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

Wildfires release a spectrum of organic matter, from environmentally persistent black carbon to rapidly degradable anhydrosugars. Levoglucosan—a soluble, biolabile anhydrosugar formed from cellulose and hemicellulose combustion—was measured across five coastal California watersheds impacted by the 2020 CZU Lightning Complex Fires. Levoglucosan concentrations peaked during the earliest post-fire flows and declined over time, independent of stream discharge. This behavior contrasts with black carbon, which was discharge-driven and increased with time-since-fire due to delayed solubilization. These findings reveal that not all pyrogenic carbon (PyC) behaves alike: labile compounds can dominate early post-fire fluxes and are efficiently transported to coastal lagoons and nearshore zones, potentially enhancing microbial processing and organic matter remineralization in downstream systems. Ignoring these fast-cycling fractions risks underestimating wildfire impacts on aquatic carbon cycling and coastal ecosystem function. Expanding focus beyond recalcitrant PyC to include labile compounds is essential to capture the full impact of terrestrial fires on carbon dynamics across the land–ocean aquatic continuum.