Decoding molecular changes in soil organic matter in heat-affected soils along latitudinal gradients

Wildfires are a global phenomenon that occur across diverse biomes, imposing deep modifications on the quantity and quality (molecular composition) of soil organic matter (SOM). Targeting SOM molecular composition is an ongoing challenge for soil researchers, since SOM is an inherently heterogeneous material with varying functionalities and interactions with the soil mineral phase. The extent and duration of fire-induced SOM alterations are closely tied to fire severity, which is influenced by environmental factors such as climate, topography and type of vegetation. Hence, by addressing SOM molecular complexity in fire-affected soils of diverse ecosystems we aim at (1) identifying factors responsible for drastic SOM transformations, and (2) predicting the occurrence of these changes according to biome of belonging.

In this study, up to 10 topsoils representative of a wide variety of biomes across the globe (from Savannah to Tropical, Mediterranean, Temperate, High-latitude and altitude and Boreal forests) were subjected to a laboratory heating (at 200 and 300 °C) aimed at mimicking the behaviour of fire. Analytical pyrolysis (Py-GC/MS) of bulk soil samples revealed a prevalence of proteins, alkylaromatics and polycyclic aromatic hydrocarbons in burnt soil samples. Conversely, less labile carbohydrate structures along with lignin-derived compounds were observed at higher temperatures. However, some differences were observed across biomes: a relatively greater abundance of compounds that promote soil water repellency (i.e., aromatics) is depicted in Mediterranean ecotone or warmer climates (savannahs), whereas a higher proportion of N-derived compounds is found in cold, wet regions. This work aims at understanding the extent of SOM transformations in fire-affected areas in relation to soil physico-chemical properties such as total nitrogen, organic carbon content and water repellency, and eventually identify the influence of environmental soil forming factors that act a broader scale, such as temperature and precipitation.

Acknowledgments: 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.