Prescribed fire effects on soil organic matter in a eucalypt forest previously affected by severe wildfire

Wildfires are integral to the dynamics of many ecosystems yet shifting fire regimes around the world pose challenges for biodiversity, carbon (C) cycling, and soil conservation. Prescribed fires (PF) are widely used to mitigate wildfire risk, yet their ecological consequences in areas previously affected by wildfires remain poorly understood. This study examines quantitative and qualitative impacts of a low-to-moderate severity PF on soil organic matter (SOM) in a dry eucalypt forest of the Greater Sydney area (Australia), 13 years after a severe wildfire. During the PF, temperatures in the litter layer and Oa horizon reached ∼900 °C, exceeding 300 °C for up to 4 min. In contrast, in the Ah horizon (0–2 cm) of the mineral soil, temperatures remained below 60 °C. The PF strongly affected soil surface SOM quantity, reducing the C stock by ∼87% (9.1 to 1.1 Mg ha −1 ). Thermal analysis (TG-DSC) and 13 C NMR spectroscopy revealed qualitative SOM transformations once the litter and the Oa horizon became ash, increasing thermal recalcitrance, reducing C-labile fractions, and a dominance of aromatic C, indicating incomplete combustion and pyrogenic C formation. By contrast, the Ah horizon (0–2 cm) showed limited changes but presented a highly recalcitrant SOM, probably reflecting the legacy of the previous severe wildfire. These findings highlight a critical trade-off presented by using PF in fire-prone eucalypt forests: while effectively reducing fuel loads, PF significantly diminished SOM stocks, but also increased the thermal recalcitrance of remaining SOM. These changes can be expected influence post-fire C cycling and enhance the persistence of this C pool in surface soil horizons. By examining both immediate PF effects and the legacy of a previous wildfire, this study contributes to a better understanding of SOM dynamics in fire-prone ecosystems and highlights the importance of considering soil C transformations when using PF as a fire-prevention tool.