A forest fire sweeps everything in its path. It drastically alters the soil environment, causing extreme temperatures, reduced moisture, spikes in pH, the release of inorganic nitrogen, a sharp decline in organic matter and disrupted nutrient cycles. Researchers have examined how wildfire smoke can carry microbes through the atmosphere, but what about the microbial life that persists beneath the charred surface-the microbes adapted to withstand extremes through successive stages of soil recovery?
When plant and soil organic matter are partially burned during wildfires, pyrogenic organic matter-a stable form of carbon, sometimes known as black carbon or charcoal-is left behind. This marks the beginning of ecological succession-a slow, complex process through which ecosystems recover and rebuild. The affected areas, or successional sites, experience distinct developmental stages as microbial and plant communities re-establish themselves. These tiny organisms do not simply endure; they pave the way for nutrient cycling and soil stabilization.
Exploring the adaptive strategies and plant interactions of pyrophilous microbes-those selectively enriched after wildfires due to their metabolic ingenuity-can reveal how these organisms drive ecosystem resilience and soil recovery. And understanding such dynamics across stages of succession can help inform targeted strategies for rehabilitating fire-affected areas.
Microbial Strategies and Ecosystem Renewal in Early Succession
Cyst-Like Resting Cells
The pioneer phase, or the initial stage of ecological succession, begins when microbial life starts re-establishing itself in the burned soil environment. Early in succession, fast-growing microbial species adapted to the harsh conditions created by fire find an opportunity to thrive. Among these tiny warriors are members of the phylum Actinobacteria. These organisms develop "cyst-like" resting cells, which help them thrive in nutrient-poor conditions and resist the high heat, drought and oxidative damage associated with wildfires. In a burned holm oak forest in Spain, researchers analyzed soil microbial community composition through 16S rRNA amplicon sequencing. They found that over 21% of the soil microbes were Arthrobacter, a resilient species from the Actinobacteria family.
Stress Tolerance Genes
Scientists also studied metagenome-assembled genomes of early successional bacteria in Eldorado National Forest, Calif., and found that they encoded stress tolerance genes involved in ectoine and mycothiol biosynthesis. Ectoine protects against osmotic stress and protects DNA from ionizing radiation. Mycothiol acts as a glutathione-like antioxidant, protecting the cells from oxidative stress after forest fires. Genes related to the SigmaB stress regulon, which protects against salt, heat and osmotic stresses, were also prevalent in early successional communities.
Metabolic Cooperation
As they fight to survive by activating stress response genes, the microbes also begin a desperate search for food. In early successional stages, the soil experiences a spike in pyrogenic organic matter, p-hydroxybenzoate and n-phenylalkanoic acid. Microbial taxa, including pyrophilic fungi, like Pyronema, and bacteria, like Massilia and Noviherbaspirillum, with the metabolic capacity to degrade these fire-derived substrates are naturally selected during this phase.
After a fire, cellulose-the main structural component of plant cells-may still be available in the short term because it is part of plant residues that can partially survive burning. The scarcity of organic carbon promotes the natural selection of microbes that produce glycoside hydrolases, which break down complex carbon compounds, like cellulose and lignin, into simpler forms for energy. Lignin-another, more complex compound in plant cells-requires more energy to break down. In deeper soil layers, members of the family Streptosporangiaceae metabolize lignin-derived fire byproducts such as protocatechuate.
In contrast, surface soil layers are enriched with catechol, another byproduct of lignin combustion. Arthrobacter metabolizes catechol effectively, giving it a competitive advantage and allowing it to dominate in these upper soil layers. Catechol and protocatechuate are metabolized into succinyl-CoA and acetyl-CoA, which feed into the citric acid cycle. The citric acid cycle generates energy and mobilizes nutrients for microbial growth. However, it is interesting to note that none of the microbes in the post-fire studies have the genetic makeup to independently and completely degrade catechol or protocatechuate. This indicates that metabolic cooperation among different bacterial community members is essential for the complete breakdown of organic carbon compounds and soil rehabilitation after wildfires. It reflects a consistent ecological principle: soil chemistry and microbial community dynamics are tightly interwoven.
The Role of Soil Microbes in Promoting Plant Growth
Arthrobacter species are not only efficient degraders of complex organic compounds commonly found in post-wildfire soils, but they also possess plant growth-promoting traits. In laboratory experiments using sterilized or agricultural soil, alfalfa and pepper inoculated with Arthrobacter show a 40% increase in growth compared to plants left to fend for themselves. What makes Arthrobacter such a versatile ally? A number of key traits:
- Capacity to produce cellulose that helps the bacteria to anchor to the roots.
- Ability to make phosphate more accessible to plants.
- Production of the natural growth hormone auxin.
- Synthesis of siderophores, which help plants access iron in nutrient-poor soils.
Notably, because plant growth promotion was observed in 2 agronomically unrelated plant species, scientists are now exploring whether the beneficial effects of Arthrobacter can be extrapolated to a wider range of plants.
Nitrogen Cycling Dominates During Early Succession
Gradually, as the soil recovers, factors such as the elevated pH begin to normalize, creating conditions that favor the growth of specific microbes and plants. These organisms often share traits that confer stress tolerance-such as the production of ectoine, mycothiol and activation of the SigmaB stress response-making them well-suited to colonize and stabilize post-disturbance environments. Herbaceous and nitrogen-fixing plants like Ceanothus and Chambaethia represent early plant colonizers. They partner with arbuscular mycorrhizae, scavenging phosphorus and other nutrients to grow stronger and faster.
In the early stages of post-fire succession, nitrification rates are high, meaning that NH4+ is rapidly converted to NO3-. This elevated nitrification is primarily driven by 3 factors: (a) fire-adapted, nitrogen-fixing shrubs colonize the affected area rapidly and add extra nitrogen to the soil, (b) limited nitrogen uptake by plants due to sparse vegetation and (c) reduced microbial immobilization of ammonium due to low organic carbon availability.
Nitrogen fixation is 2.7 times higher in the first few decades after fire compared to later successional stages. At the same time, denitrification remains relatively low because the microbial processes that drive it are constrained by both insufficient carbon-needed as an energy source-and high oxygen availability, which is unfavorable for the anaerobic microbes responsible for denitrification.
Mid-Successional Stages: Toward Building Stable Post-Fire Ecosystems
Over time, as plants grow back and organic material accumulates, carbon becomes more available, soil parameters stabilize, and microbes shift to different metabolic strategies. Microbial growth rates reach the maximal rate in the mid-successional phase, supported by increased soil carbon and nutrient availability. Over time, Proteobacteria and Acidobacteria became more common, while Actinobacteria-which were more abundant shortly after the fire-gradually decline. Nitrification rates begin to decline. This shift occurs as plant cover increases.
Simultaneously, organic carbon levels begin to recover, supporting the growth and activity of heterotrophic microbes. With more carbon available, microbes are better able to immobilize ammonium, incorporating it into their biomass rather than leaving it available for conversion to NO3-. As a result, there is less nitrogen available for denitrification. Trees gradually start succeeding the pioneer shrubs and herbs. They can flourish in the symbiotic association of ectomycorrhizal fungi that become more abundant and diverse, thus, heralding the transition toward more stable soil ecosystems.
Late Succession: Tree-Fungi Partnerships and Microbial Competition Shape Stable Forest Soils
The community composition shifts from herbaceous and N-fixing shrubs, like Ceanothus spp., which form symbioses with arbuscular mycorrhizae, to trees like Quercus and Pinus, which form symbioses with ectomycorrhizal fungi. The changes in mycorrhizal dominance likely reflect post-fire vegetation succession. They promote establishment and growth of trees by enhancing water and nutrient uptake. This results in a feedback loop where the recovery of plants and microbes support one another.
In contrast to early phases, microbes dominant in advanced successional stages, such as Mycobacterium abscessus, have lesser abundance of stress-response genes and a greater prevalence of genes (like hfl operon) that are essential for microbial competition (e.g., those that confer resistance to antibiotics).
Plant growth begins to slow, largely due to increasing limitations in light and moisture availability within the maturing ecosystem. As plant nitrogen uptake declines, more NH4+ remains available in the soil. This renewed availability of NH4+ allows nitrification rates to increase once again. Concurrently, microbial communities continue to adjust and diversify, supporting higher levels of denitrification. With both nitrification and denitrification processes operating in tandem, the system achieves a more balanced nitrogen cycle, characterized by stable and relatively low levels of NO3- in the soil.
Harnessing Plant-Microbe-Soil Interactions for Post-Fire Ecosystem Restoration
Defining the plants-microbes-soil interplay across the successional phases opens up possibilities for post-fire ecosystem restoration. The initial carbon limitation can be addressed through methodical organic amendments-such as compost or leaf litter-and thus can support faster microbial recovery. Further, the adaptive strategies of microbes-in early stress-tolerant phases and later symbiotic partnerships-can help inform the design of bioinoculants tailored to wildfire ecological contexts.
One key approach is the reintroduction of lost pioneers and mutualists that are critical for the survival and nutrient uptake of native plants. For example, re-establishment of obligate ectomycorrhizal (EM) host plants like Pinus contorta can be supported by commercial inoculants containing EM symbionts, Cenoccum, Pisolithus, Rhizopogon. Using no-till methods, like plug planting or seed broadcasting, can ensure that delicate underground networks of fungal hyphae are not disturbed. To further enhance effectivity of commercial inoculants, host-specific formulations can be used. Once fast-growing native plant species, especially nitrogen-fixing species like Lupinus or Acacia, are established, it can stimulate microbial activity and restore soil fertility.
Learn more about the microbiology of wildfires and how they affect the microbial ecosphere, from soil communities to the spread of species through bioaerosols.
