Rapid assessment of post-fire soil health recovery using thermal and molecular analysis

Wildfires, which impact millions of hectares globally each year, are among the most significant natural disturbances to terrestrial ecosystems [1]. Their effects on soil organic matter (SOM) and soil organic carbon (SOC)—critical indicators of soil health and ecosystem resilience [2,3]—are profound yet not fully understood. This study investigated the recovery of SOC and its molecular composition over a year following wildfires of varying severity (high, moderate, and low/moderate) in the Caldera de Taburiente National Park, La Palma, Spain. Using evolved gas analysis/mass spectrometry (EGA/MS) and thermogravimetry (TG), we analyzed quantitative and qualitative changes in SOM, focusing on the distribution of active and stable carbon pools across two soil depths (0–2 cm and 2–5 cm). The results revealed that high-severity fires caused substantial SOC losses, particularly in the topsoil layer (0–2 cm), with minimal recovery observed within one year. Moderate- and low/moderate-severity fires preserved a greater proportion of active carbon promoting faster and more consistent recovery patterns [4]. The subsurface layer (2–5 cm) showed greater resilience, exhibiting minimal SOM changes regardless of fire severity, likely due to its thermal insulation capacity. Molecular analyses by EGA/MS indicated that fire severity shaped SOC recovery dynamics and influenced the relative abundance of organic compound families, such as lipids, lignins, polysaccharides, and nitrogen compounds. High-severity fires led to the accumulation of more recalcitrant compounds, while lower severities retained higher proportions of labile fractions, accelerating soil regeneration [4]. These findings underscore the critical role of fire severity and soil depth in determining SOC recovery rates and composition. The study highlights the value of combining EGA/MS and TG as a rapid, reliable approach to assessing fire-induced changes in SOM, offering practical insights for post-fire soil health assessment and ecosystem management.

References:
[1] Pausas, J.G., Keeley, J.E., 2009. A Burning Story: The Role of Fire in the History of Life, BioScience 59, 593–601.
[2] González-Pérez, J.A., González-Vila, F.J., Almendros, G., Knicker, H., 2004. The effect of fire on soil organic matter—a review. Environ. Int. 30, 855–870.
[3] Jiménez-Morillo, N.T., De la Rosa, J.M., Waggoner, D., et al., 2016. Fire effects in the molecular structure of soil organic matter fractions under Quercus suber cover. Catena 145, 266–273.
[4] Jiménez-Morillo, N.T.; Almendros, G.; De la Rosa, J.M.; et al., 2020. Effect of a wildfire and of post-fire restoration actions in the organic matter structure in soil fractions. Sci. Total Environ. 728, 138715.

Acknowledgements: This work received support from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the research project FIRE2C (ref. CNS2023-143750). N.T. Jiménez-Morillo acknowledges the “Ramón y Cajal” contract (RYC2021-031253-I) funded by MCIN/AEI/10.13039/501100011033 and the European Union “NextGenerationEU”/PRTR. The authors would like to thank the Caldera de Taburiente National Park (La Palma, Spain) for the sampling permits and logistic assistance during the experiment.