Molecular characterization of dissolved organic matter from two eucalypt forests in southwest Australia - legacies of...

Wildfires are increasingly affecting forest ecosystems worldwide, with potentially long-lasting impacts on soil organic matter and carbon fluxes. Transformations of dissolved organic matter (DOM) by fire are particularly important, as they regulate many biogeochemical and physical processes in soils. We examined changes in DOM from two eucalypt forest ecosystems in southwest Australia that were variably impacted by a large wildfire five years earlier. DOM was extracted from loamy soils under karri forest (Eucalyptus diversicolor F. Muell) and from sandy soils under jarrah forest (Eucalyptus marginata Donn ex. Smith). Soils were collected from low and high severity burned areas and adjacent, unburned sites. To assess molecular characteristics, we combined ¹HNMR with excitation-emission matrices (EEMs). Our results show that fire legacy was more evident in DOM from loamy soils, where the concentration of dissolved organic carbon (DOC) and the molecular weight of DOM was lower in sites burnt at high severity than unburnt sites. The ¹H NMR spectra of unburnt sites in the loamy soils were characterised by oxygenated structures, whereas sites exposed to high severity fires exhibited a dominance of aliphatic structures. By contrast, sandy soils showed little change in ¹H NMR spectra or fluorescence signatures across burned and unburned areas. Importantly, there was no evidence of aromatic structures in burned soils from either forest type, as indicated by the absence of increased aromaticity (SUVA₂₅₄) or aromatic ¹H NMR signals. Our findings highlight that wildfire can impart long-term changes to DOM, particularly in loamy soils, with implications for carbon cycling and ecosystem recovery.