Fire’s Hidden Carbon: Labile Sugars Race Ahead of Black Carbon After Wildfire

When wildfire sweeps through a forest, it does far more than scorch trees and darken soils. It fundamentally rewrites the chemistry of carbon moving through the landscape, and a new study from coastal California suggests that scientists have been paying attention to only half the story. Research published in the journal Biogeochemistry reveals that in the aftermath of the 2020 CZU Lightning Complex Fires, two very different classes of fire-derived carbon behaved in strikingly different ways as they traveled from burned hillsides into streams, lagoons, and ultimately the nearshore ocean. The findings challenge a long-standing emphasis on the most persistent forms of pyrogenic carbon and highlight the overlooked importance of compounds that vanish almost as quickly as they appear.

The study, led by Riley Barton of the Marine and Coastal Research Laboratory at Pacific Northwest National Laboratory and Rensselaer Polytechnic Institute, together with Sasha Wagner, Christina M. Richardson, Alex Collins, Margaret A. Zimmer, Maya Montalvo, and Allison N. Myers-Pigg, focused on five small coastal mountain watersheds in California that were burned by the CZU Lightning Complex Fires of 2020. That fire event, ignited by an extraordinary lightning storm, tore through the Santa Cruz Mountains and left steep coastal drainages draped in ash and charred vegetation. Because these watersheds drain directly toward the Pacific, they offered an ideal natural laboratory for tracking how fire-altered carbon moves from land to sea.

The researchers zeroed in on a molecule called levoglucosan, an anhydrosugar that forms when cellulose and hemicellulose, the structural carbohydrates of plant tissue, are burned at moderate temperatures. Levoglucosan has long served as a molecular fingerprint for biomass burning in atmospheric science, because it is produced in abundance during combustion and can travel long distances in smoke. But in aquatic systems, levoglucosan has a very different personality: it is highly soluble, which means it dissolves readily in the first rains after a fire, and it is biolabile, meaning microbes can consume it quickly as a carbon and energy source. In other words, levoglucosan is the ephemeral, fast-cycling counterpart to the stubborn, centuries-persistent black carbon that has dominated discussions of fire-derived carbon in rivers.

Black carbon, by contrast, is the charred, condensed aromatic material that gives burned soils their dark color. It is famously recalcitrant, resisting microbial decay and persisting in soils, sediments, and the open ocean for hundreds to thousands of years. Because of this stability, black carbon has been treated as the primary long-term sink in the fire-carbon equation, and much of the aquatic research on pyrogenic carbon, often abbreviated PyC, has centered on quantifying these durable fractions. The new study asked a deceptively simple question: if you measure both the labile and the recalcitrant fire-derived carbon in the same streams at the same time, do they behave the same way?

The answer, emphatically, is no. Levoglucosan concentrations in the streams peaked during the earliest post-fire flows, the very first storm-driven runoff events after the flames were out, and then declined steadily over time. Remarkably, this decline was independent of stream discharge. It did not matter how much water was coursing through the channels; the levoglucosan signal faded because the supply was being exhausted, flushed out of the burned landscape during those initial pulses and consumed by microbes along the way. Black carbon told the opposite story. Its concentrations were discharge-driven, rising when flows rose, and, counterintuitively, they increased with time since the fire. The researchers attribute this delayed pattern to gradual solubilization: the charred material left on the landscape does not dissolve immediately but slowly becomes available to water over months and years as physical and chemical weathering break it down.

This divergence matters because it means that the two major categories of pyrogenic carbon operate on fundamentally different clocks. Labile anhydrosugars are a flash flood of bioavailable carbon, delivered within the first post-fire storm seasons and efficiently transported all the way to coastal lagoons and nearshore zones. Black carbon is a slow-release reservoir, accumulating in export pathways over longer timescales. A monitoring program that samples a burned watershed a year or two after a fire, or one that focuses only on the recalcitrant fraction, would completely miss the early, intense pulse of labile carbon, and with it, a significant chapter in the post-fire carbon budget.

The downstream consequences could be substantial. When levoglucosan and other soluble, easily degraded combustion products arrive in coastal lagoons and nearshore waters, they provide a fresh fuel source for microbial communities. Enhanced microbial processing means enhanced remineralization, the conversion of organic carbon back into carbon dioxide, potentially altering the carbon balance of these transitional ecosystems and influencing oxygen demand, nutrient cycling, and food web dynamics. Coastal lagoons along the California coast are already sensitive environments, subject to seasonal sandbar closure, variable salinity, and increasing fire pressure in their upstream catchments. A sudden influx of labile pyrogenic carbon could ripple through these systems in ways that measurements of black carbon alone would never reveal.

The broader implication is that wildfire’s impact on the global carbon cycle may be underestimated. Wildfire frequency and severity are increasing across the western United States and many other regions of the world, and each fire leaves behind a spectrum of organic matter ranging from the environmentally persistent to the rapidly degradable. If carbon accounting and biogeochemical models focus on the recalcitrant end of that spectrum, they will capture the long-term storage story but miss the fast-cycling one. The authors argue that expanding the focus beyond recalcitrant pyrogenic carbon to include labile compounds is essential to capture the full impact of terrestrial fires on carbon dynamics across the land-ocean aquatic continuum, the connected chain of rivers, lakes, estuaries, and coastal waters through which carbon travels from continents to the sea.

Methodologically, the study demonstrates the value of pairing molecular tracers with hydrologic context. By measuring levoglucosan and black carbon side by side across multiple burned watersheds and multiple flow conditions, the team could disentangle the effects of supply exhaustion from the effects of dilution and transport. The independence of the levoglucosan decline from discharge is a particularly telling signature: it points to a finite, depletable pool of soluble combustion products rather than a continuous source regulated by water flux. Such distinctions are exactly what models of post-fire carbon export need in order to predict how landscapes will behave as fire regimes intensify.

The research, supported by the National Science Foundation and the U.S. Department of Energy’s Office of Science, and conducted as part of Barton’s doctoral dissertation, arrives at a moment when fire science is racing to keep pace with a changing fire regime. As more watersheds burn and more rain falls on fresh ash, the first flush of labile carbon into rivers and coasts may prove to be one of the most dynamic and underappreciated consequences of wildfire. Recognizing that not all fire-derived carbon behaves alike, and that the most fragile molecules may carry the biggest early signal, is a crucial step toward understanding what burning landscapes mean for the carbon cycle from mountaintop to ocean.

Subject of Research: Post-fire mobilization of levoglucosan and black carbon in coastal California watersheds

Article Title: Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds

Article References: Barton, R., Wagner, S., Richardson, C. M., Collins, A., Zimmer, M. A., Montalvo, M., & Myers-Pigg, A. N. (2026). Early post-fire mobilization of levoglucosan distinctly contrasts black carbon dynamics in small coastal mountain watersheds. Biogeochemistry. https://doi.org/10.1007/s10533-026-01367-3

Image Credits: AI Generated

DOI: 10.1007/s10533-026-01367-3

Keywords: levoglucosan, black carbon, pyrogenic carbon, wildfire, watersheds, biogeochemistry, carbon cycle, anhydrosugar, coastal lagoons, organic carbon, CZU Lightning Complex Fire, carbon export