Mediterranean Wildfires' Effect on Soil Quality and Properties: A Case from Northern Euboea, Greece

Physical and chemical soil properties are affected by wildfires. Post-wildfire runoff may contain nutrient loads and particulate matter that negatively impact soil, surface water, and groundwater. According to data from the Copernicus Emergency Management Service, devastating wildfires on Northern Euboea Island, Greece, in August 2021 destroyed more than 50,910 ha. Coniferous and broad-leaved forests mostly covered the affected area, according to CORINE. Topsoil and subsoil samples were collected from burned areas and analyzed for physicochemical parameters: pH, electrical conductivity, and organic carbon. After digestion with aqua regia, the Pb, Zn, Cd, Cu, Mn, Fe, Cr, Ni, Co, and As 'pseudo total' contents were determined. Leaching experiments were conducted to evaluate the levels of potentially toxic elements leaching from soils and the impact of environmental conditions. The leachates were analyzed for Pb, Zn, Cd, Cu, Mn, Fe, Cr, Ni, Co, As, Ca, Mg, Na, and K. Overall, most of the concentrations of the studied elements were higher in fire-affected soils than in unburned ones. Similar findings for element concentrations have been confirmed between topsoils and subsoils, with the latter exhibiting lower values. The increased 'pseudo total' values of Cr, Ni, Fe, Co, and Mn in all the soil samples, along with the medium to high positive correlations between them, indicate that geogenic factors play a major role in controlling element enrichment. High concentrations of Mn, Ni, and As in soil leachates exceeded the EU maximum permissible limits, indicating a potential ecological risk to natural water quality and, subsequently, to human health. The correlation coefficients between elements in fire-affected and unburned soils suggested that their geogenic origins were mainly associated with the ultramafic rocks and related ores of the study area. The elements' concentrations in the leachates were significantly lower than their 'pseudo total' contents in soil, with no correlation between them. The reducing order of elements leachability did not coincide with the decreasing order of elements 'pseudo total' median values in soils. The decreasing order of element abundance in soil leachates coincides with their relative extractability and differs from the decreasing order of their 'pseudo total' median values in soils. Neutral to alkaline, soil pH conditions and organic carbon content, which substantially influence the retention and mobility of elements, presented different patterns among the studied elements, with only Mn, Cr, and Co showing correlations. The increased content of organic matter in fire-affected soils suggests that the combustion of vegetation was incomplete.