Fire Consumes Ancient Charcoal and Makes Rest More Vulnerable to Decay

When a forest burns, the fire leaves behind more than ash and scorched trunks. It also produces pyrogenic organic matter, or PyOM, the charcoal-like material created when plant biomass is heated under low-oxygen conditions. Because microbes break it down only slowly, PyOM has long been treated as one of the most persistent reservoirs of carbon in terrestrial ecosystems, locking away carbon for centuries or even millennia. A new laboratory study, published in the journal Biogeochemistry, now shows that this apparent permanence is more fragile than assumed. Subsequent fires, the researchers found, do not merely sit atop old charcoal deposits; they actively remove preexisting PyOM from the ecosystem through two distinct mechanisms: direct combustion of the material itself, and chemical alteration that makes the surviving charcoal far more susceptible to microbial decomposition.

The research team, led by Mengmeng Luo and Thea Whitman of the University of Wisconsin-Madison, together with Kara Yedinak of the USDA Forest Service Northern Research Station and Keith Bourne of the USDA Forest Service Forest Products Laboratory, set out to answer a deceptively simple question: what happens to charcoal that is already in the soil when the next fire arrives? Fire ecology has traditionally focused on how PyOM forms, but the fate of PyOM during reburning has remained poorly quantified. This gap matters for the global carbon cycle, because models of long-term carbon storage often assume that once charcoal enters the soil, it stays there. If reburning erodes that assumption, carbon budgets for fire-prone regions could be significantly misestimated.

To simulate a reburn under controlled conditions, the team used a mass loss calorimeter, an instrument that delivers precisely regulated heat fluxes to samples while allowing researchers to track mass and chemical changes. Their experimental material was PyOM produced from jack pine (Pinus banksiana Lamb.) at 350 degrees Celsius, a charring temperature typical of many wildland fires. The choice of jack pine is ecologically deliberate: it is a boreal and northern conifer whose stands are adapted to frequent stand-replacing fires, making its charcoal a realistic stand-in for the PyOM accumulating in fire-prone North American forests.

The experimental design was fully factorial, crossing three heat flux profiles with three burial depths. The high heat flux profile delivered heating for 4.72 hours with a peak flux of 85 kilowatts per square meter, conditions comparable to intense flaming fronts. The low profile delivered heating for 2.5 hours with a peak of 45 kilowatts per square meter, representing milder reburn conditions. A control profile received no heating. Each thermal treatment was applied to PyOM placed at the surface, buried at 1 centimeter, or buried at 5 centimeters, reflecting the natural stratification of charcoal in forest soils, where some fragments lie exposed on the litter layer while others sit deeper in the mineral horizon.

The first headline result was straightforward: both heat flux and burial depth significantly affected the amount of carbon remaining after the simulated reburn, with statistical significance at p less than 0.05. Carbon losses were greatest at higher heat fluxes and at shallower exposure depths. In other words, charcoal sitting on the soil surface during an intense reburn was substantially consumed, while buried charcoal was partially shielded by the insulating properties of the overlying soil. This direct combustion pathway confirms that fire is not only a producer of PyOM but also a destroyer of it, a feedback that carbon cycle models have rarely captured explicitly.

The second mechanism proved more surprising. The researchers predicted that treatments with high heat exposure, specifically the combinations of high heat flux with surface placement, high heat flux with 1-centimeter burial, and low heat flux with surface placement, would show characteristic post-fire chemistry. That prediction held: these treatments showed increases in pH and decreases in both total dissolved organic carbon and mineralized carbon after the simulated reburn. Intense heating appears to strip away the more labile, soluble fractions of the charcoal, leaving behind a more refractory residue that microbes can access less readily. Under these conditions, reburning actually re-concentrates the persistent character of PyOM.

But the intermediate treatments defied expectations. When PyOM experienced conditions that were intense enough to chemically alter it but not intense enough to fully combust it, namely high heat flux at 5-centimeter depth and low heat flux at 1-centimeter depth, the researchers observed decreases in pH alongside increases in dissolved organic carbon, mineralized carbon, modelled decomposable carbon, and the modelled carbon decomposition rate, all significant at p less than 0.05 relative to unburned controls. In plain terms, moderate reburning made the surviving charcoal more edible to microbes. The fire appears to crack open the aromatic structures and release soluble, easily decomposed compounds, converting a fraction of what was once stable carbon into material that can be mineralized to carbon dioxide on much shorter timescales.

This dual mechanism, combustion plus increasing mineralizability, has profound implications for how scientists account for carbon in a world of changing fire regimes. As climate warming lengthens fire seasons and increases burn frequency in boreal and temperate forests, the interval between fires is shrinking. Charcoal that once might have accumulated over centuries of fire-free conditions now faces reburning within decades or less. The new findings suggest that each reburn chips away at the PyOM pool twice over: once by burning a portion of it directly into the atmosphere, and again by priming the remainder for faster microbial decay. The net effect is that PyOM may contribute less to long-term carbon storage than previously believed, particularly in regions where reburning is becoming the norm rather than the exception.

The study also refines our understanding of the chemistry involved. The opposing responses of pH and dissolved organic carbon between the high-exposure and intermediate treatments indicate that PyOM does not respond to fire as a single homogeneous substance. Its surface layers, soluble fractions, and mineral-associated components respond differently depending on the thermal dose received. The calorimeter approach allowed the team to disentangle these responses with a precision that field studies, where heat fluxes and soil moisture vary unpredictably, cannot easily achieve. By modelling decomposable carbon pools and decomposition rates from their measurements, the researchers could translate raw chemical changes into estimates of microbial accessibility, connecting fire physics to soil biology.

For the global carbon cycle, the message is clear: persistence is not a permanent property of charcoal, but a conditional one. PyOM remains highly persistent in fire-free conditions, yet it can be readily combusted and chemically altered in subsequent fires. As fire regimes intensify across the boreal zone and beyond, the stability of one of the planet’s most durable carbon reservoirs will depend not just on how much charcoal fires create, but on how often fires return to consume and destabilize what previous burns left behind. The charcoal in the ground, it turns out, is only as permanent as the next fire allows it to be.

Subject of Research: Biogeochemistry

Article Title: Fire removes preexisting pyrogenic organic matter from the ecosystem through the mechanisms of both direct combustion and increasing mineralizability

Article References: Luo, M., Yedinak, K., Bourne, K., & Whitman, T. (2026). Fire removes preexisting pyrogenic organic matter from the ecosystem through the mechanisms of both direct combustion and increasing mineralizability. Biogeochemistry. https://doi.org/10.1007/s10533-026-01380-6

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01380-6

Keywords: pyrogenic organic matter, carbon cycle, fire ecology, reburning, soil carbon, mineralizability, biogeochemistry, Fire, removes, preexisting, pyrogenic, organic