Introduction
The increase in air pollution is currently one of the major problems of civilization. This problem also concerns Krakow and its surroundings. For several months a year, Krakow, the second largest city in Poland, and its surroundings suffer from ever-higher pollution levels. In Krakow, the pollution of air and soil is one of the highest in Europe. The main components of air pollution in Krakow include carbon dioxide, sulfur dioxide, nitrogen oxides, suspended dust and polycyclic aromatic hydrocarbons (PAHs). The geochemical and/or magnetic methods have been successfully applied by many researchers to assess the impact of air pollution on the environment (Chodak et al.
2013; Klamerus-Iwan et al.
2015; Szuszkiewicz et al.
2016). In assessing the degree of soils contamination, the content of heavy metals is most often used. The contamination of soil by heavy metals is a significant problem, which leads to a negative influence on soil characteristics and limitations of productive and environmental functions (Friedlová
2010). Polycyclic aromatic hydrocarbons (PAHs) have strong toxic, carcinogenic and mutagenic effects, thus posing a risk to both soil organisms and people, and therefore, recent studies devote more attention to them (Maliszewska-Kordybach et al.
2008; Srogi
2007). PAHs are emitted to the atmosphere mainly from anthropogenic sources (Kwon and Choi
2014). A large amount of PAHs emitted to the atmosphere accumulates in soil via dry and wet deposition (Srogi
2007). Soil is a geoindicator of long-term environmental pollution because PAHs are strongly associated with soil organic matter (Wang et al.
2013). The sorption process plays a key role in controlling transport of PAHs during leaching, as well as in transformation and bio-accumulation processes (Reeves et al.
2004). Magnetic susceptibility is an effective, easy, fast and inexpensive method which can be used as a heavy metal pollution detection tool (e.g., Heller et al.
1998; Petrovský et al.
2000; Chaparro et al.
2006; Magiera et al.
2007; Xia et al.
2014), in particular for areas with a strong fly ash deposition. Łukasik et al. (
2016) used magnetic susceptibility to assess the state of pollution of the upper horizons of soil forest reserves of Polish lowlands and uplands. Topsoil horizons can be significantly affected by airborne supply of elements, in particular from anthropogenic sources. The majority of urban–industrial dusts contain anthropogenic iron oxides (i.e., magnetite, maghemite, hematite) which are formed during high-temperature technological and metal extraction processes as well as during combustion of solid fuels, and they are accompanied by heavy metals (Hulett et al.
1980). These anthropogenic iron minerals are called technogenic magnetic particles (TMPs) and are significantly different from the minerals formed by natural processes because of stoichiometry and crystallographic structure as well as morphology (Magiera et al.
2011).
Monitoring by means of the characteristics of microbiological and biochemical properties of soils is successfully used in assessing the degree of soil contamination (Friedlová
2010). Biochemical properties reflect the changes, and they are directly related to the amount and activity level of soils microflora (Błońska and Januszek
2013). Soil enzymatic activities are sensitive bio-indicators of any natural and anthropogenic disturbance (Kumar et al.
2013). The activity of soil enzymes is one of the approved parameters used for the quality evaluation of soil polluted with PAHs (Lipińska et al.
2014). Klamerus-Iwan et al. (
2015) confirmed that oil contamination with PAHs modified the physical properties of forest soils (air porosity, water resistance of aggregates) and had a negative impact on enzyme activity in soil. According to Sardar et al. (
2010), high concentration of heavy metals (Cd and Pb) changes soil microbial community structure and activities. The soil enzymatic activities decreased significantly with the increasing contamination by heavy metals, especially dehydrogenase and urease activities (Chen et al.
2005).
Changes in the species composition of forest stands may lead to modifications in soil properties. Tree species affect pH and cation-exchange capacity (CEC) of soil (Mueller et al.
2012; Gałka et al.
2014; Gruba and Mulder
2015). Species composition of trees affects the quantity and quality of soil organic matter (SOM) (Augusto et al.
2002; Hobbie et al.
2007) on which the accumulation of heavy metals (Biernacka and Małuszyńska
2006) and PAHs (Komprdová et al.
2016) depends. Heavy metals bound on insoluble humic substances are relatively immobile (Borůvka and Drábek
2004).
The distance from the pollution source has a significant impact on the degree of soil pollution. Šmejkalová et al. (
2003), Wang et al. (
2007) observed a statistically significant correlation between soil properties and the distance from the source of contamination. The heavy metals are deposited into soil at various distances depending on wind velocity and direction (Agrawal et al.
2010; Ogunkunle and Fatoba
2014). PAHs are adsorbed onto fine particles (<2.5 μm), which are deposited slowly and, owing to atmospheric conditions, may be transported over long distances (Srogi
2007). According to Larsen and Baker (
2003) heavy molecular weight PAHs, typical products of motor vehicle exhausts are not transported far from their source.
Few papers about soil pollution around urban areas in relation to the forest stands composition have been written so far. The primary objective of the research was to determine the degree of soil pollution in different forest types around Krakow, using biochemical, chemical and magnetic methods. The impact of the forest species composition by comparing the surfaces of broadleaf and coniferous forest stands was taken into account in assessing the degree of soil contamination. In addition to assessing the degree of accumulation of pollutants, the position of the surveyed plots in relation to Krakow, the direction of prevailing winds and the distance from the pollution sources were included. An attempt was made to use enzymatic activity of soils in assessing their pollution. Enzymatic activity was related to other measures of pollution, i.e., heavy metal concentration, content of PAHs and level of magnetic susceptibility.
Discussion
The obtained results confirm the high diversity of forest soil pollution around Krakow agglomeration. Significant differences in both the content of heavy metals, polycyclic aromatic hydrocarbons and values of the magnetic susceptibility were observed in the soils of forest complexes located in different directions away from Krakow and at various distances from Krakow. The level of contamination is related to the historical and current deposition of industrial and urban dusts from Krakow and Silesia. As for the magnetic susceptibility and heavy metal content, the most divergent research plots were located west and east of Krakow. The level of soil pollution in those locations was affected by climatic conditions, i.e., the mainly western direction of the prevailing winds. Pollutants transferred from the Upper Silesian Industrial Region are deposited in the west while 35 km east of Krakow are deposited contaminants from Krakow, primarily from the steel producer ArcelorMittal Poland Unit in Krakow. According to Zhang et al. (
2012), wind direction and speed might change the heavy metal contaminants of soil. Ogunkunle and Fatoba (
2014) studied the level of soil contamination by Pb, Cu, Cr, Cd and Zn and showed that these heavy metals are deposited at various distances depending on wind velocity. Additionally, they indicated that the heavy metals which are found in the soil do not only come from cement production (second works of Lafarge—Cement WAPCO factory in Nigeria) but also from road traffic. According to Kabata-Pendias (
2011), the admissible level of Cd amounts to 1 mg kg
−1, Cu to 30 mg kg
−1, Ni to 20 mg kg
−1, Pb to 50 mg kg
−1 and Zn to 100 mg kg
−1. The permissible content of heavy metals was exceeded most to the east and west of Krakow. The acceptable standards for cadmium are exceeded three times to the east and twice to the west of Krakow. The content of copper and lead exceeded the admissible level in deciduous and coniferous stands 35 km west of Krakow. The highest nickel content was determined 35 km to the east of Krakow in the soils of deciduous and coniferous stands (36.13 and 31.85 mg kg
−1, respectively). The values reported there exceeded the admissible levels. The permissible content of zinc in soils was exceeded 35 km to the east of Krakow: The zinc content in the soils of coniferous stands was 166.98 mg kg
−1, whereas in the soils of deciduous stands it amounted to 182.41 mg kg
−1. The reported exceeded values are connected not only with industry but also with the impact of motorization. One of the main traffic routes in Poland, the Medyka–Zgorzelec motorway, runs east and west of Krakow. Moreover, west of Krakow there is Balice Airport. This Krakow Airport handles about four million passengers a year. Many studies have demonstrated the potential heavy metal pollution load of airfields, such as high Pb concentration in mosses and lichens in the Helsinki-Vantaa Airport area, Finland, elevated Pb concentration in the air at Heathrow Airport, London, UK, traffic emissions around the International Airport in Delhi, India (Nichols et al.
1981; Ray et al.
2012).
A high value of
χ indicates significant concentration of ferrimagnetic minerals (Thompson et al.
1980). Magnetic susceptibility enhancement observed in topsoil samples suggests urban–industrial dust deposition as a source of TMPs in the studied area. Localization of the sampling sites in areas of significant emitters of air pollutants (i.e., the city of Krakow, the steel producer ArcelorMittal Poland Unit in Krakow, the “Łęg” power station and the Upper Silesian Industrial Region) as well as the prevailing wind direction revealed the spatial distribution of the observed magnetic susceptibility anomalies. These findings are in agreement with previous studies, e.g., by Łukasik et al. (
2016), who reported that high χ values were found in mineral (A) horizons of the forest under the influence of Krakow’s urban–industrial emissions due to the presence of the large steel producer (the steel producer ArcelorMittal Poland Unit in Krakow), located in Nowa Huta district. Moreover, many authors (e.g., Strzyszcz and Magiera
1998; Fürst et al.
2007; Blaha et al.
2008; Szuszkiewicz et al.
2016) reported that the highest contents of TMPs and accompanying heavy metals are observed in the topsoil organic (O) and mineral (A) horizons. According to previous findings (e.g., Zawadzki et al.
2009; Fürst et al.
2010), magnetic susceptibility is a suitable method for prediction of the spatial extent of heavy metal pollution in fly ash impacting forest areas.
The
χ
fd reflects the occurrence of ultra-fine superparamagnetic particles in the material (Dearing et al.
1996b); it has been applied to measure pedogenic and/or biogenic magnetic enhancement (
χ
fd > 5 %)—cf. Dearing et al. (
1996a). The values of
χ
fd < 5 % suggest a natural or anthropogenic source of magnetic particles. However, the surroundings of the study area (an urban–industrial setting) lead to a conclusion that
χ
fd ≤ 4 % measured in the examined soil samples is a result of the presence of TMPs.
The highest content of polycyclic aromatic hydrocarbons was observed to the east and south of Krakow. The most important sources of PAHs around Krakow are low emissions whose sources are coal combustion in individual home furnaces and traffic emissions. Additionally, industrial emissions (heavy industry, cogeneration plants) are PAH sources. For identifications of PAH sources, we used diagnostic ratios. Petrogenic sources and coal/biomass burning were identified. In Krakow surroundings, a lot of houses are heated using hard coal. The investigated soils are influenced by the emission from industrial activities in Krakow and Upper Silesian Industrial Region. The obtained values of diagnostic ratios suggest that the sources of PAHs could be not only industrial facilities but also vehicles. PAHs may be formed during incomplete combustion of organic materials including coal and wood. PAHs get into soils predominantly from dusts and rainfall; part of the dust becomes deposited on the aboveground parts of plants and gets into the soil after their death (Maliszewska-Kordybach
1999). Xiao et al. (
2014) noted that traffic emissions and coal burning were the primary contributors to forest soil PAHs in the Pearl River Delta. Kwon and Choi (
2014) indicated that diesel vehicle emission is a major source of PAHs. In addition, industrial emission sources, such as heavy oil combustion, coke ovens and coal burning, were identified. The type of stand had a considerable influence on the content of polycyclic aromatic hydrocarbons and the level of magnetic susceptibility. A higher content of aromatic hydrocarbons was observed in the soils of deciduous stands located to the east, north and south of Krakow. In soil situated east of Krakow, the higher content of PAHs was noted under coniferous stands. Physiological features of leaves play an important role in determination of the scavenging efficiency and retention of airborne particles on leaf surfaces (Howsam et al.
2000). The average scavenging efficiency was defined as the ratio of the number of particles deposited on the tree to the dose corresponding to the tree (Hidy
1973). Howsam et al. (
2000) noted that leaves with hairs (oak, hazel) had a higher sum of PAH concentrations than those with recessed hairs. The role of other leaf components, such as lipids and epicuticular waxes, in leaf accumulation of PAHs, which are lipophilic compounds, was highlighted. Differences between the content of PAHs in the soil of coniferous stands and the soil of deciduous stands may be related to the surface area of leaves and needles. The annual litterfall in deciduous stands increases the deposition fluxes of aromatic hydrocarbons. In addition, according to Terytze et al. (
1995), PAHs are characterized by a strong sorption affinity with respect to the soil organic matter. Yang et al. (
2014) suggest that pine needle litter-derived dissolved organic matter (DOM) can have a substantial effect of inhibiting PAH sorption and promoting PAH desorption, thus leading to enhanced leaching in soil, which should be taken into account in risk assessment of PAHs accumulated in forest soil.
The applied biochemical parameters, i.e., the enzymatic activity, correlated negatively with the content of selected polycyclic aromatic hydrocarbons and their sum. Polycyclic aromatic hydrocarbons can inhibit the development and metabolic activity of microorganisms (Baran et al.
2004). In the present study, a negative correlation was noted between urease and dehydrogenase, and the selected aromatic hydrocarbons. Urease activity correlated with the contents of Acy, Pyr, BbF, BaP, Chr and BghiP, while dehydrogenase activity correlated mainly with four-, five- and six-ring hydrocarbons (Phe, BaA, Chr, BbF, BaP, DahA, BghiP and IcdP). According to Thavamani et al. (
2012), PAHs in long-term contaminated soils could have a strong and complex impact on microbial community structure activities. It may be concluded that the differences in enzymatic activity between the research plots result from different contents of PAHs and different amount of soil organic matter as a result of species composition. The chemical composition of SOM largely results from microbial transformation of tree litter, whose composition, in turn, influences the activity and diversity of soil microorganisms (Pérez-Bejarano et al.
2010; Ushio et al.
2008). Deciduous species had a positive impact on soil organic matter and the most beneficial impact on the intensification of microbial activity. At the same time, a weaker relation was noted between enzymatic activity and heavy metal content. The inhibition of urease and dehydrogenase activity as a result of heavy metal pollution has been reported by many scientists (Chen et al.
2005; Shen et al.
2005; Sardar et al.
2010). The sequence of inhibition of urease activity was generally in the decreasing order of Cr > Cd > Cu > Zn > Mn > Pb. However, the stimulating effects of heavy metals have also been documented (Bååth
1989; Błońska et al.
2015). In the present study, a positive correlation was noted between urease activity and the content of Cd, Ni and Zn as well as between dehydrogenase activity and the content of Ni. Metals detected in small amounts in the soil are not harmful and can even stimulate certain enzyme processes. It may be the case that a metal could be one of the components of the catalytic center and may activate selected enzymes by creating a metal-substrate complex (Balicka and Varanka
1978). The described relationships concerned the soils of deciduous stands.