Enhanced Release and Reactivity of Soil Water-Extractable Organic Matter Following Wildfire in a Subtropical Forest

Climate-driven increases in wildfire frequency may disrupt soil carbon dynamics, potentially creating positive feedback within global carbon cycle. However, the release and lability of soil carbon following wildfire remain unclear, limiting our ability to predict fire impacts on carbon cycling. Here, we investigated chemical alterations in soil water-extractable organic matter (WEOM) following a subtropical forest wildfire by comparing burned soils to an adjacent unburned site. The consensus is that fire-altered DOM is aromatic and less reactive. However, we found that 10 months postfire, burned soils contained nearly three times more water-extractable organic carbon (WEOC) than the control site. Reactomics analysis further revealed an overall 8-fold increase in potential reactivity of this carbon, identified by higher abundances of molecular formulas involved in identified microbial reaction pathways. Specifically, burned soils exhibited elevated potential oxidative enzyme reactions, linked to a higher nominal oxidation state of carbon (NOSC) in WEOM. Metagenomic analysis revealed an enrichment of microbial taxa specialized in degrading aromatic compounds in burned areas, supporting the occurrence of potential microbial reaction pathways acting on WEOM in postfire soils. These findings highlight that wildfires may accelerate soil carbon loss through reactive WEOM mobilization and microbial response, with implications for long-term carbon-climate projections.