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

Abstract

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.

1. Introduction

Wildfire and forest fire are terms used interchangeably to refer to major catastrophes that occur in forests, grasslands, or prairies. They can be caused by natural factors, such as lightning strikes or human activities such as smoking cigarettes, campfires, or arson. Wildfires are destructive fires that occur under uncontrolled conditions, while a forest fire is a specific type of wildfire that causes damage to forests globally. Wildfires often develop when weather and environmental conditions make it impossible to manage fires [1,2]. So, wildfires and climate change are closely interconnected, with climate change playing a significant role in the increased frequency, intensity, and severity of wildfires observed worldwide, especially in the Mediterranean [1,2]. Several factors including temperature, soil moisture, the presence of trees, shrubs, and other potential fuel sources strongly impact wildfire risk. Climate change enhances reduced rainfall, longer summer dry seasons, warmer and windier conditions, and eventually extended fire periods [3,4].

Many physical, chemical, mineralogical, and biological soil properties can be affected by wildfires [5]. The quantity and quality of the fuel load determine fire behavior. Different vegetation species in burned forest land have been shown to significantly affect soil characteristics [6].

Low-severity fires can benefit soil properties due to lower temperatures and the loss of nutrients due to volatilization and reduce smoke [7]. The production of carbon-rich ash increases soil organic matter, pH, electrical conductivity, and extractable cations such as Ca, Mg, Na, and some forms of nitrogen such as ammonia, which are crucial for vegetation recovery [7]. Low- and moderate-intensity fires do not consume vegetation entirely. Dehydrated plant parts that are partially burned fall to the ground and are added to the litter biomass, increasing soil organic matter content [8].

Contrary to low-severity fires, high-severity fires consume a considerable amount of fuel and negatively impact the soil. Two important effects include a significant reduction in soil cover [7] and transformations in organic matter composition [9]. Major changes in soil organic matter composition occur at temperatures between 250 and 450 °C [7]. Substantial combustion of organic matter starts around 200–250 °C and ends at 460 °C [5]. Nutrients such as calcium, magnesium, aluminum, and manganese, which require higher temperatures to be volatilized, can be lost by evacuation with ash and smoke when wildfire temperatures reach 1100 °C. Other elements with low volatilization temperatures, such as potassium (>774 °C), sodium (>880 °C) and phosphorous (>700 °C) may be lost by direct volatilization. High-severity fires increase soil pH due to the denaturation of organic acids, increased sodium and potassium oxides, carbonates, and hydroxides, and elevated electrical conductivity due to the mineralization of organic matter. An increase in pH raises the solubility of some cations such as calcium, magnesium, sodium, and potassium while reducing the solubility of others such as copper and zinc [7]. The soil-neutralizing capacity of ash correlates well with the sum of its K, Ca, and Mg contents [5]. The increase in pH is one of the most beneficial effects of prescribed burning, as it affects the availability of the most essential nutrients, especially in low-pH soils [10].

Regardless of fire severity, the combustion of burning organic matter increases soil nutrients such as Fe, Al, and Zn, as well as potentially toxic trace elements such as Ni, Cd, and Pb. As soil organic carbon (SOC) decreases, nutrients increase [11]. Their concentration depends on the severity of the fire and the type of vegetation burned [6].

Wildfires have been shown to release and deposit metals on the soil’s surface [3], either directly through vegetation combustion and the mineralization of soil organic matter or indirectly through ash’s interactions with the underlying soil [8].

As previously stated, the burned material’s composition is one of the variables determining the nature of ash. According to the type of wood wasted in combustion, element concentrations in biomass can vary [3]. Ash produced by wildfires is typically a heterogeneous material comprising oxides, hydroxides of base cations (Ca²⁺, Mg²⁺ and K⁺), which have high adsorption affinity for other metals and metalloids, and, to a lesser extent, major and trace metals that can contaminate soil [7,8,12]. Furthermore, these contaminants can leach into the soil profile or be transported downslope and downstream by runoff, affecting surface water and groundwater bodies and generating a potential ecological risk to benthic biota [8,13].

In 2019, a high-severity wildfire in Croatia was followed by noticeably elevated Cu and Zn concentrations in soils [6]. According to a recent soil study conducted after a wildfire in Greece, Al, As, Co, Cr, Fe, Mn, Ni, and V in burned topsoil were higher than the residential soil-screening thresholds or soil-quality requirements for residential land use, threatening potential harm to human health and the environment [14]. Regarding fire-induced temporal variations of element concentrations in soils, a study conducted in Portugal found that Cu and Pb levels barely changed throughout the study period, whereas Cd, Co, Mn, Ni, and V contents varied depending on the sampling time following the wildfire [8]. A study in Lithuania showed that higher values of some major elements, such as Ca and Mg, were only recorded in the burned area shortly after the fire, regardless of the sampling date. However, the potassium content was always much higher in the burned area [7].

Scientists studying the effects of fire on soil properties are interested in the extent to which heat generated by fire alters the chemical and physical characteristics of soils and affects post-fire properties, such as soil fertility, soil erodibility, or runoff generation [15]. For high-severity wildfires, burning biomass over soil can transfer heat to the soil surface, reaching depths of up to 30 cm [16]. Temperatures can reach about 850 °C at the soil–litter interface; however, in mineral soils, temperatures at a depth of 5 cm do not exceed 150 °C, since dry soil is a good insulator [15,16]. Clay content is a physical characteristic affected by soil heating [17,18]. In general, the mineralogical assemblage is not significantly affected by fire, since the initial step of disruption in most minerals, such as dihydroxylation, occurs at >500 °C [5]. Clay minerals begin to change when soil temperatures reach approximately 400 °C, as clay hydration and the clay lattice structure begin to collapse. In severely burned soils, the complete decomposition of kaolinite can occur in the uppermost layers (1–8 cm depths) and at temperatures of 700–800 °C. At temperatures >300 °C, clays lose OH⁻ and H₂O, whereas temperatures >850 °C result in the formation of anhydrous minerals [19].

According to the technical report Wildfires in Europe, Middle East, and North Africa by the European Commission’s Joint Research Centre (JRC) [1], the year 2021 was regarded as significant, with growing international concern about wildfires, their impact on the European Union, and its neighboring countries. More than 5500 km² of land was burned, 1000 km² of which was in the Natura 2000 network of protected areas in Europe. The damage caused to many of these invaluable ecosystems will take many years to restore.

Among the five Southern EU countries (Greece, Italy, Spain, Portugal, and France) with longer time series’ of available forest fire data (1980–2021), Greece had the worst season compared to previous years. The average fire size in Greece was the largest documented and has increased nearly tenfold since 2020 [1]. A total of 131,254 ha was mapped from 222 fires, including the second-largest fire of over 51,000 ha recorded by EFFIS (European Forest Fire Information System), which burned a significant portion of the island of Euboea. Almost half of the area burned by wildfires that year occurred in August, which was considered the most destructive month. Extremely high temperatures, >39 °C, prevailed in Greece from mid-June until the end of August 2021. Combined with the longest heat wave in 35 years and a prolonged period of drought, these conditions increased the risk of fire danger, resulting in the North Euboea wildfires recorded on 3 and 11 August 2021. Although wildfires encompass uncontrolled prescribed fires too, this is not the case in north Euboea, since ‘prescribed fires’ have not been employed in the study area as a practice of land management.

In this work, we assessed the impact of Mediterranean wildfires on soil quality and soil properties by (a) evaluating and comparing topsoil and subsoil compositional changes in wildfire-affected areas; (b) estimating the potential effects of fire-impacted soils on human health and the environment through leaching tests simulating natural conditions; (c) discriminating the impact of different origins, besides fire, on soil geochemistry. According to our knowledge, this is the first integrated work examining the overall effects of fire on soil geochemistry (major and trace element composition and leachability capacity), mineralogy, and various soil properties, such as soil pH and organic carbon content.

2. Materials and Methods

2.1. Study Area

The study area was strongly affected by wildfires in the northern part of Euboea Island, the second-largest Greek island, Central Greece, in 2021. The fire extended from the Agriovotano municipal district (in the north) to Kamatriades (in the west), Agia Anna (in the east), and Rovies (in the south) (Figure 1). That area is adjacent to Megalo kai Mikro Livari–Delta Xeria in the northwest, a European Union protected area of Natura 2000, and a world-class magnesite mine (Mantoudi municipal district) in the east (Figure 1).

The primary landcover categories heavily impacted by the wildfires included predominantly coniferous woodlands, a blend of mixed and broad-leaved forests, and to a lesser degree, areas designated for agroforestry practices. Unfortunately, the studied area lacks consistent implementation of targeted forest management practices that could effectively prevent or enhance wildfire management.

Abrupt slopes, downcutting erosion, and a dense drainage system characterize the fire-affected area. Most of the drainage basins were small with small hydrographic networks; however, there are a few large drainage basins covering a significant part of the fire-affected area, including at Gouves, Rovies, Neochori, and Krya Vrysi [20,21]. All studied areas are characterized by a Mediterranean climate on its northern shore, i.e., a decrease in annual rainfall, dry summers, and mild winters [21].

The main geological outcroppings in the study area are peridotites and mafic volcanic rocks, covered by extensive Neogene deposits [20]. Alluvial deposits occur along the drainage networks’ major channels and at the mouths of the rivers [22].

2.2. Soil Sampling and Analysis

The soil-sampling campaign was conducted one month after the wildfires. Sampling sites were selected in accordance with the severity and degree of the fire spread in most affected areas (Figure 1). According to the burn severity map of the area [20], all samples came from impacted areas of moderate to high severity. A total of 34 topsoil (0–10 cm depth) and subsoil (15–30 cm depth) samples were collected from the burned areas. The samples mainly comprised loam-type soils.

The collected soil samples were dried overnight at 45 °C, disaggregated with a porcelain pestle and mortar, and sieved to <2 mm. The finer fraction (<2 mm) was homogenized and pulverized in an agate mill. The soil pH was measured from the sieved fraction of the soil samples. Organic carbon, mineralogical and chemical composition, and leaching tests were performed on the pulverized fraction of the soil samples.

Soil organic carbon content was assessed using the classical Walkley and Black (1934) method. Soil pH was determined by mixing the soil with deionized water in a solid-to-liquid ratio of 1:2.5. A slurry was created by shaking 8 g of soil with 20 ml of deionized water in an orbital shaker at 200 rpm for 15 min, allowing it to settle for 30 min. The pH was measured in this mixture using a Consort 561 portable multiparameter analyzer (Consort, Turnhout, Belgium).

Aqua regia solution extracts ‘pseudo total’ contents refer to element amounts associated with soil phases such as clays, Fe and Mn-oxides, and organic matter, but not to insoluble minerals such as chromite. A portion of 0.5 grams of soil sample was digested with Aqua Regia (1:3 HNO3:HCl) solution, and the Pb, Zn, Cd, Cu, Mn, Fe, Cr, Ni, and Co contents were determined by FAAS [Perkin Elmer PinAAcle 500 (PerkinElmer, 217 Inc., Waltham, MA, USA)] and ICP- MS [THERMO ICAP-Qc (Thermo Fisher Scientific 218 Inc., Bremen, Germany).

In most extraction schemes, water-soluble metals are extracted as part of the exchangeable fraction, which includes weakly adsorbed elements retained on the solid surface by relatively weak electrostatic interactions. In other words, these are elements that can be released using ion exchange processes and elements that can be co-precipitated with carbonates [23]. Two different batch leaching procedures using natural mineral water (pH 7.67) and surface water (pH 8.40) were used to determine the relative extractability of potentially toxic elements from soil samples at a 4:1 w/s water-to-solid ratio. Solutions were maintained under agitation for 48 h in an orbital shaker at 120 rpm. Subsequently, the leachates were centrifuged for 12 min at 3000 rpm and filtered over 0.45 μm membrane filters using a vacuum filtration device. Finally, the leachates were analyzed for Cr, Mn, Fe, Cu, Ni, Zn, Cd, As, Co, Pb, Ca, Mg, Na, and K by ICP-MS [THERMO ICAP-Qc (Thermo Fisher Scientific Inc, Bremen, Germany)].

Multivariate (factorial) statistics were applied to the soil dataset to explain the observed variation in elemental contents (MINITAB ver. 15 for Windows). The input focused on 10 elements, namely Cr, Mn, Fe, Cu, Ni, Zn, Cd, As, Co, and Pb. The data were processed using R-mode factor analysis and applying the varimax-raw rotational technique.

The mineralogical composition of the soil was investigated using an X-ray diffraction spectroscopy powder D5005 diffract meter (Siemens, Munich, Germany). Cu K(alpha) radiation was performed at 40 kV, a 40 nA, 0.020° step size, and a 1.0 s step time. The obtained XRD patterns were evaluated using Bruker DIFFRAC, EVA 2.2 software (Bruker AXS GmbH, Karlsruhe, Germany).

3. Results and Discussion

High-intensity wildfires can profoundly affect soil quality and properties, especially when they occur in characteristic and unique ecosystems such as those found in the Mediterranean context. The effects of the 2021 wildfires on soil erosion dynamics for multiple fire-affected areas in Greece, the study area being one of the main case studies, demonstrated a shift toward erosion hazard from very low and low (pre-fire) to severe and very severe (post-fire) [24]. The burnt areas were more susceptible to erosion events than the surrounding non-burnt areas [21]. Valkanou et al. [22] demonstrated a significant post-fire change in mean annual soil loss that corresponded to an increase of 114% compared to the pre-fire natural conditions. The dominant landcover types ravaged by the wildfires were mostly coniferous woodland (31%), mixed- and broad-leaved forest (21.30%), and to a lesser extent, agroforestry land [20,22,25,26].

The Supplementary Materials (Table S1) present ‘pseudo total’ compositions of soil samples. Topsoils are enriched in Pb, Zn, Cd, and Cr, compared to subsoils (Figure 2). This observation concurs with earlier studies on burnt areas of similar lithology (e.g., [14]).

All soil samples exhibited elevated median ‘pseudo total’ values of Pb, Zn, Cd, Mn, Cr, Ni, and Co compared to those from adjacent unburned areas [6,27,28] (

Table 1. Median values (μg/g) of element contents in topsoils and subsoils collected from the study sites compared to values obtained from the literature (n = number of samples; n.d = not detected; n.m = not measured).

	Burned of Study Area		Unburned of North Euboea [27]
	Topsoils (n = 17)	Subsoils (n = 17)	Topsoils	Subsoils
Pb	77	20	18	16
Zn	91	64	67	55
Cd	1.00	0.29	0.6	n.d
Cu	27	28	35	46
Mn	834	845	480	360
Fe	19,400	29,455	27,400	24,100
Cr	334	269	78	40
Ni	174	564	44	34
Co	62	63	12	9
As	n.m	8	10	9

). However, they were analogous to those in burned areas with similar lithology [14] or vegetation [8]. Previous studies also reported fire-induced increases in Mn, Ni, Cd, and Pb in soil concentrations [12,29]. Cd and Pb accumulation in soils is typically related to its sorption by organic matter, fixation by clay minerals, association with Mn oxides, and Fe and Al (hydro) oxides [30]. Manganese, among all elements, exhibited the highest increase in burnt soils. Copper did not vary significantly in burned and unburned areas. This observation aligns with other authors’ findings [8,29].

Both topsoils and subsoils were enriched with Fe, Mn, Cr, Ni, and Co. The decreasing order of elements’ median ‘pseudo total’ values Fe > Mn > Cr > Ni > Zn > Pb > Co > Cu > Cd in topsoils did not differ significantly from those detected in subsoil Fe > Mn > Ni > Cr > Zn > Co > Cu > Pb > Cd. In both affected and unaffected soils, statistically significant (p < 0.05) medium to high positive correlations between Cr, Ni, Fe, Co, and Mn ‘pseudo total’ values in topsoils and subsoils (Figure 3) [27,28] imply their geogenic origins are from ultramafic rocks and related ores in the study area. Multivariate statistics applied to the soil dataset confirmed this finding. The first factor included Cr, Ni, Fe, and Mn with high positive loadings, accounting for 49%, 56%, and 55% of the total variance in topsoils, subsoils, and the whole-soil dataset, respectively. Enrichment of the above metals appears to be consistent with the soil mineral phases of antigorite and chrysotile, reflecting the contribution of ultramafic rocks and related ores (Fe-Ni-laterites and chromites). In general, the soil mineralogy did not differ significantly from adjacent areas that were unaffected by fire [27,28]. The presence of clay minerals, such as kaolinite, smectite, and micas, in the subsoils and corresponding topsoils implies that the fire temperature at the mineral soil surface did not exceed 500 °C [5,16].

Table 2. Major and trace element median, min, and max concentrations in topsoil and subsoil leachates by different leaching agents compared to EU parametric values for human usage [32] and the leaching agents’ composition (NMW = natural mineral water, NSW = natural surface water). The red font denotes values above EU-regulated values.

Major and Trace Elements		TOPSOILS LEACHATES						Blank of NMW Leaching Agent	Blank of NSW Leaching Agent	Parametric Values
		With NMW Leaching Agent			With NSW Leaching Agent
		Median	Min	Max	Median	Min	Max	Median Values
Pb	μg/L	0.143	BDL	1.48	0.166	BDL	1.24	0.009	0.024	10 μg/L
Zn		13	4.08	42	25	BDL	33	3.24	1.69
Cd		0.059	0.012	0.143	0.047	0.009	0.112	0.014	0.009	5 μg/L
Cu		7.23	3.31	11	6.47	2.81	9.45	0.03	1.28	2mg/L
Mn		151	19	772	216	8.31	681	6.21	8.73	50 μg/L
Fe		30	2.32	142	25	2.38	140	0.025	0.025	200 μg/L
Cr		2.9	BDL	35	2.50	BDL	36	0.003	2.92	50 μg/L
Ni		78	9.31	291	48	7.42	293	0.025	1.80	20 μg/L
Co		23	9.26	195	26	9.20	214	0.296	0.278
As		23	4.48	37	24	4.99	40	0.392	0.418	10 μg/L
Ca	mg/L	159	90	375	149	87	354	71	50
Mg		36	24	84	51	36	101	3.79	47
Na		13	8.63	28	18	13	28	2.83	11	200 mg/L
K		39	26	76	54	28	87	0.159	0.070
BDL = Below Detection Limit
Major and Trace Elements		SUBSOILS LEACHATES
		With NMW Leaching Agent			With NSW Leaching Agent
		Median	Min	Max	Median	Min	Max
Pb	μg/L	0.048	0.001	0.191	0.005	0.001	0.373
Zn		9.89	0.725	33	9.27	0.504	48
Cd		0.020	BDL	0.089	0.022	0.006	0.169
Cu		3.56	0.598	7.86	3.35	0.656	7.67
Mn		48	3.63	2662	56	3.02	2423
Fe		6.69	BDL	47	6.82	BDL	48
Cr		1.18	BDL	12	4.87	0.003	10
Ni		15	0.639	468	22	1.74	461
Co		8.62	0.666	65	6.18	1.41	78
As		5.87	2.14	18	5.99	2.40	15
Ca	mg/L	81	31	139	77	31	154
Mg		19	8.04	56	39	21	57
Na		9.34	6.60	13	15	13	18
K		16	5.82	21	13	6.00	22
BDL = Below Detection Limit

presents the range and median concentration values of the investigated elements in soil leachates for each leaching agent applied. Leaching procedures using water as an extraction agent provide information on the available water-soluble metal ions that can easily transfer into aquatic systems and groundwater via water infiltration and readily bioavailable element fractions [31].

Approximately 80% of topsoil and 50% of subsoil leachates exhibited Mn, Ni, and As concentrations, exceeding the EU’s maximum limits for water intended for human usage [32] (parametric values in Table 2), regardless of the leaching agent.

Calcium concentrations in both natural mineral and surface waters used as leaching agents are elevated, as are magnesium concentrations in natural surface water, compared to median values in soil leachates and those elements’ average concentrations in natural waters [33]. However, due to geogenic factors, such as lithology and mineral speciation, actual levels of element concentrations in groundwater and natural surface waters can vary greatly. Elevated Mg concentrations in natural surface water leaching agents can be attributed to Mg-bearing minerals in ultramafic rocks. Topsoil leachates exhibited higher median Ca and Mg values than subsoil leachates (Figure S1 in Supplementary Materials).

The decreasing enrichment of soil leachates in trace element concentrations, presented in

Table 3. Decreasing order of soil leachate enrichment in trace element concentrations.

Leaching Agents	Topsoils	Subsoils
Natural Mineral Water	Mn > Ni > Fe > As > Co > Zn > Cu > Cr > Pb > Cd	Mn > Ni > Zn > Co > Fe > As > Cu > Cr > Pb > Cd
Natural Surface Water	Mn > Ni > Co > Fe > Zn > As > Cu > Cr > Pb > Cd	Mn > Ni > Zn > Fe > Co > As > Cr > Cu > Cd > Pb

, differs from the reducing order of their ‘pseudo total’ contents and lacks correlation. The topsoil leachates exhibited a high abundance of Mn, Ni, Fe, and Co, while the subsoil leachates contained high levels of Mn, Ni, and Zn. Other researchers have also reported increased water-soluble Mn values after fires [34]. The observed decrease in subsoil leachates’ Mn, as well as the statistically positive correlation (R² = 0.54, p < 0.05) between ‘pseudo total’ Mn content in subsoils and organic carbon, may indicate that precipitation events quickly bond and immobilize Mn after it is released during vegetation incineration [8].

The enrichment of arsenic in soil leachates is reflected by its high extractability. Besides reduction conditions [35], the presence of organic matter acts as a catalyst by forming carbon from biomass incineration in fires [36], causing As(V) reduction to the more mobile form of As(III). Additionally, the presence of organic matter accelerates the release of As associated with Fe minerals under environmental conditions [35]. The mobility of As at high pH values [37,38], such as those measured in the study area’s soil samples (approximately pH 8.0), is emphasized by the statistically positive correlations between As total leachability and soil pH values.

Table 4. Decreasing order of the studied elements’ relative extractability by leaching agents.

Leaching Agents	Topsoils	Subsoils
Natural Mineral Water	Co > Mn > Ni > Cu > Zn > Cd > Cr > Pb > Fe	As > Co > Cu > Zn > Cd > Ni > Mn > Cr > Pb > Fe
Natural Surface Water	Co > Ni > Zn > Mn > Cu > Cd > Cr > Fe > Pb	As > Zn > Cu > Cd > Co > Ni > Mn > Cr > Pb > Fe

shows the relative extractability of the examined elements as the ratio of the extractability of all leaching agents (natural mineral and natural surface water) to their ‘pseudo total’ soil content.

The ratios of element concentrations in leachates to their ‘pseudo total contents’ in topsoils leached with natural mineral and surface water were very low, ranging from 0.001 to 0.15% and 0.0007 to 0.17%, respectively. In subsoil leachates, they ranged from 0.0001 to 0.30% and from 0.00007 to 0.32%, respectively. Despite the leachates’ different pHs, there was no significant variation in the ratios of ‘pseudo total’ contents extracted by both leaching agents in topsoils and subsoils (Figure 4). Thus, natural mineral water can be used instead of ‘local’ natural surface water or rainwater as a leaching agent to estimate trace elements’ environmental availability.

Although the soil samples displayed a specific enrichment order in trace element ‘pseudo total’ contents (Fe > Mn > Cr > Ni > Zn > Pb > Co > Cu > Cd in topsoils and Fe > Mn > Ni > Cr > Zn > Co > Cu > Pb > Cd in subsoils) and leachate concentrations (Table 3), other metal sequences were easily extracted from soil leachates (Table 4). Iron and chromium, enriched in all soil samples, exhibited low relative extractability from both leaching agents, whereas Ni and Mn, also abundant in all soil samples, presented higher relative extractability in topsoils than subsoils.

However, statistically significant medium (in topsoil samples) to high (in subsoil samples) positive correlations have been found between Cr, Ni, Cu, and Mn ‘pseudo total’ contents in soil and their concentrations in leachates, implying that soil enrichment may influence their extractability (Figure 5).

Nickel and cobalt, derived from the breakdown of phyllosilicate minerals in the study area, such as antigorite and chrysotile, are also associated with manganese and iron oxides [39]. A statistically significant positive correlation between Mn and Co (Figure 3) is consistent with Co’s affinity for Mn oxides. This finding may explain the high ratios of Co extracted by both leaching agents, especially in topsoil leachates where Mn concentrations were higher than in subsoils. Furthermore, Ni ‘pseudo total’ content positively correlates with organic carbon, implying that its distribution and mobility in soil are related more to soil organic matter than to clay minerals and Fe-oxides/hydroxides [30].

Compared to their ‘pseudo total’ enrichment in soil samples, the low extractability of elements such as Fe and Cr is likely due to their affinity for organic matter. Confirming this suggestion, a statistically significant (p < 0.05) positive correlation between the proportion of Fe and Cr ‘pseudo total’ contents extracted by both leaching agents and organic carbon, possibly due to insoluble organic compounds with these metals. Mn typically accumulates in tree leaves, especially needles of resinous species [30]. Manganese’s high extractability and abundance in soil leachates (especially in topsoil leachates) may be related to heat-induced physicochemical breakdowns of Mn-complexes with soil organic matter as well as vegetation combustion [40] and ash deposition [41].

Organic carbon ranged from 2.75 to 10.8% in topsoils and from 0.19 to 6.75% in subsoils. In most sampling sites, topsoils exhibited higher organic carbon values than subsoils and topsoils in nearby unburned areas of similar lithologies and vegetation types [27,42] (Figure 6). Compared to adjacent unburned areas’ soils, organic carbon increased by almost 80% in fire-affected areas. As observed by other authors [8], incomplete combustion of plants is responsible for the increase in organic matter content in burned soils.

Soil pH values ranged from 6.91 to 8.46 in topsoils and 7.26 to 8.73 in subsoils, indicating neutral to alkaline conditions for all topsoils. All soil datasets demonstrated a statistically significant negative correlation between both agents’ low Mn relative extractability and soil pH at more than −0.65. Compared to Mn, the higher relative extractability of As revealed a statistically significant positive trend in soil pH values. The leachability of most elements in solid samples is known to highly depend on the acidity of the solution [43]. Lowering soil pH promotes exchange. In other words, the lower the soil pH, the higher the leachability of elements from solid samples [8,43].

4. Conclusions

Mediterranean wildfires have significantly impacted soil quality and soil properties in the affected areas. Northern Euboea, Greece, exemplifies these impacts.

Overall, the concentrations of almost all the studied elements values were higher in fire-affected than unaffected soils. Similar variations in element concentrations were observed between topsoils and subsoils, with the latter exhibiting lower values.

However, the increased ‘pseudo total’ values of Cr, Ni, Fe, Co, and Mn in all soil samples, along with the medium to high positive correlations between them, indicate that geogenic factors play a major role in controlling element enrichment.

In soil leachates, Mn, Ni, and As concentrations that exceed the EU’s maximum permissible limits indicate a potential ecological risk to natural water quality and, subsequently, human health.

Correlation coefficients between elements in both fire-affected and unaffected soils suggests their geogenic origin, which is mainly related to ultramafic rocks and associated ores in the study area.

Element concentrations in the soil leachates were significantly lower than their ‘pseudo total’ contents in the soil, with no correlation between them. The reducing order of element leachability does not coincide with the decreasing order of elements’ ‘pseudo total’ median values in soils.

The decreasing order of element abundance in soil leachates is consistent with their relative extractability from both agents (natural mineral and surface water). However, it differs from the decreasing order of their ‘pseudo total’ median values in soils.

Neutral to alkaline soil pH conditions and organic carbon content, which significantly influence element retention and mobility, revealed patterns that were not evident for all the studied elements. Correlations were only found for Mn, Cr, and Co.

The increased organic matter content in fire-affected soils suggests that vegetation combustion was incomplete.