Wildfire-Altered Soil Water-Extractable Organic Matter Drives Divergent Greenhouse Gas Emissions in Anaerobic...

Intensifying global climate change has increased wildfire frequency. Wildfire-altered soil water-extractable organic matter (burned-WEOM) is hydrologically transported to unburned areas, profoundly affecting cross-ecosystem carbon-nitrogen cycling and greenhouse gas (GHG) emissions. Taking soils from unburned subtropical forests as the research object, this study combined anaerobic incubation with high-resolution mass spectrometry and metagenomic sequencing to elucidate the regulatory mechanisms of burned-WEOM on soil GHG emissions under anaerobic conditions. The results showed that burned-WEOM increased CO₂ emissions by 17.0%, induced a 164.6% surge in N₂O emissions, and simultaneously inhibited CH₄ emissions by 52.9%. With unique properties of high unsaturation and strong electron exchange capacity, burned-WEOM not only reshapes soil organic matter composition but also drives differential GHG emissions by enhancing complete carbon fixation pathways and recalcitrant carbon decomposition, increasing the abundance of anaerobic methane oxidation (AMO) genes and methanotrophs, enriching denitrifying microorganisms (especially fungi), and boosting N₂O-generating gene activity without altering the reduction pathway. Moreover, WEOM molecular characteristics drive differences in GHG emissions: CH₄ is mainly fueled by reduced, unsaturated lipid-like compounds, N₂O is associated with nitrogen-rich, complex aromatic compounds, and CO₂ has a broader range of source substrates. This study provides insights that may improve mechanistic understanding of postfire GHG dynamics and inform process representations in climate models.