Impact of Fire Severity and Vegetation Cover on Soil Biogeochemistry in Mediterranean Holm Oak Forests

Wildfires are increasing in frequency and severity across Mediterranean ecosystems. However, the immediate soil biogeochemical responses that determine shortly post-fire resilience remain poorly understood. This study assessed how contrasting fire severity levels influence soil physicochemical, nutrient, and biochemical properties in ecologically relevant vegetation microsites—beneath Quercus ilex L. canopy, Stipa tenacissima L. tussock, and open interspaces—in a Mediterranean holm oak woodland in central Spain. Soils were sampled early after a wildfire and analyzed for organic matter, nutrient pools, water repellency, microbial respiration, nitrogen mineralization, and enzyme activities. Fire severity was the dominant driver of immediate post-fire soil responses. High-severity fire reduced soil organic matter, cation exchange capacity, total C and N, nitrate, microbial respiration, and all measured enzyme activities, with the most pronounced losses occurring beneath Q. ilex canopy. In contrast, ammonium, labile phosphorus, pH and soil water repellency increased under high severity, mainly in this microsite. Low-severity fire generally preserved biological functioning, with values comparable to unburned soils. Microsite identity modulated the magnitude of fire effects, with soils beneath Q. ilex cover microsite showing the greatest sensitivity, and open interspaces the least. The microsite × severity interaction detected for key nutrients and biochemical variables suggests that high-severity fire might destroy the microsite-specific fertility islands that constitute the functional core of Mediterranean woodland soils. These findings should be considered in management strategies prioritizing their monitoring and protection.