Although many microbes are adapted to high-temperature environments (we’re all fascinated by reports of weird microbes growing right at the edges of geysers and undersea vents), no physiologically active microorganism can survive fire.

However, we’ve learned that fire is a powerful regenerating force. This is why prescribed burns are useful management tools in forests and rangelands to clear out old growth, stimulate new growth and recycle nutrients.

We can see benefits of fire on plant communities that rebound from dormant seeds and surviving roots, but there are pluses for the unseen as well. The microorganisms that are killed by fire near the soil surface, where temperatures are greatest during a fire, become food for the survivors that escape by living deeper down or by being protected within the occasional soil aggregate (bound-up clusters of soil particles that may encase microbes and protect them from high temperatures).

The duration and intensity of the peak temperature is what affects the soil organisms most. If it gets hot enough, the soil can actually be sterilized, at least in the top few inches. Emissions of carbon dioxide normally rising from the soil microbes may be briefly suppressed due to the reduced population.

Some of the most sensitive microbes are the mycorrhizal fungi, which are adapted to a symbiotic (literally “living together”) relationship with plant roots. The fungi may be reduced near the soil surface, where new plant roots eventually take hold. Those plants may struggle at first, in the absence of these fungi that aid in nutrient uptake. But the microbe population will recover once the heated soil cools down and microbial cells and spores re-enter on wind and in water. Cycling of nutrients ramps up fast, and their availability to newly emerging plants allows recolonization to begin.

Fire is a disturbance in nature, but life is strangely hard to deny. Individual types of soil microorganisms may be extremely sensitive to the conditions created by fire and its immediate aftermath, but as a group microorganisms are the most successful on the planet. Once they begin the recovery process, the plants and animals are soon to follow.

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