Apparent Soil Carbon and Nitrogen Stocks in the Mediterranean Forest Ecosystem over a Six-Year Post-Fire Chronosequence

Mediterranean ecosystems are particularly exposed to wildfire because seasonal summer drought, high temperatures, and flammable vegetation favour fire ignition and spread. Because post-fire nutrient patterns are context dependent, this observational study compared soil organic carbon (SOC) and nitrogen (N) stocks and selected physicochemical properties among sampling campaigns conducted before and after a wildfire in Vesuvius National Park (Italy). Surface mineral soil (0–10 cm) was sampled beneath two vegetation-cover categories (shrubs and trees) before fire (2015–2017) and approximately 22, 46, and 70 months after fire. Soils were analysed for pH, water content (WC), soil organic carbon concentration (Corg), total N concentration, bulk density, Corg:N ratio, and apparent fixed-depth soil organic carbon (SOC) and N stocks. Soil properties varied primarily among sampling periods, whereas vegetation category showed a more limited association. The short-term campaign was characterised by the highest pH and the lowest WC and total N concentration, whereas total N concentration and apparent fixed-depth N stock reached the highest median values during the long-term campaign. Vegetation category was associated only with WC and Corg:N ratio, which were generally higher under trees, and no significant sampling period × vegetation interaction was detected. Corg concentration and apparent fixed-depth SOC stock did not differ significantly among sampling periods. These differences cannot be attributed exclusively to wildfire or elapsed time because contemporaneous unburnt controls were unavailable and sampling period, calendar year, and spatial variability were not fully separable. The findings nevertheless show the value of jointly monitoring element concentrations and soil physical properties when assessing Mediterranean forest soils after wildfire.