Effects of wildfire on soil nutrients in Mediterranean ecosystems

High-intensity and fast-spreading wildfires are natural in the Mediterranean basin. However, since 1960, wildfire occurrence has increased because of changes in land use, which resulted in extensive land abandonment, increases in the fuel load and continuity in the landscape. The level of soil degradation related to wildfire occurrence depends on fire recurrence, topography of the site, intensity of the soil erosion processes and plant cover post-fire regeneration rate. Therefore assessing fire impacts on soil properties is critical to quantify land degradation processes and to assess post-fire restoration plans. This article reviews the changes in soil nutrient status of Mediterranean ecosystems affected by wildfires by focusing on the interactions between the different drivers and factors, and the underlying processes of these changes. Articles dealing with wildfires in areas belonging to the Mediterranean basin and characterized by an annual average rainfall of 300–900mm and a mean annual temperature around 14–19°C, have been reviewed. The data show that the soil nutrient content in Mediterranean drylands affected by wildfires depends on the vegetation type, fire recurrence and fire intensity. Immediately after a fire, the nutrient content in both the O and A horizons often increases because of ash deposition, nutrient release from the burnt vegetation and formation of stable nutrient forms. Ash deposition persistence on the soil surface is one of the most important factors in determining the soil nutrient content both immediately after a fire and for the long-term. For the restoration of burned habitats it is important to know the content and the spatial distribution of nutrients in the soil because this can act as a limiting factor to vegetation recovery. Carbon and nitrogen pools in the soil have been recognized as fundamental to vegetation recuperation after a fire. To promote the accumulation and retention of nutrients in soil after a fire, it is important to stabilize the burnt site by applying post-fire measures that limit soil erosion, surface runoff and wind loss of the ash. Depending on the plant species and the time elapsing between consecutive wildfires, fire is responsible for the transition from mature ecosystems (i.e. conifer forests) to shrublands, which are poorer in soil nutrient status. Wildfire occurrence can be reduced by planting fire-resilient plants in fire-prone areas. To define the best post-fire and restoration treatments, the impacts of fire on both the O and the A horizon as well as the impacts of different post-fire treatments on the soil nutrient content require further study.