Thermal alteration of soils belonging to highly diverse forest ecosystems: what’s behind the non-univocal aggregate stability and water repellency response?

Climate change drawbacks have recently induced major variations in fire regimes, and projections for future years forecast even more frequent and disruptive events [1]. As wildfires impose deep modifications on soil organic matter (OM) and mineral phase [2,3], soil water repellency (WR) and aggregate stability (AS) can be heavily affected, with fallouts on fertility and erosion. Yet, the extent of these alterations may vary according to soil type and development degree. Thirty topsoils representative of a wide variety of forest ecosystems across the globe (from savannah to tropical, Mediterranean, temperate and boreal forests) were subjected to laboratory heating at temperatures of 200° and 300°C (for 30 minutes) and characterized for WR, AS, and main drivers of organomineral interactions, such as pH, texture, organic C (OC) and total N content, and abundance of Fe oxyhydroxides. Selected samples were also analyzed by FT-IR, GC/MS, TGA-DSC and XRD. Water repellency, despite being highly variable among the samples, was always drastically lost when samples were exposed to temperatures>200°C. After separation into macro (2–0.250 mm) and micro (<0.250 mm) aggregate size classes, determination of macro-aggregate stability revealed that, upon growing temperatures, OM-rich high-elevation soils (from temperate and boreal forests) were far more subjected to AS loss than highly-developed Mediterranean and savannah soils, where aggregation is possibly ruled mostly by clay and heat induced Fe-Al oxyhydroxides re-crystallization phenomena [4]. In tight interlink with ecosystem resilience, the present study clearly evidenced the vulnerability of certain biomes towards thermal-induced soil degradation.