Wildfire drives lasting changes in the topsoil chemistry of Alpine forests

Background Wildfires are natural disturbances that impact the soil altering the biogeochemical cycling of many elements. The European Alps are being increasingly affected by extreme wildfires, but little is known about the persistence of fire-induced changes in these soils, and the relationship with fire severity in this environment. We sampled an Alpine Scots pine (Pinus sylvestris L.) forest affected by a mixed-severity wildfire (tree mortality range 15–100%) and assessed the recovery of several soil properties three years after the event in surface and sub-surface horizons of unburnt and burnt plots belonging to different severity classes (mapped with a combination of remotely sensed and field data). Results The burnt plots differed from unburnt plots by having greater bare soil exposure, and the dark-colored surface horizon was characterized by significantly higher pH and pyrogenic carbon content compared to the unburnt plots. The sub-surface horizons had significantly lower organic carbon content than the unburnt plots, higher pH, and less sand-sized aggregates. There was no proportionality between fire-induced changes in soil properties and fire severity class. The soils with the greatest water repellency (water drop contact angle > 110°) were mostly located in low severity plots. Conclusions Most soil properties, including those that usually recover quickly (e.g., pH), were still different in burnt plots three years after the wildfire. Severity maps produced in these environments, although detailed, can fail in identifying the sites where the soil was most severely burnt, and recovery of soil properties is not proportional to the severity degree obtained with this approach.