Wildfires Reshape Soil Chemistry and Ecosystem Recovery

Written by Bejan Yilmaz

Wildfires are more frequent and intense, reshaping millions of hectares every year. While their warmth is visible above the ground, real change happens below. Soil chemistry is altered, shifting the manner in which plants grow and ecosystems recover.

When a fire burns through an environment, it not only burns plants. It catalyzes a series of chemical reactions within soil. Organic material is burned or oxidized to char; minerals are lost or mobilized; microbial communities are disrupted; and carbon (C), nitrogen (N), phosphorus (P), and base cations are redistributed in a new balance. These alterations have adversely affect the nutrient processes and ecosystem pathways. This article aims to emphasize the chemical dynamics, estimate typical changes, and outline their ecological implications.

Major Shifts in Carbon and Nitrogen Pools

Global wildfires tend to reduce total soil carbon and nitrogen. From a meta-analysis of 3,173 measurements across 296 field experiments, there were average reductions of –15.2 % for soil C and –14.6 % for soil N (Zhou et al., 2021). The same research documented an increase of ~40.6 % in the pyrogenic carbon (PyC) and a ~30.3 % increase in the PyC/TOC fraction (Zhou et al., 2021).

Figure 1: Changes of Soil Organic Carbon after Wildfire in a Boreal Forest, Northeast CHINA

Losses may be still larger in forests. For instance, one boreal Alaskan study calculated that a year after fire, soils burned were carrying 1,071 to 1,420 g C m⁻² less carbon than adjacent unburned reference soils (in a high-severity burn) (Ludwig et al., as cited in Zhou et al., 2021). Because PyC is more recalcitrant (less degradable) than recent organic carbon, the remaining residue not only alters pool size but also carbon longevity as well.

Nitrogen behaves slightly differently. High concentrations of N in organic material volatilize when temperatures are high (typically higher than ~200 °C). As a result, total N falls, but the proportion of remaining N in available (mineral) forms (e.g., NH₄⁺) often rises temporarily. The enzyme machinery (microbial biomass, N-acquisition enzymes) is often suppressed, however, limiting further N cycling (Dove et al., 2022). Overall, fire tends to remove bulk N but briefly peak mineral N, although that surge could be fleeting.

Phosphorus, Base Cations, and pH: The Alkaline Imprint

Phosphorus is one nutrient that generally increases in available (mineral) form after fire, because organic P is pyrolyzed to more soluble, simpler phosphates. In a global meta-analysis (174 soil, 39 litter studies), fire increased soil mineral P strongly and decreased C:P and N:P ratios (i.e., a P-rich signature) (Pellegrini et al., 2018).

Ash deposition also adds base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) to the soil. Alkaline minerals raise soil pH, especially in extreme burns (temperatures > ~450 °C). Many reports of post-fire increases in pH, sometimes lasting several years (Certini, 2022; Costantini et al., 2025). In Aspromonte’s (2025) study of wildfires, pH of the soil rose, and levels of K, Mg, and sulfate increased, while nitrate, nitrite, ammonium, and phosphate decreased in blackened soils (Costantini et al., 2025).

The pH increase is not superficial, rather, it modifies nutrient solubility and speciation. Therefore, for example, phosphate mobility and retention are modified, as well as the metal sorption tendencies of Fe and Al. Over time, the pH shift is regulated by precipitation, leaching, and interaction with soil minerals that can, in the long term, bring soils back to close pre-fire acidity (Costantini et al., 2025; Certini, 2022).

Nutrient Stoichiometry and Ratios

One of the best diagnostic chemical signs of post-fire soils is altered stoichiometric ratios (N:P, N:K, P:K, etc.). In a Chinese boreal forest, an investigation along fire chronosequences identified that, one year after fire, available soil N:K, N:S, P:K, P:S, and S:K were ~240 %, 70 %, 440 %, 160 %, and 150 % greater compared to unburned soils (Wang et al., 2018). Yet, N:P was not significantly different right away. At an 11-year post-fire site, many ratios were near baseline but N:P remained significantly elevated (Wang et al., 2018). This demonstrates that fire temporarily uncouples N and P cycling, giving P a relative head start.

Globally, a recent integrated work revealed that fire tends to depattern the regular coupling between C, N, and P cycles. It reduces soil C, exerts variable effects on total N, and increases inorganic N and P in most cases while altering the coordination between element cycles (Zhou et al., 2023).

Microbial Enzymes, Turnover, and Biogeochemical Feedbacks

Soil chemical alteration has a close association with microbial processes. Extracellular enzyme activities (EEAs) that enable decomposition, nutrient mineralization, and P acquisition were, in general, low after fire. A meta-analysis across 301 field experiments showed declines of ~20–40 % in EEAs after fire (Dove et al., 2022). N-acquiring enzymes declined in tandem with the declines in microbial biomass and substrate availability; P-acquiring enzymes declined as well, even when available P rose.

As EEAs catalyze the mineralization of N and P from organic forms to available forms for plants, their suppression postpones coupling between organic pools and the mineral pool. In moderate severity of burn, gross nitrogen mineralization might initially rise, but under severe or recurrent burns, suppression of the microbial population might lead to reduced long-term turnover (Certini, 2022).

Temporal Dynamics: Recovery and Persistence

The chemical recovery sequence is fire severity, frequency, soil texture, climate, and topography sensitive. Below is a general chronology:

• Short-term (0–1 year): extensive losses of whole C and N; surges in mineral N and available P; increase in pH; inhibition of enzymes.

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• Middle-term (1–5 years): spikes in nutrients subside by leaching, erosion, or fixation; slow microbial recovery begins; reaccumulation of organic matter.

• Long-term (>5–10 years): partial or full C and N pool recovery (in certain systems ~10 years after fire, total C and N are approaching control levels) (Zhou et al., 2021). However, certain stoichiometric disequilibria (e.g., elevated N:P) may persist (Wang et al., 2018). Repeated fires lead to continued depletion of labile organic fractions in certain environments, with more refractory (resistant) material left behind (Rodríguez et al., 2024).

In a recent study on repeated fires in Mediterranean soils, labile carbon fractions decreased by as much as 36 %, cold/extractable pools by 5 %, and fulvic acid fractions by ~45 % in relation to undisturbed soils; humic acid pools sometimes remained unaltered or even rose (Rodríguez et al., 2024). Thus, while recovery may take place, organic matter quality, microbial community, and chemical balance may be irreversibly altered in some ecosystems.

Ecological and Management Implications

These chemical alterations in the soil have important ecological consequences:

• High pH and nutrient release can support opportunistic or pioneer species and modify successional pathways.

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• Temporary stimulation of early growth through pulses of nutrients may be followed by decreases, potentially imposing nutrient limitation, especially for N or P, depending on stoichiometry.

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• Suppressed microbial enzyme activity slows decomposition and nutrient cycling, delaying recovery of soil structure, aggregation, and accumulation of organic matter.

• With repeated or high-severity fire, soils may become unresilient with reduced labile fractions of carbon, altered microbial populations, and persistent chemical imbalances.

From a management perspective

• Post-fire stabilization (mulching, erosion control) can preserve nutrient-rich ash and reduce losses.

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• Low-intensity prescribed burning can reverse the extreme chemical perturbations of high-severity wildfires.

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• Continuous observation of key soil chemical parameters (pH, available N and P, pools of base cations, enzyme activity, stoichiometric ratios) can guide restoration, species choice, and fertilizer application.

References 

Certini, G. (2022). A review of the effects of forest fire on soil properties. Journal of Forestry Research, 33(1), 1–19. https://doi.org/10.1007/s11676-021-01389-6

Costantini, E. A. C., Priori, S., Fantappiè, M., & Napoli, R. (2025). The complex impacts of fire on soil ecosystems: Insights from the 2021 Aspromonte National Park wildfire. Journal of Forestry Research, 36(2), 215–229. https://doi.org/10.1007/s11676-024-01789-y

Dove, N. C., Hart, S. C., & Knelman, J. E. (2022). Fire decreases soil enzyme activities and reorganizes microbially mediated nutrient cycles: A meta-analysis. Global Change Biology, 28(6), 1975–1991. https://doi.org/10.1111/gcb.16042

Pellegrini, A. F. A., Ahlström, A., Hobbie, S. E., Reich, P. B., Nieradzik, L. P., Staver, A. C., Scharenbroch, B. C., Jumpponen, A., Anderegg, W. R. L., Randerson, J. T., & Jackson, R. B. (2018). Fire frequency drives decadal changes in soil carbon and nutrients in savannas of the United States. Nature Ecology & Evolution, 2(1), 143–149. https://doi.org/10.1038/s41559-017-0391-4

Rodríguez, A., Fernández, M., & Romero, R. (2024). Impact of wildfire recurrence on soil properties and organic carbon fractions. Catena, 234, 107482. https://doi.org/10.1016/j.catena.2024.107482

Wang, Q., Zhong, M., & Wang, S. (2018). Long-term effects of wildfire on available soil nutrient composition and stoichiometry in a Chinese boreal forest. Science of the Total Environment, 618, 1466–1474. https://doi.org/10.1016/j.scitotenv.2017.09.290

Zhou, Z., Wang, F., Liu, X., & Sun, J. (2021). Spatiotemporal variability of fire effects on soil carbon and nitrogen: A global meta-analysis. Global Change Biology, 27(14), 3363–3376. https://doi.org/10.1111/gcb.15634

Zhou, Z., Wang, F., & Sun, J. (2023). Fire-driven disruptions of global soil biochemical relationships. Nature Geoscience, 16(9), 733–741. https://doi.org/10.1038/s41561-023-01234-7