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Article

Effect of Forest Species Canopy on the Accumulation of Toxic Metals in the Soil Within and Around Macedonia Airport, Northern Greece

by
Ioannis Mousios
1,
Marianthi Tsakaldimi
1,
Evangelia Gkini
1,
Theocharis Chatzistathis
2 and
Petros Ganatsas
1,*
1
Laboratory of Silviculture, School of Forestry and Natural Environment, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
2
Institute of Soil and Water Resources, Hellenic Agricultural Organization ‘DIMITRA’ (ELGO-DIMITRA), 57001 Thessaloniki, Greece
*
Author to whom correspondence should be addressed.
Urban Sci. 2025, 9(6), 191; https://doi.org/10.3390/urbansci9060191
Submission received: 19 March 2025 / Revised: 14 May 2025 / Accepted: 22 May 2025 / Published: 27 May 2025
(This article belongs to the Special Issue Integrating Ecosystem Services into Urban Planning: A Novel Approach for Natural Resources Management in Metropolitan Areas)
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Abstract

Soil pollution at airports is a critical environmental issue that affects not only the local ecology but also the health of people living near these infrastructures. The main causes of pollution include the use of chemical products such as de-icing agents, fuels, and lubricants, as well as waste from aircraft and ground vehicles. These substances often seep into the soil, leading to the accumulation of toxic elements. However, due to security reasons, there is a great scarcity of real data on the impact of airport operations on ecosystems and the role trees could play in pollutant limitation. Thus, the aim of this study was to determine whether airport operations have toxic effects on soils within and around Macedonia Airport, Thessaloniki, Northern Greece, by determining the concentrations of potentially toxic elements (Cu, Ni, Pb, Mn, Fe, Co, Cr, Cd, and Zn) in soil samples taken within the airport and near the airport. Furthermore, this study aimed to investigate the effect of the canopies of forest species on the accumulation of toxic metals in the soil inside the airport and in the peripheral zone. The results show that, overall, no important pollution was detected in the soil of the Thessaloniki Airport, Northern Greece, both inside and outside the airport area. Some differences were observed in the content of toxic metals studied between the samples taken inside and outside the airport, and some effects of tree canopy were noted. However, all values were lower than the defined permissible limits according to international standards (except for iron). It is important, however, to perform regular re-checking of soil quality with new samples in order to prevent soil contamination and mitigate any contamination found.

1. Introduction

Air transportation is a major source of environmental pollution; more than 99.9% of flights are powered by the burning of fossil fuels, adding over a billion tons of CO2 to the atmosphere each year [1]. Air and soil pollution in airports is caused by aircraft operations and ground activities as well as access to road transport, and its impacts could be hazardous to organisms and humans [2,3]. A wide variety and quantity of sources of emissions are noted at airports due to aircraft operational emissions, sources of infrastructure or fixed assets, and sources of road traffic [4]. Some specific activities affecting soil conditions are the application of de-icing and anti-icing substances to aircraft during winter operations. In addition, to ensure the safety of aircraft operations and the durability of runways, runoff from the pavement is drained as soon as possible and collected in an adequate drainage system [5]. Surface spills of fuel, which usually occur at aircraft stands and usually result from overfilling of aircraft and vehicles, are a significant source of contamination. Although large spills are usually cleaned up, smaller ones may be carried into the stormwater drainage system, some of which may be permeable, thereby allowing contaminated water to seep into the ground [6,7]. The repair of composite parts generates waste containing Kevlar, glass fibers, and organic solvents, including toluene and acetone. Electrical circuit repair produces waste with cleaning fluids to remove glue, wax, oil and grease from electronic components, metals, and other materials. Engine maintenance and wheel, tire, and brake repair produce waste with vitreous and non-vitreous solvents, fuel, hydraulic, brake and lubricating oils, greases and specialty oils (e.g., dielectric heat transfer) [8]. Activities for fire training with different types of fuel (kerosene, butane, propane, wood) may contribute to soil pollution. There are also important sources of emissions from non-air traffic mainly due to vehicular traffic, motorcycles, cars, semi-trucks, trucks, buses, and coaches connected to the airport on access roads, pavement, and parking areas on or off the premises (including engine shutdown, starting and fuel tank exhaust emissions). Thus, because of the airport operation, soil pollution is expected mainly in the airport area; however, it probably occurs in the surrounding area as well, including pollution due to high concentrations of (toxic) metals.
Toxic metal pollution of the soil refers to the excessive deposition of toxic metals in the soil mainly caused by human activities. The most common chemicals involved are petroleum hydrocarbons, solvents, pesticides, lead, and other toxic metals of biological toxicity, such as mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), and arsenic (As). These also include some other metals of certain biological toxicity, such as zinc (Zn), copper (Cu), nickel (Ni), tin (Sn), vanadium (V), etc. In recent years, with the development of the global economy, both the type and content of toxic metals in soil caused by human activities have gradually increased, resulting in environmental degradation [9,10,11,12]. Toxic metals are particularly dangerous for the environment and organisms. Once the soil is affected by contamination, it is difficult to restore it. When it exceeds the environmental tolerance or when environmental conditions have changed, toxic metals in soil can be activated and cause serious ecological damage [13,14]. They exhibit toxic effects toward soil biotic beings, affecting basic microbial processes and reducing the number and activity of soil microorganisms [15]. Chen et al. [16] reported that toxic metals cause a decrease in bacterial species diversity, a relative increase in soil actinomycetes, or even a decrease in the biomass and diversity of bacterial communities in contaminated soils. Karaca et al. [17] reported that enzyme activities are affected differently by various metals due to the different chemical relationships of enzymes in the soil system. Ashraf and Ali [18] also reported that toxic metals exert toxic effects on soil microorganisms, resulting in changes in the diversity, population size, and overall activity of soil microbial communities. It was also observed that metal (Cr, Zn, and Cd) pollution affected the metabolism of soil microorganisms in all cases. Cd is more toxic to enzymes than Pb because of its greater mobility and lower affinity for soil colloids. Cr (VI) is a strong oxidizing agent and is highly toxic, while Cr (III) is quite less hazardous [19]. Cr (VI) toxicity in plants includes delayed seed germination, damaged roots, reduced root growth, reduced biomass, reduced plant height, photosynthetic impairment, membrane damage, leaf chlorosis, necrosis, low grain production, and ultimate death of the plant [13,20]. In general, an increase in metal concentration negatively affects the properties of soil microorganisms [21].
The high concentration of toxic metals in soil also has many negative effects on plants grown in polluted soils. Most plant species cannot adapt when the physical, chemical, and biological properties of the soil change due to soil pollution. Soil fungi and bacteria begin to decline, creating an additional problem of soil degradation. Soil fertility slowly declines, making the soil unsuitable for agriculture and for the survival of any vegetation. Soil contamination makes large tracts of land hazardous to health [22]. Low concentrations of toxic metals in soil does not affect plant growth. However, if the concentration is too high, toxic metals can be absorbed by the plant, causing them to become poisoned and even die. One of the most important factors for metal toxicity is the pH of the soil; an alkaline pH of the soil could restrict the mobilization of the metals in soil matrix, controlling the metal uptake by crop plants, thereby reducing the risk of metal toxicity [23]. The limit values of toxic metals in the soil for good production are listed in Table 1 [24]. Similar target values were specified by the WHO (World Health Organization) for the soils to be characterized as unpolluted soils [13] (Table 1). While in Table 2, the limit values of toxic metals in some European countries are presented [25].
On the other hand, tree functions affect soil chemical properties as well as the content of soil metals [26]. Tree presence influences soil content of toxic metals. However, this influence is not similar for all elements, and it greatly depends on the morphological and functional characteristics of the tree species [27]. Generally, each element follows a different pattern that depends on the functions of the tree species. For example, Tőzsér et al. [28] reported that different Populus species present different effects on soil metals, while Alahabadi et al. [29] reported that (toxic) metal accumulation patterns vary among different tree species and plant tissues.
The environmental impacts of air transport have become increasingly important [30], and the role of trees in pollution mitigation by capturing them has been greatly recognized [31]. At the same time, there is a great scarcity of real pollution data on the impact of airport operations on ecosystems as well as the role the trees could play in pollutant limitation. Thus, the aims of this study were as follows: (i) to determine whether airport operations have toxic effects on airport soils, (ii) to determine if there is an impact of airport operations on the soils of the surrounding area, and (iii) to determine the effect, if any, of different tree species on the degree of soil pollution by the potentially toxic elements, in the area within the airport of Thessaloniki, Greece, and outside the airport near the runway. The overall goal was to assess whether airport traffic had actual toxic effects in the territory of the airport area, but also in the surrounding area. The hypothesis is that trees with different morphological and functional characteristics can modify the degree of soil pollution through their crown. The forest species Pinus brutia and Populus alba were selected both inside and outside the airport, and the concentrations of toxic metals (Cr, Cu, Co, Zn, Ni, Pb, Fe, Mn, Cd) in soils and how they is affected by tree canopy of the different forest species were examined.

2. Materials and Methods

2.1. Research Area

The research was carried out in two locations. The first area is within the airport of Thessaloniki, Macedonia Airport (SKG), Greece. It is the only airport that serves the area of Thessaloniki, the second largest city in Greece. The airport is located in the coastal area of Mikra at a position of latitude 40°31′11″ N and longitude 22°58′15″ E at an altitude of 1–2 m asl, just near the sea (Figure 1) and at a distance of 16 km from the city of Thessaloniki. The airport occupies an area of approximately 2306.7 ha and is surrounded in the west-southwest by areas of agricultural and semi-urban use and in the north-northwest by the sea. It has two runways with dimensions of 3440 m × 50 m and 2410 m × 60 m, separately [32]. The annual traffic of the airport for the year 2023 is 7,029,957 million passengers and approximately 65,000 flights [33]. The area around the airport was the second part of the study area. Sampling outside the airport was performed at sites in the area near the airport in the direction of airport runway close to the settlements Neo Rysio and Trilofos. These two areas are located within a radius of 10 km from the airport, while their airspace is a traffic corridor for aircraft that take off or land at the airport of Macedonia.
The urban complex of Thessaloniki extends over an area where the altitude varies from 0 to 350 m. The climate of the Thessaloniki region can be considered Mediterranean. The temperature shows its highest values in July and the lowest in January, with the annual temperature range exceeding 20 °C. In addition, there are observed some mild and sunny days during the winter, as well as a relatively large number of summer days and low rains in summer. According to the data of the Meteorological Station of the Aristotle University of Thessaloniki, the average annual air temperature for the period 1960–2007 [34] amounts to 15.8 °C with an average monthly low temperature of 5.9 °C (January) and high of 25.9 °C (July). The absolute minimum temperature is –12.6 °C (January) and the absolute maximum 41.8 °C (July). During the year, about 140 days have a maximum temperature above 25 °C and about 70 days above 30 °C, while 107 are clear and 73 cloudy. Sunshine hours range between 2400 and 2600 [34,35].

2.2. Field Data Sampling

Sampling was carried out at the end of summer 2023 (August–early September). Soil samples were taken in relation to two forest tree species Pinus brutia and Populus alba that both appear inside the airport and in the areas near the airport. The selection of the two species was based on their silvicultural characteristics and functional traits. Both are high trees, native to Greece, and seen in many airport areas in Greece. They form trees of high dimensions and thus can affect metal deposition in soil in the place they grow. One of them is an evergreen tree with a medium growth rate (Pinus brutia), while the other one, Populus alba, is a fast growing deciduous tree that is characterized by high growth rates and quick rates of litter decomposition. A total of 12 individuals (trees) were selected for sampling, i.e., six individuals for each species and three per species for the area inside and outside the airport (Figure 2), and their locations were recorded with a handheld GPS (Table 3). The trees selected were of similar age and without any sign of damage or unhealthy conditions.
For each selected tree, four points were selected for soil sampling, including two points under the tree crown and near the trunk and two outside the tree crown (Figure 3). To avoid possible contamination, clean tools were used when collecting soil samples in the field. Thus, stainless steel tools as well as plastic transport bags were used for sampling, since their transport was achieved in less than 48 h [36]. Sampling was carried out at a depth of 0–25 cm, firstly removing the surface litter. A quantity of one kilogram was collected and taken to the laboratory. A total of four samples were collected from the area of each individual (tree) on each sample site, resulting in a total sampling size of 24 samples inside the airport and 24 samples outside, yielding 48 samples in total. Using GPS and the Google Earth satellite application, version 2024, the sampling points were marked in the map (Figure 2).

2.3. Preparation and Chemical Analyses of the Soil Samples

All sample preparation procedures were carried out at the Laboratory of Silviculture and the Soil Science Laboratory of the Department of Forestry and Natural Environment of the Aristotle University of Thessaloniki. Soil samples were dried at room temperature, and the stones were removed. Then, they were sieved through a 10-plexus dredge, prior to chemical analysis. Here, pH, organic matter content, total (Kjeldahl) N, as well as the concentrations of micronutrients and metals (Fe, Mn, Zn, Cu, Cd, Co, Cr, Ni, and Pb) were defined under lab conditions. All the chemical analyses concerning soil properties were carried out at the Hellenic Agricultural Organization ‘DIMITRA’ (ELGO-DIMITRA), and particularly at the Soil & Water Resources Institute (Thermi, Thessaloniki, Greece). pH was determined in a soil-distilled water paste at a 1:1 ratio [37]. The organic matter content was estimated using the potassium dichromate method [38], and the Kjeldahl N method was employed using the methodology described by Bremner and Mulvaney [39]. For the determination of concentrations of micronutrients and metals, the DTPA method (pH 7.3) was used [40]. In order to determine nutrient and metal concentrations, an ICP (OPTIMA 2100 DV optical emission spectrometer, Perkin Elmer, Waltham, MA, USA) spectrometric method was applied [41].
The respective detection wavelengths used and the lowest detectable metal value, according to the instrument specifications, are given in Table 4.

2.4. Statistical Analysis

Analysis of variance (ANOVA) was applied to test if there were differences in the mean values of the toxic metal content between the soil samples of the two studied species Pinus brutia and Populus alba both inside and outside of the airport area as well as under the tree canopy and outside the canopy. The data were analyzed with the ANOVA method in the methodological framework of general linear models. Statistical analysis was performed with the statistical SPSS 29 for Windows package. For all tests, the significance level was pre-set at p = 0.05.

3. Results

3.1. Silvicultural Characteristics of the Trees

Both tree species, which have their effects examined in this study, where the soil samples were taken, are represented with trees of a height over 10 m, having mean heights of 10.5 ± 0.8 and 11.3 ± 1.0 m and diameters at breast height of 45.1 ± 4.0 cm and 36.7 ± 5.5 cm for the species Pinus brutia and Populus alba, respectively (Table 5). Their max crown widths were 9.0 ± 0.4 m and 7.3 ± 0.5, and their crown lengths were 7.0 ± 0.7 and 9.7 ± 0.5 m, respectively.

3.2. Soil Main Chemical Characteristics (pH, Organic Matter, and Total Nitrogen)

According to data analysis, the following results were obtained and are presented in Table 6, Table 7 and Table 8, for soil pH, organic matter, and total nitrogen, respectively. Soil pH was found constant for all sampling points, presenting an average value of 7.77. The values slightly varied between 7.48 and 8.15, indicating the alkaline character of the airport soils, as a result of the location of the airport, on a flat alluvial area. Therefore, no statistical differences were found for soil pH, either between the soil samples inside and outside the airport or between the crown and outside the canopies of the two studied forest species. These constant values of soil pH in turn suggest a similar bioavailability of metals and metal ion activity in solutions of the soils studied.
In contrast to soil pH, significant differences were recorded between the soil samples in regard to soil organic matter (Table 7). Higher values of organic matter were recorded in the soil outside the airport in the case of Pinus brutia sampling (10.8% and 15.5%, respectively). This was probably due to the fact that sampling was carried out under or close to the canopy of dense forest stand of the species and under isolated pine trees inside the airport. However, in the case of the deciduous Populus alba, higher values were recorded in the soil sampled inside the airport, regardless of sampling under or outside the tree crown (7.2% and 6.9%, respectively), while the values of soil organic matter are relatively low inside the airport, regardless of the tree crown (2.1% and 2.8%, respectively).
Also, some differences in values of organic nitrogen were observed in soils samples, especially for the samples related to the Populus alba trees (Table 8). Even though no differences were recorded under the tree crown and outside the tree canopy, there were observed significant differences in organic nitrogen for the soil sampled inside and outside the airport, both for samples under the canopy and outside the canopy, with values recorded outside the airport to be significant lower (mean value of 0.19% compared with 0.42% inside the airport).

3.3. Airport Operation Impacts on Toxic Metals in the Soils Inside and near the Airport

All data concerning the concentrations of the studied toxic metals (Cr, Cu, Co, Zn, Ni, Pb, Fe, Mn, Cd) are presented in Table 9 and Figure 4 and Figure 5; each figure indicates the mean values and the corresponding standard errors for each element identified in the soils in relation to the forest species (under the tree crown and outside the tree canopy) and airport area (inside and outside the airport). In addition, the statistical differences between the soil element content based on the sampling positions are presented. Based on the data collected during the study, all the recorded values of the studied toxic metals, even the maximum ones, are lower than the contamination limits set by countries or international organizations (values in Table 1 and Table 2), except for iron. This finding shows that airport operation does not significantly alter the environment of the airport with regard to the studied toxic metals. For iron, both maximum and average values are slightly above the limits in soils inside and outside the airport, indicating a significant effect of airport operation for this element.
Examining the differences inside and outside the airport, the data analysis revealed statistical differences in soil contents of five toxic metals, iron (Fe), copper (Cu), cadmium (Cd), chromium (Cr), and nickel (Ni), between the soils inside and outside the airport (Table 9) when the effect of tree presence is not taken into account. For all these elements, except for copper, the soil content was statistically higher in the soils inside the airport. This means that the operation of the airport burdens the soil with higher concentrations of these four toxic metals. Only in the case of copper was the opposite result obtained (the values were higher outside the airport). On the contrary, no effect was observed for soil levels of manganese (Mn), zinc (Zn), cobalt (Co), and lead (Pb). However, as previously mentioned, all values were under the contamination limits, except iron. The contribution of each source (and their interactions) to each metal is presented in Table 10. Based on the data presented in Table 10, it seems that, among the factors studied, the tree position (sampling site) has a higher contribution to differences among metals.

3.4. Influence of Forest Trees on Toxic Metal Accumulation in Soil

Data analysis showed that not all metals follow the same pattern of the tree crown effect. For some metals (Zn, Fe, Cu, Cd, Cr, Pb), there was a tree crown effect on their concentration in soil, while others were not affected by trees (Mn, Co, Ni). The effect of tree species was also significant for some elements (Cu, Cd, Co, Cr).

3.5. Metals Affected by Trees

3.5.1. Zinc

The mean zinc (Zn) soil content was higher inside the airport in samples related to the species Populus alba, while the opposite was observed for the species Pinus brutia (Figure 4). A significant crown effect was observed only for the species Populus alba inside the airport. The highest average concentration was found in the samples taken under the crown of Populus alba trees inside the airport (7.02 ppm), and this significantly differs from all the other values recorded both inside and outside the airport. In contrast, in the samples related to the species Pinus brutia, the mean zinc soil content in both sampling locations was higher outside the airport, and these differences are statistically significant compared with the values recorded inside the airport, but they are still significantly lower than those recorded under the tree crown of Populus alba inside the airport.

3.5.2. Iron

The higher average soil concentration of iron (Fe) (9.88 ppm) was recorded inside the airport in the soil samples taken outside the crown of the trees of Populus alba species (Figure 4). This value is significantly higher than all other values of iron recorded both inside and outside the airport. On the other hand, the lowest average iron concentration was recorded outside the airport in sampling related to the trees of Populus alba, and these values are significantly lower (p < 0.05) compared to all other values.

3.5.3. Copper

Values of soil copper (Cu) content were generally low in all sampling sites related to the trees of Pinus brutia, without any significant differences (Figure 4). On the contrary, in the case of Populus alba, there were significant differences both between samples taken inside and outside the airport as well as under and outside the tree canopy outside the airport. The highest concentration of soil copper (mean value 5.17 ppm) was recorded in the site outside the airport and close to the Populus alba trees. The correspondingly lowest average concentration (mean value 0.9 ppm) was recorded at the off-airport site in samples related to Pinus brutia trees. In general, we observe that inside the airport the average copper concentration in all sampling sites does not show much variation, while outside the airport there is a difference between the species, as well as there is a crown effect (in the case of Populus alba sampling).

3.5.4. Cadmium

The values of soil cadmium content were found generally low (mean value for all samples 0.04 ppm), both inside and outside the airport (Figure 4). However, the mean concentration of cadmium (Cd) inside the airport was found significantly higher in the samples related to the Populus alba trees, and taken under the tree crown, followed by samples taken outside the tree canopy of the same species. These differences were found to be statistically significant. In all other sampling cases, the recorded values were low and there were no significant differences observed. The lower mean concentration was observed outside the airport in the soil samples related to Populus alba species. For the soil samples related to Pinus brutia species, the recorded cadmium values are low in all cases both inside and outside the airport with insignificant differences.

3.5.5. Chromium

The concentration of chromium (Cr) in the soils of the airport area was found low (less than 0.01 ppm) in almost all cases (Figure 4), except for one case in samples taken inside the airport and near the Populus alba trees (outside their crown). In this case, the recorded values were significantly higher (0.029 ppm), and much higher than in all other cases, ca. triples. The lowest value was recorded outside the airport, in sites close to Pinus brutia trees.

3.5.6. Lead

For lead (Pb), the concentration values are generally low in all soil samples (Figure 4), presenting a fluctuation related to the sample sites. However, there was recorded as significant tree crown effect (p <0.05) for both species inside and outside the airport. The highest mean concentration was found within the airport area, in the soil samples taken under the crown of the Populus alba trees, followed by the samples taken outside the airport and outside the canopy of Pinus brutia trees. On the contrary, the lowest mean concentration was found within the airport near the Pinus brutia trees.

3.6. Metals Not Affected by Tree Crown

3.6.1. Manganese

There were no differences in manganese content between the soil samples taken inside the airport, both under and outside the tree canopy (no effect of tree crown). The values recorded ranged between 3.98 and 6.39 ppm, with an average value of ca. 4 ppm (Figure 5). This means that the overall encumbrance of airport operations in terms of soil manganese within the airport area is similar, and it is not affected by the trees. Outside the airport, the recorded values are higher, with the highest concentration (6.39 ppm) observed under the Pinus brutia trees, with a slight lower value (with insignificant difference) noted outside the tree canopy of this species (p < 0.05). A significantly lower value was observed in samples taken under the tree crown of the species Populus alba; however, these values are significantly higher compared with those recorded in all soil samples taken inside the airport, as well as compared to the soils taken outside the tree canopy of Populus alba outside the airport.

3.6.2. Cobalt

The values of soil cobalt (Co) content were found generally low, presenting a mean value (for all soil samples) 0.03 ppm both inside and outside the airport (Figure 5). No tree crown effect was observed for both studied tree species. In the case of the Populus alba, the cobalt content does not present a statistically significant difference (p < 0.05) in any location inside and outside the airport. However, for the soil samples related to the Pinus brutia trees, significantly higher values were recorded outside the airport, both under and outside the tree canopy.

3.6.3. Nickel

The average concentration of soil nickel was significantly higher inside the airport, both under and outside of the trees canopy (Table 9, Figure 5). In addition, it was higher outside the tree canopy of the species Pinus brutia in both cases (outside and inside the airport); however, these differences were statistically insignificant. On the other hand, in the samples related to the trees of deciduous Populus alba, there were no differences between the values under the tree crown and outside the tree canopy, both inside and outside the airport. The values recorded in sites related to Pinus brutia were similar to those related to Populus alba inside the airport, which differ from values recorded outside the airport related to the trees of Populus alba.

4. Discussion

The current research was carried out with the aim of investigating the soil conditions and the concentrations of toxic metals in the area of Thessaloniki Airport, Northern Greece, and in the surrounding area. Generally, the results of the soils studied showed that no concentration was found to exceed the defined permissible limits according to international standards (Table 1 and Table 2) [13,24,25], with the exception of iron. The soil pH was slightly alkaline (average values of approximately 7.5, with no differences between sampling sites inside and outside the airport). This constancy of the soil pH suggests a similar bioavailability of the studied toxic metals and metal ion activity in solution of the soils of the airport area. Soil total nitrogen and organic matter were quite high, both inside and outside the airport (mean values for organic nitrogen and organic matter of over 0.3% and 4.5%, respectively) and greatly varied between the sampling sites.
Concerning the analysis of toxic metals concentrations in the soil, first, we must point out that some differences were revealed in the content of the toxic metals studied between the samples taken inside and outside the airport. These differences concerned the following elements (toxic metals), namely iron (Fe), copper (Cu), cadmium (Cd), chromium (Cr), and nickel (Ni). Based on the data analysis, statistical differences in the soil content were found for these five toxic metals. For all these elements, except for copper, the soil content was found statistically higher in the soils inside the airport, which means that the operation of the airport burdens the soil with higher concentrations of these four toxic metals. It is worth pointing out that Thessaloniki Airport has been in operation for over fifty years. Thus, the higher levels of these elements is due to the accumulated impacts of operation, and these findings indicate the current environmental conditions of the area. Only in the case of copper was the opposite result observed (the values were higher outside the airport). On the contrary, no effect was observed for the following soil toxic metals, manganese (Mn), zinc (Zn), cobalt (Co), and lead (Pb). This means that there is an overall low encumbrance of airport operation within the airport area for some toxic metals, but not for others.
This encumbrance differs depending on the presence of tree species as well as on elements studied. For the following five elements, namely, zinc, iron, cadmium, nickel, and lead, the highest average concentrations (7.021 ppm, 9.883 ppm, 0.108 ppm, 0.699 ppm, and 2.99 ppm, respectively) were found in soil samples related to Populus alba inside the airport. For copper and chromium, the highest average concentrations (5.17 ppm and 0.029 ppm, respectively) were found in soil samples related to the Populus alba species outside the airport and near the runway. While, for manganese and cobalt, the highest average concentrations (6.385 ppm and 0.048 ppm, respectively) were found in the samples referring to the trees of Pinus brutia, also outside the airport. The analysis of source contribution to metal differences presented in Table 10 indicates that, among the factors studied, the tree position (sampling site) has the higher contribution to metal differentiation. From the comparison of the above values, with the values in Table 1 and Table 2, we conclude that no value of any elements exceeds the permissible limits except for that of iron (Fe). In fact, all other concentration values, even the maximum ones (Table 9), are quite low and do not cause concern for soils both inside and outside the airport.
In comparison with the worldwide literature data, the results obtained in this study from the Macedonia Airport, Northern Greece, are similar to those of the other airports where similar studies were conducted. More specifically, similar investigations have been carried out at Athens International Airport, Delhi International Airport, Warsaw Airport, and Hatay Airport. By comparing the results of the existing literature and the toxicity limits in the soils, we can give a first estimate of the concentrations of toxic metals. Massas et al. [42] assessed the quality of the environment after 13 years of operation of the airport and measured the concentrations of toxic metals, including lead (Pb), copper (Cu), zinc (Zn), nickel (Ni), chromium (Cr), manganese (Mn), and iron (Fe), in its peripheral soils. They concluded that the average concentrations were at low levels in general but observed an accumulation of metals in the northern and eastern parts of the airport. Despite the small number of samples, they concluded that the daily flight load likely burdens the areas with greater accumulation and that periodic research is required for the above elements.
In another case, Ray et al. [43] conducted a similar soil sampling of the airport perimeter and measured the concentration of eight metals, including lead (Pb), copper (Cu), zinc (Zn), nickel (Ni), chromium (Cr), manganese (Mn), and iron (Fe). From the results of the research, it was initially found that the concentrations of toxic metals, excluding iron and manganese, were higher inside the airport by one and a half times compared to the remote surface. At the same time, regarding the points inside the airport, a greater concentration was found in the landing part of the aircraft, compared to that of take-off and taxiing. At Warsaw Chopin Airport, Brtnický et al. [44] examined soil pollution with the help of indicators and showed low soil pollution in the vicinity of the airport runway. The airport soil was found to be slightly polluted, but due to the constant growth of the airport traffic, an increase in contamination can be expected in the future. In addition, regarding Hatay Airport, Turkey, Ozkan et al. [45] concluded that the agricultural land around the airport had not been contaminated by toxic metals except for iron, which could come from soil fertilizers.
Tree presence was found to influence the soil content of toxic metals, but this influence is not similar for the studied elements or the same for the studied tree species. Each element follows a different pattern that depends also on the presence of tree species. Similar conclusions were made by Tőzsér et al. [28] for Populus species as well as for different tree species. Alahabadi et al. [29] reported different patterns for metal accumulation in tree plant tissues. According to worldwide data, it is well documented that morphological and functional characteristics of the tree species affect their influence on soil metals [26,27]. This trend was confirmed in this study in the area of Thessaloniki Airport, Northern Greece. More specifically, it is reported that the concentration of metals in soil depends, among other factors, on the presence of plant species, especially trees, and their morphological and functional traits [46]. The results of our study confirm the aspect that different tree species result in the cycling of different elements (Figure 4 and Figure 5), which are attributed to their different morphological and functional traits, such as leaf area index, leaf form or needles, leaf decomposition rate, element cycling, tree growth rate, crown surface area, crown form, and biological processes [47,48]. However, although it is generally suggested that evergreen species have higher capacity and retain a larger number of metals in their foliage than deciduous trees, other studies report that broadleaves play an important role in removing air pollution [46,48,49], and among broadleaves trees, those with rough leaf surfaces are more efficient [50].
To sum up, based on the analysis of the field data collected in this study, no important pollution was detected in the soil of the Thessaloniki Airport, Northern Greece, both inside and outside the airport area, despite the observed significant differences for some toxic metals in soils sampled inside and outside the airport. It is important, however, to perform the temporary re-checking of the values with new samplings in order to prevent and optimally manage the natural wealth of the region. It is a given that the region will experience greater prosperity, and air transport is expected to increase rapidly in the near future [51,52], and these factors cumulatively may affect air pollution and the accumulation of toxic metals in the soil and vegetation [44]. We have to point out that any differences found in toxic metal concentrations inside the airport and outside the tree canopies are probably not related to tree species but to the position of sampling in regard to any pollution sources.
The results provide information on potentially toxic concentrations that affect the soils of the area and, by extension, the vegetation and the water table. The results obtained allow us to assess the current conditions and form the basis for the optimal management and protection of soils from pollution originating from human activities. Land pollution from air transport is a topic that has not been studied in depth; thus, further research is needed, especially under the expected significant increase in air transportation across planet earth [53].

5. Conclusions

Even though soil pollution at airports is a critical environmental issue that could affect not only the local ecology but also the health of people living near these infrastructures, according to the results of this study, and in comparison to the findings of other similar studies, it is concluded that, overall, no important pollution was detected in the soil of the Thessaloniki Airport, Northern Greece, both inside and outside the airport area, with regard to the following toxic metals: Cu, Ni, Pb, Mn, Fe, Co, Cr, Cd, and Zn. Some differences were observed in the soil content of toxic metals studied between the samples taken inside and outside the airport, and some effects of the tree canopy were noted. However, all values were lower than the defined permissible limits according to international standards (except for iron). It is important, however, to perform regular re-checking of soil quality with new samples in order to prevent soil contamination and mitigate contamination if found.

Author Contributions

I.M.: Conceptualization; Investigation; Methodology; Formal analysis; Data curation; M.T.: Writing—original draft; Data curation; Formal analysis; Validation; E.G.: Investigation; Formal analysis; Methodology; Data curation; T.C.: Methodology; Investigation; Writing—review and editing; P.G.: Conceptualization; Project administration; Data curation; Writing—review and editing; Supervision; Resources. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are available upon request.

Acknowledgments

Authors would like to thank FRAPORT company for their permission and support for field data sampling inside the area of the Thessaloniki Airport as well as all the Wildland Management team of the Thessaloniki airport.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Map indicating the study area, namely, the Thessaloniki Airport, Northern Greece.
Figure 1. Map indicating the study area, namely, the Thessaloniki Airport, Northern Greece.
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Figure 2. Sampling sites in the area of Thessaloniki Airport (Google Earth). Up: inside airport sites; down: outside sites.
Figure 2. Sampling sites in the area of Thessaloniki Airport (Google Earth). Up: inside airport sites; down: outside sites.
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Figure 3. Sampling design for under the tree crown and outside the tree canopy; sampling points 1 and 4 are outside the tree canopy, and 2, 3 are under the tree crown.
Figure 3. Sampling design for under the tree crown and outside the tree canopy; sampling points 1 and 4 are outside the tree canopy, and 2, 3 are under the tree crown.
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Figure 4. Soil content of the six metals (zinc, iron, copper, chromium, cadmium, and lead) significantly affected by the tree crown. Values are mean and s.e.m. of the samples taken under the tree crown (blue color) and outside the canopy (grey color) both inside and outside the airport area. For each metal, values followed by a different letter denote statistical differences.
Figure 4. Soil content of the six metals (zinc, iron, copper, chromium, cadmium, and lead) significantly affected by the tree crown. Values are mean and s.e.m. of the samples taken under the tree crown (blue color) and outside the canopy (grey color) both inside and outside the airport area. For each metal, values followed by a different letter denote statistical differences.
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Figure 5. Soil content of the three metals (manganese, cobalt, and nickel) not significantly affected by the tree crown. Values are mean and s.e.m. of the samples taken under the tree crown (blue color) and outside the canopy (grey color) both inside and outside the airport area. For each metal, values followed by a different letter denote statistical differences.
Figure 5. Soil content of the three metals (manganese, cobalt, and nickel) not significantly affected by the tree crown. Values are mean and s.e.m. of the samples taken under the tree crown (blue color) and outside the canopy (grey color) both inside and outside the airport area. For each metal, values followed by a different letter denote statistical differences.
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Table 1. Limit values of toxic metals in the soil for good production [24] and outlined by the WHO [13].
Table 1. Limit values of toxic metals in the soil for good production [24] and outlined by the WHO [13].
Toxic MetalspH 5–6 mg/kgpH > 6 mg/kgWHO
mg/kg
Pb5030085
Cd130.8
Ni307535
Cr100100100
Co8080
Cu5014036
Fe4.54.5
Zn15030050
Mn7070
Table 2. Limit contamination values of toxic metals in some European countries, in mg/kg [25].
Table 2. Limit contamination values of toxic metals in some European countries, in mg/kg [25].
CountryCdCrCuNiPbZn
Austria17015060120500
Belgium1.5709020120300
Denmark0.4-1000301204000
Germany1.510010050150400
Ireland1.510010050150350
Luxembourg1.510010050150400
Netherlands1506020100200
Spain210010060150400
Sweden110010050100300
UK110020050150400
Table 3. Coordinates of the sampling sites.
Table 3. Coordinates of the sampling sites.
Sampling SiteLatitude LongitudeLocation
Pinus brutia 140.5124422.98312Inside airport
Pinus brutia 240.5122122.98275Inside airport
Pinus brutia 340.5119722.98237Inside airport
Populus alba 140.5149422.98657Inside airport
Populus alba 240.5135922.98403Inside airport
Populus alba 340.5131522.98387Inside airport
Sampling siteLatitudeLongitudeLocation
Pinus brutia 140.4932922.98805Outside airport
Pinus brutia 240.4925422.98856Outside airport
Pinus brutia 340.4921322.98871Outside airport
Populus alba 140.4972222.98278Outside airport
Populus alba 240.4834022.98838Outside airport
Populus alba 340.4843622.98571Outside airport
Table 4. Detection wavelengths used and the lowest detectable metal values, according to the instrument specifications.
Table 4. Detection wavelengths used and the lowest detectable metal values, according to the instrument specifications.
ElementDetection Wavelength, nmLowest Detectable Metal Value, ppm
Manganese (Mn) 257.610 0.0014
Zinc (Zn)206.2000.0059
Iron (Fe)238.204 0.0046
Copper (Cu)327.3930.0097
Cadmium (Cd)228.8020.0027
Cobalt (Co) 228.6160.007
Chromium (Cr) 267.7160.0071
Nickel (Ni)231.6040.015
Lead (Pb)220.3530.042
Table 5. The means with the corresponding standard errors of the mean (s.e.m.s) of the main silvicultural characteristics of the studied trees.
Table 5. The means with the corresponding standard errors of the mean (s.e.m.s) of the main silvicultural characteristics of the studied trees.
Forest SpeciesTree Diameter (cm)Tree Height (m)Crown Height (m)Crown Max Diameter (m)Crown Min Diameter (m)
Pinus brutia45.1 ± 4.0 10.5 ± 0.87.0 ± 0.79.0 ± 0.45.0 ± 0.3
Populus alba36.7 ± 5.511.3 ± 1.09.7 ± 0.57.3 ± 0.53.7 ± 0.6
Table 6. Soil pH under and outside the canopies of the two species (mean and s.e.m.) and inside and outside the airport. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Table 6. Soil pH under and outside the canopies of the two species (mean and s.e.m.) and inside and outside the airport. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Soil pHPinus brutia Populus alba
Airport AreaUnder the CanopyOutside the CanopyMeanUnder the CanopyOutside the CanopyMean
Inside7.5 ± 0.2 aA7.7 ± 0.2 aA7.6 ± 0.2 A7.5 ± 0.2 aA7.5 ± 0.2 aA7.5 ± 0.2 aA
Outside7.8 ± 0.2 aA7.8 ± 0.2 aA7.8 ± 0.2 A7.9 ± 0.2 aA8.1 ± 0.2 aA8.0 ± 0.2 aA
Table 7. Soil organic matter under and outside the canopies of the two species both and inside and outside the airport. Values are mean and s.e.m. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Table 7. Soil organic matter under and outside the canopies of the two species both and inside and outside the airport. Values are mean and s.e.m. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Organic Matter %Pinus brutia Populus alba
Airport AreaUnder the CanopyOutside the CanopyMeanUnder the CanopyOutside the CanopyMean
Inside 5.2 ± 0.6 bB5.5 ± 0.7 bB5.4 ± 0.6 B7.2 ± 0.9 aA6.7 ± 0.8 aA6.9 ± 0.8 A
Outside 10.8 ± 1.2 bA15.4 ± 1.4 aA13.1 ± 1.0 A2.1 ± 0.4 cB2.8 ± 0.5 cB2.4 ± 0.4 B
Table 8. Total (Kjeldahl) nitrogen content, under and outside the canopies of the two species, and inside and outside the airport. Values are mean and s.e.m. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Table 8. Total (Kjeldahl) nitrogen content, under and outside the canopies of the two species, and inside and outside the airport. Values are mean and s.e.m. In the same row, values followed by a different lower case letter denote statistical differences between the two species as well as between the position of sampling. Values in the same column followed by a different capital letter denote statistical differences within and outside the airport for the same species and the same position with regard to the tree crown.
Total Nitrogen %Pinus brutiaPopulus alba
AirportUnder the CanopyOutside CanopyMeanUnder the CanopyOutside CanopyMean
Inside0.34 ± 0.05 aA0.39 ± 0.06 aA0.37 ± 0.05 A0.39 ± 0.06 aA0.45 ± 0.07 aA0.42 ± 0.05 A
Outside0.36 ± 0.06 aA0.46 ± 0.08 aA0.41 ± 0.05 A0.20 ± 0.04 bB0.17 ± 0.04 bB0.19 ± 0.03 B
Table 9. Toxic metal content in mg/kg in the surface soil inside and outside the airport in comparison to the limit values (standards) set by several European countries and international organizations; in all cases, we applied the lowest suggested limit. Values are mean and s.e.m., and maximum recorded values are reported (in separate row). In the same column, values followed by a different lower case letter denote statistical differences between the two sampling sites.
Table 9. Toxic metal content in mg/kg in the surface soil inside and outside the airport in comparison to the limit values (standards) set by several European countries and international organizations; in all cases, we applied the lowest suggested limit. Values are mean and s.e.m., and maximum recorded values are reported (in separate row). In the same column, values followed by a different lower case letter denote statistical differences between the two sampling sites.
SiteMn Zn Fe CuCd Co Cr NiPb
Inside airport4.03 ± 0.4 a3.38 ± 0.6 a7.59 ± 0.7 a1.37 ± 0.3 b0.037 ± 0.0 a0.025 ± 0.0 a0.019 ± 0.0 a0.63 ± 0.06 a1.47 ± 0.14 a
Max value6.210.123.62.30.300.050.141.044.36
Outside airport4.95 ± 0.5 a4.16 ± 0.8 a4.55 ± 0.3 b3.02 ± 0.9 a0.026 ± 0.0 b0.034 ± 0.0 a0.008 ± 0.0 b0.42 ± 0.06 b1.91 ± 0.42 a
Max value9.810.89.212.10.050.080.021.003.79
Limit70504.5360.4080502085
Table 10. Contribution (%) of sources and their interactions regarding differences in each toxic metal.
Table 10. Contribution (%) of sources and their interactions regarding differences in each toxic metal.
SourceMn Zn Fe CuCd Co Cr NiPb
Inside airport/outside airport effect16.130.014.296.447.9110.761.6315.990.83
Tree crown effect2.090.856.210.831.230.241.152.850.22
Tree position sources 22.365.9915.7027.246.127.648.3018.0516.01
Inside airport/outside airport effect X Tree crown effect1.551.755.240.406.150.012.720.299.04
Inside airport/outside airport effect X Tree position 4.4720.356.9422.748.7011.819.038.574.42
Tree crown effect X Tree position 18.094.142.690.5510.840.009.5510.913.81
Inside airport/outside airport effect X Tree crown effect X Tree position 4.264.574.091.580.820.000.002.229.52
Natural sources31.0562.3454.8440.2260.2869.5467.6251.1256.15
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Mousios, I.; Tsakaldimi, M.; Gkini, E.; Chatzistathis, T.; Ganatsas, P. Effect of Forest Species Canopy on the Accumulation of Toxic Metals in the Soil Within and Around Macedonia Airport, Northern Greece. Urban Sci. 2025, 9, 191. https://doi.org/10.3390/urbansci9060191

AMA Style

Mousios I, Tsakaldimi M, Gkini E, Chatzistathis T, Ganatsas P. Effect of Forest Species Canopy on the Accumulation of Toxic Metals in the Soil Within and Around Macedonia Airport, Northern Greece. Urban Science. 2025; 9(6):191. https://doi.org/10.3390/urbansci9060191

Chicago/Turabian Style

Mousios, Ioannis, Marianthi Tsakaldimi, Evangelia Gkini, Theocharis Chatzistathis, and Petros Ganatsas. 2025. "Effect of Forest Species Canopy on the Accumulation of Toxic Metals in the Soil Within and Around Macedonia Airport, Northern Greece" Urban Science 9, no. 6: 191. https://doi.org/10.3390/urbansci9060191

APA Style

Mousios, I., Tsakaldimi, M., Gkini, E., Chatzistathis, T., & Ganatsas, P. (2025). Effect of Forest Species Canopy on the Accumulation of Toxic Metals in the Soil Within and Around Macedonia Airport, Northern Greece. Urban Science, 9(6), 191. https://doi.org/10.3390/urbansci9060191

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