Introduction
Nowadays, coexistence and accumulation of trace elements in soil are considered to be very important issues in ecotoxicology [1-5]. This is because of the fact that the specific participation of individual elements’ compounds in the soil can influence the process of their cumulation in the so-called critical plant parts (usually underground organs or leaves, but also other morphological parts of the plant), and leads to their undesirable elevated levels [6-10]. As it is known, excess exposure to both essential and toxic elements either via the food chain or through direct exposure by dermal contact can be hazardous to human health [11-14].
In the process of accumulation of bioavailable compounds by the plant, a phenomenon of elements co-occurrence in the roots layer of soil occurs, and selected morphological parts of the plant will play a role [15- 19]. However, the physiological properties of plants to discriminate between selected elements, as well as the availability in the so-called toxicological phase, will have a decisive role [20].
The nature of the co-occurrence is largely determined by the geochemical composition of soil [21-23], as well as its pH, and the mobility of elements depends on the content of organic matter and the presence of manganese and iron oxides, which have strong sorptive properties [24-27]. A previously published paper studied the influence of the type of tree habitat on the character of co-occurrence of selected metals in the soil of Tatra National Park. It was reported that, because of different metal quantitative composition and their form of occurrence in the soil of habitats of three different species of trees, metal co-occurrence varied, and the biggest changes were observed in the root area of spruce. The highest content of all the elements examined in the soil from all habitats was observed in the soil form of their organic compounds.
In this paper, the contents of specific forms of nine chosen elements in the soil of urbanized and non-urbanized regions of Tatra National Park and metal co-occurrence in all four valleys, in respect to three forest complexes, are presented [28, 29].
Table 1.
Statistical characteristics of metals occurrence in selected forms in soil [µg/g] dry weight in non-urbanized and urbanized regions of Tatra National Park.
Material and Methods
6 samples (L/W/H 10 cm/10 cm/5 cm) taken from a surface layer in an open area without undergrowth were collected from each of four investigated sites located in the Tatra National Park’s valleys: Chochołowska, Strążyska, Mała Łąka, and Kościeliska (Fig. 1).
After air-drying, milling, and sieving with 1 mm sieve, each sample was subjected to sequential extraction procedure according to Rudd’s method (Rudd et al. 1988). In brief, 1 g of soil was exposed to the following solvents: - for exchangeable form: 1,0 M KNO3 ratio 1:40, - for adsorbed form: 0,5 M KF (pH = 6,5) ratio 1:40, - for organic solution: 0,1 M Na4P2O7 ratio 1:40, - for carbonates: 0,1 M EDTA (pH = 6,5) ratio 1:40, - for sulfides: 6,0 M HNO3 ratio 1:40. - for residues: 5 ml HNO3 (twice)
for 24 hours, centrifuged for 10 minutes (4000 rpm), rinsed with 25 cm³ of redistilled water, and centrifuged again for 5 mins (4000 rpm). Obtained extracts were acidified with 1% HNO3 and subjected to measurements, and the residue treated with the next solvent. Three blank tests were prepared parallelly. All the reagents used in this study were spectrally pure and delivered by Merck.
An AAS spectrometer SP9 (Pye Unicam) was used to measure analytes concentrations in extracts – each result was calculated as an average of 6 samples after subtraction of the blank. Accuracy of the measurements amounted to 0,01 µg/g, and the recovery between 97-102%. Precision was controlled by using pattern solutions produced by WZORMAT, containing Pb, Cd, and Ni within the range of concentration in soil. Used methods were validated in collaboration with the Department of Inorganic Chemistry, The Silesian University of Technology, and varied in the range from 3.8% for Zn to 5.3% for Cr.
Results and Discussion
The remarks mentioned in the introduction regarding the bioavailability of selected metals in the soil should be considered as actual resources for the flora in selected types of forest complexes in Tatra National Park. In the four examined valleys, areas dominated by different tree species were identified in the following order: fir, beech, and spruce.
The original geochemical composition of the substrate, along with the involvement of wet atmospheric precipitation and metal compounds over a long period of time, has shaped the appropriate amounts of mobile compounds of individual metals in soils in uncontaminated areas (A), compared to soils in areas directly affected by industrial emissions (B) (Table 1) [30-35].
This has resulted in the occurrence of specific coexistence patterns. The presented relationships regarding exchangeable forms, adsorbed forms, organic compounds, and carbonates were statistically significant at p≤0.05 for values of r = 0.30 – 0.45 and r = 0.5 – 0.80 at p≤0.01, as well as r>0.90 at p≤0.008, suggesting the presence of different relationships between the amounts of ions of individual elements in the soil, not only due to the nature of the given area (polluted, unpolluted), but also the chemical forms in which they occur.
In these areas, there is a characteristic interdependence between the changes in the content of Cd compounds and the changes in the content of Cr compounds. In polluted areas, this interdependence is described by a significant correlation coefficient of 0.77, while no interdependence was observed between these elements in exchangeable form in unpolluted areas. In unpolluted soils, the importance of the geochemical composition is emphasized by exchangeable forms of Mn and Co, Pb and Cd. The changes in the occurrence of exchangeable forms of Pb and Cd in unpolluted areas are described by a correlation coefficient of r = 0.84, while in polluted areas, it is r = 0.68 [36]. Similarly, the ion-exchange forms of Mn and Cd in unpolluted areas are described by a correlation coefficient approximately twice as high, equal to 0.5, compared to industrially unpolluted soils. In this regard, the common industrial origin of Cr and Pb is indicated by a correlation coefficient of 0.53 for polluted soils, whereas no such relationship is observed in unpolluted soils. This tendency in polluted soils also applies to the relationship in the occurrence of ion-exchange forms of Zn and Cd (0.78), which is not observed in unpolluted soils (exchangeable and adsorbed forms) [37]. The correlation coefficients regarding exchangeable forms, adsorbed forms, organic compounds, and carbonates listed in Table 2, confirm the distinct nature of selected interdependencies, which may affect the degree of availability of the investigated elements in their speciation forms during the toxicological phase.
A comparison of correlation coefficients also reveals the occurrence of characteristic mutual changes in the content or characteristic elements in selected chemical forms of their occurrence. The changes in the content of Mn, Cu, and PB in the adsorbed form in polluted soils, compared to unpolluted soils, are a good illustration of this observation. Another example of the distinctiveness of soils from industrially polluted areas compared to unpolluted areas is exemplified by the changes in the content of organic and carbonate forms of Zn with other investigated elements [30].
Using the example of soils subject to permanent industrial emissions and soils located at a large distance from the industrial center (80 km), it has been demonstrated that distinct chemical changes in the content of individual elements occur in different chemical forms in soils in areas with negligible industrial emissions, where sporadic long-range dust emissions occur. Such exemplary areas include the habitats of beech, spruce, and fir trees. Here, the soil in Tatra National Park (TPN) is characterized by separate correlation coefficients, and clearly different total contents of individual metals in selected forms of occurrence, roles, and contributions of compounds of individual elements in shaping chemical equilibrium are observed. This diversity deepens the different nature of interdependencies in the tree-root-soil system between individual chemical forms of elements in the TPN area (Table 3).
Table 2.
Correlations between metals in the soil in selected forms in non-urbanized and urbanized regions of Tatra National Park (p≤0.05 for r≥0.30).
Table 3.
Co-occurrence of metals in bioavailable form in the soil in habitat of selected species of trees in non-urbanized and urbanized regions of the Tatra Mountain National Park. Table 3. Table continued.
In general, the largest amount of significant directly proportional changes in element content in the soil in the TPN area concerned carbonates. When comparing the respective correlation coefficients (Table 3), the greatest orderliness in occurrence changes was related to indirectly bioavailable forms of individual metal ions. For plants in the rhizosphere, changes in the content of exchangeable forms of metal ions Pb, Cd, and Mn with Zn compounds, are of significant importance. The role of these relationships regarding the adsorbed form of Pb is much weaker (r = 0.26 – 0.36).
The specified interdependencies of a certain magnitude (Tab. 4) determine the significant chemoeconomic differences between these forest complexes in the TPN [38-42].
Mineralogical studies generally indicate the significant role of the system of Fe and Mn oxides in soils in the TPN area in the process of accumulating other ions in relation to changes in soil mineral structure (Tab. 4). This is confirmed by the results of bivariate correlation analysis describing changes in the content of Fe and Mn compounds in the beech forest complex in the Chochołowska Valley, Kościeliska Valley, Strążyska Valley, Mała Łąka Valley, and beech habitats. However, this observation was not confirmed in the spruce forest complexes in the Strążyska Valley.
Another characteristic set of elements participating in common content changes in the soil is Fe-Zn and Fe-Cu. The Fe-Zn relationship strongly occurred in soils in all valleys, regardless of the tree species composition, except for the spruce forest complex in the Strążyska Valley.
Uniform changes in Fe and Pb content (r = 0.64 – 0.95) occurred in soils in the beech forest complexes in the Strążyska Valley and Mała Łąka Valley, and in the case of beech forest complexes in the Kościeliska Valley and Strążyska Valley (r = 0.84), proportional changes in the content of Fe and Pb compounds were observed. Proportional changes in the content of Fe and Cr compounds were observed in beech and spruce habitats in the Chochołowska Valley (r = 0.74 and 0.53) and beech and spruce habitats in the Mała Łąka Valley (r = 0.65 and 0.87). In general, the greatest role of Fe compounds in relation to the compounds of individual elements in the soil was observed in the Kościeliska Valley, regardless of the type of forest complex. Significant correlation coefficients characterizing directly proportional changes ranged from 0.38 to 1.0.
Physiologically important relationships between Fe compounds and other metals, observed at the level of absorption by the underground organs of trees, were found in the beech forest complex in the Kościeliska and Strążyska Valleys [43-47]. Notably, a significant Fe and Cu antagonism in soil was observed in the habitat of beech trees in the Kościeliska Valley (-0.59). A similar antagonism between Fe and Cu ions was found in soils of the beech forest complex in the Strążyska Valley. Strong antagonistic relationships, described by values ranging from r = -0.67 to (-0.98), were observed between Fe and Ni in the spruce forest complexes in the Strążyska Valley and Mała Łąka Valley. Additionally, in the beech forest complexes in the Chochołowska Valley (-0.46) and the spruce forest complex in the Kościeliska Valley (-0.55), antagonistic relationships between Fe and Ni occurred in the soils [48]. The ions Zn, Cu, Cd, and Ni showed high affinity for Mn compounds in the beech forest complex in the Strążyska Valley. On the other hand, in the soils of the Mała Łąka Valley, Mn compounds changed directly proportionally to the variations in the content of Zn, Pb, Cd, Ni, and Cr. The simultaneous changes in the content of Mn and Zn compounds in the habitat of beech trees were only absent in the soils of the Chochołowska Valley. For comparison, a significant Mn-Zn relationship was confirmed in the beech and spruce forest complexes in all examined valleys. Furthermore, in the beech forest complex in the Kościeliska Valley, changes in the content of Mn compounds encompassed all the analyzed elements (r = 0.49), with Ni and Cu exhibiting a negative correlation coefficient (r = -0.97). The spruce and beech forest complexes in the Chochołowska and Kościeliska Valleys, as well as the spruce and beech forests in the Mała Łąka Valley, simultaneously displayed characteristic relationships between Mn and Cd ions. The changes in Co content were ambiguous in relation to Mn and Fe compounds.
In summary, it should be emphasized that each forest complex will have a specific characteristic regarding the total content of selected metal compounds. Regardless of the geochemical characteristics of the substrate, these large forest complexes can, over a long period of time, develop distinct patterns of co-occurrence of metals in the soil within the tree’s rooting zone [49]. The specific interaction of soil solution, and indirectly the soil pH, can significantly influence the accumulation of selected metals in the soil, concerning the content of Fe and Mn compounds. Consequently, this can differentially affect their content in the soil and, as a result, shape the specific co-occurrence of the analyzed elements in the soil. This specific co-occurrence often determines the mobility and bioavailability of individual elements in competition for underground tree organs as receptors [50].
In the context of these remarks and the presented statistical calculations, a significant chemical-ecological distinctiveness of Tatra National Park is evident in comparison to polluted natural environments. This confirms that each natural area should primarily be characterized by the amount of bioavailable forms of individual metals and the nature of their co-occurrence in the rooting zone within one ecosystem [51-53]. Tatra National Park exhibits a significant variation in the individual forest complexes concerning their secondary interactions and species composition of trees.
Conclusions
The mineral composition of the soils of Tatra National Park varies in different valleys and the impact of human activities in the Kościeliska Valley can be clearly seen. Elevated levels of all speciation forms of Cu, Pb, and Cr, as well as of Cd and Co in their exchangeable forms were revealed in the urbanized part of the park. It was observed that with a change in the elemental soil composition, as well as in different trees’ complexes, different relationships between chemical forms of various metals occurred.
