1. Introduction
Podzols are the direct effect of a podzolization process, which assumes the release of iron and aluminum oxides, as well as complexes of organic acids from the eluvial horizon (albic material) downward the soil profile, into the illuvial (spodic) horizon [
1,
2,
3,
4]. The podzolization process reveals different intensity, especially in mountain areas, where many factors may influence its rates [
2,
5].
Formation of Podzols in the mountain areas is mainly driven by cool and humid climate conditions, topography, as well as specific vegetation [
1,
3,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. It is widely accepted that coniferous forests promote the formation of proper Podzols [
6]. According to Zwanzig et al. [
15],
Ericaceae and some oak species may increase the podzolization rates. Furthermore, Musielok et al. [
10] stated that the combination of berry (
Vaccinium sp.) and mosses (
Polytrichum strictum and
Sphagnum sp.) might facilitate the podzolization process even on calcareous materials.
Together with the leaching of organic and inorganic substances, except Fe and Al, other trace elements are involved in the transport within the profiles of Podzols and accumulate in the illuvial horizons [
16]. Different rates of the podzolization process under various vegetation covers may be expected, thus intensity of trace elements mobilization in spodic horizons can vary among Podzols [
4]. Nonetheless, deep storage of trace elements, originating from natural sources in the subsoil of Podzols, is a fact, and may involve storage of pollutants.
Mountain soils, in general, are very vulnerable to accumulation of contaminations as they form “orographic barrier” that limit and stop air masses, which might lead to increase retention of pollutants, such as trace elements [
17,
18,
19]. Many studies point out that the southern mountainous part of Poland is extremely exposed to contamination, although it is not situated in the center of pollutant emissions. The main reason of anthropogenic pollutions is long-distance transport, including transboundary sources [
19,
20]. The example of such area, highly exposed on the trace elements pollution, are Tatra Mts.—the highest mountain range in the Carpathians. Some studies, based on various types of soils in Tatra Mts. and surrounding areas, confirmed the pollution with trace elements, e.g., [
18,
21,
22]. These authors reported that high, unnatural values of trace elements could be caused by various anthropogenic activity from industrial plants, mine, and areas of intensified transport that might be located in the Upper Silesia region (to the west from Polish Carpathian), but also related with many industrial facilities on the Slovak side of Tatra Mts., mostly mining activities [
23]. Similar findings and proof of long-distance pollutants transport was noted by the authors dealing with the soils in the surrounding regions of Tatra Mts., but also in other Carpathians ranges, e.g., Little Beskids [
24], and in the Sudety Mts., e.g., Karkonosze Mts. [
19].
In view of the above, it seems that soils in Tatra Mts. are exposed to the accumulation of trace elements. The assessment of pollution in mountain areas is very important. Up until now, there have been few, detailed analyses based on pollution indices on soils from Polish Carpathians Mts., e.g., [
25]. According to many authors, it seems reasonable to track not only the total content of trace elements, but also use favorable pollution indices [
26,
27,
28] to comprehensively evaluate the degree of contamination, or its lack, in the various ecosystems [
28,
29]. They may reflect the dynamics of pollution changes that seem important in the context of the functioning of sensitive environments [
20,
30].
The influence of the podzolization process on the deep storage of trace elements in the soil profile, as well as the influence of external factors, such as human activity, have never been comprehensively combined in one research studyso far. Therefore, we decided to (i) estimate if the podzolization process of soils under various vegetation covers leads to the deep storage of trace elements in the subsoil, and (ii) assess the potential contamination of studied soils with trace elements using pollution indices.
4. Discussion
In the studied soils, the evident translocation of various forms of Fe and Al, which is typical for podzolization process [
49,
50,
51], was confirmed by three indicators. First, the podzolization index (Al
o + 0.5 Fe
o) exceed values higher than 0.5% (
Table 3), indicating the clear development of a spodic horizon [
40]. These features correspond with requirements of World Reference Base for Soil Resources [
40], allowing these soils to be classified as Podzols (
Table 1). Furthermore, the molar ratio of organic carbon to the sum of Al
p and Fe
p (C
p/Al
p + Fe
p), met the requirements for spodic horizon given by Mokma and Buurman [
2], which have to be with a range of 5.8 to 30 (
Table 3). Statistically, the podzolization indices did not indicated significant difference in terms of rates of this process itself (
Figure 4); however, all of them confirmed the occurrence of this process in studied soils.
The podzolization process in studied soils is supposed to be strongly connected with vegetation cover [
1,
9,
10,
14,
52]. However, the pedogenic indices did not point unequivocally under which plant habitats the podzolization process was more advanced. Highest values of podzolization index as well as Al and Fe extracted with pyrophosphate solution (
Table 3) were found in the soils under
Plagiothecio-Piceetum (tatricum). This finding is in good agreement with data of Nikodem et al. [
9] who stated that podzolization intensity is higher under coniferous forest. Similarly, Zwanzig et al. [
15] reported that the podzolization process is more intensive under coniferous forest or coniferous-mixed than under grass-herb cover or deciduous vegetation. Moreover, Lundström et al. [
1] noted that Podzols development may be favored by coniferous and ericaceous shrubs; thus, also other advantageous vegetation conditions might lead to increased podzolization. Of course, we should keep in mind that the vegetation cover in the Tatra Mts. change over the decades and hundreds of years. For instance, vast areas where
Hieracio (vulgati)–Nardetum and Vaccinium myrtillus are common nowadays, have been overgrown by coniferous forest in the past, and there were (at least partially) either spruce or ecotone communities present, which favored podzolization [
9,
53].
The trace element distribution in studied soils should indicate the pattern typical for Podzols, clear peak of trace elements accumulation in B horizons. Nonetheless, the studied shallow mountain Podzols did not always show this trend so clearly, mostly because these horizons represent the deepest (BsC horizon) (
Table A1). Thus, in such situations, it is difficult to estimate the exact point (horizon) where the enrichment of trace elements occurred as a direct result of podzolization. However, considering the site-specificity of mountain soils, high abundances of rock fragments and dense soil horizons cause limitations to percolation [
1,
4]; these may be a possible factors that hamper maximum depth of podzolization [
54]. Consequently, trace elements in the BsC horizons of the studied soils should indicate the maxima related to the podzolization process itself [
48].
Analyzing the trace element arrangements in studied soils, it can be noted that, in most profiles, podzolization influenced the accumulation of Pb, Zn, Ni, and less Cr (
Table 5) in the Bs and/or BsC horizons that met the criteria for a spodic horizon. In those profiles, the effect of the trace elements distribution is rather undisturbed, and their deep storage occurs as a result of podzolization (
Table 4) [
1,
55]. Although, the difference between the content of trace elements in eluvial and illuvial horizons according the Mann–Whitney was statistically insignificant (
p > 0.05), it was the tendency, especially the case of Pb, Zn, Ni, and less Cr, their higher content in the illuvial horizon (
Table 5). This may suggest the deep storage of these elements. However, we should keep in mind that their high contents in the subsoil might also be an effect of local lithology (e.g.,
Figure 3e). This is especially a case issue in soils developed from slope deposits where we expect multiple sources of trace elements, e.g., in the lowermost horizons, not related to podzolization.
Nonetheless, the studied Podzols also showed other patterns of the trace elements distribution, which for the purposes of this work were called “anomalies”, and represent deviations from the general trend triggered by podzolization process. Anomalies in studied soils were distinguished with regard to the distribution of trace elements that should be present in the podzolization process. In the traditional arrangement within Podzols profiles, two maximum values of trace elements concentration should be observed—in the O horizon, where natural bioaccumulation occurs, and in Bs horizon, where the enrichment with trace elements may be expected due to translocation [
4,
16]. Where the content of trace elements in the Bs horizon exceeds the maximum content of trace elements in the O horizons, this means that the effect of podzolization itself cannot be longer considered.
However, because the content of trace elements is much higher in the BsC horizon compared to the upper O horizons, it is rather doubtful that the entire pool of trace elements was leached and accumulated in the illuvial horizon. The trace elements probably have not been washed out entirely, and their displacement from Olf/ Oh into Bs horizons was not related with podzolization only. It thus seems right to conclude that it is not the pedogenic factors, but the lithological factors that have a greater effect on soil enrichment with trace elements [
56,
57]. It is reasonable to state that at least partially the source of trace elements in BsC horizon consists of the parent rock with varying degrees of weathering or representing sudden changes of lithology [
58,
59].
The parent material of the studied Podzols, e.g., Pleistocene moraine deposits, granite gneisses and sandstone slope deposits, could be reworked by a glacier [
60]. Such mixed deposits, with different degree of weathering, could be the main cause for the presence of geochemical anomalies reflected in specific trace elements distribution, not connected with podzolization. However, the input of trace elements coming from weathering of the lowermost soil horizons and those being translocated due to podzolization is hard to measure [
57], especially in case of low concentration of trace elements in topsoil compared with the subsoil (BC horizons). Such anomalies were present in soils under each of vegetation types, e.g., Zn content in soil P1 (
Plagiothecio-Piceetum (tatricum), and soil P8 (
Vaccinium myrtillus), Cr content in case of soil P11(
Hieracio (vulgati)–Nardetum,
Figure 3e,
Table 4)
The studied soil represents also other unusual trends of trace elements. Besides the two main trends, such as (i) traditional distribution resulting from the podzolization process (
Figure 3a), and the (ii) enrichment of Bs/BsC horizons due to parent material weathering (
Figure 3e), three additional cases could be considered in this study. Some of the soils reveal (iii) decrease of the content of trace elements with soil depth (
Figure 3b), (iv) quite homogenous distribution of trace elements (
Figure 3c), and (v) enrichment in E horizon (
Figure 3d). As far as (iii) is concerned, the decrease of trace elements down the soil profile (
Figure 3b) is quite common, especially due to bioaccumulation or when the high accumulation of trace elements in the surface horizons is considered as the result of the anthropogenic activity [
27,
29].
The other above-mentioned cases—homogenous distribution and enrichment of trace elements in the E horizon—could be related mostly with the erosion and translocation processes of soil material, since this could disturb their natural accumulation in Bs/BsC horizon. These results are consistent with the data in Zhu et al. [
61], where the relationship between topography and the distribution of some trace elements in the soil profile was noted. Similarly, according to Henkner et al. [
62], slope erosion triggered redistribution of trace elements what is well visible in those two cases of anomalies as mentioned above (
Figure 3c,d).
It was reported that the effect of podzolization on trace elements distribution may be additionally controlled by plant communities [
63,
64]. Generally, the greater potential for the transfer of Fe/Al-organic complexes and more decomposed organic matter in the soil, the easier translocation of trace elements down the soil profile [
64,
65]. Simultaneously, sites with coniferous forests should be less enriched by trace elements than grass and blueberry habitats, because one, they decompose more slowly and two, with a cover made by upper branches of a coniferous tree they can constitute a natural protective barrier.
Thus, in view of the above, the studied soils developed under
Hieracio (vulgati)–Nardetum and
Vaccinium myrtillus ecosystems should reveal a higher accumulation potential of trace elements in the spodic horizon, since the organic debris from these plant communities can degrade faster than, for example, coniferous forests [
63]. However, in this study, no direct relationship was found between the vegetation type and trace elements translocation. Similar results were obtained by MacDonald and Hendershot [
57] in a study over Podzols in northern forest ecosystems (Rouyn-Noranda, Canada). The accumulation of trace elements in all of the studied soils has more or less the same intensity in the Bs/BsC horizons, which was also well visible on “whisker plot” (
Figure 6) diagrams for Zn, Pb, Cd, showing large spreads between the minimum and maximum values and the median (
Figure 6). However, the “whisker plot” for Pb of Bs horizons of soils under
Vaccinium myrtillus (
Figure 6) showed a different relationship—the spread was lower compared to other habitats—but this could be influenced rather by very local conditions, e.g., microrelief or the deposition of Pb from the atmosphere [
19].
On the other hand, Zn unexpectedly showed the greatest accumulation in the spodic horizon in soils under the coniferous forest (
Table 4). This can be related to the possible change in the vegetation cover resulting from the gradual disappearance of coniferous forests over the years [
52]. Because of this, the inflow of substances acidifying the soil as well as leaching within the solum could have stopped [
66]. The remained litter can be overgrown, by, e.g., grass and shrubs, and often favors an increase in trophic index through that returning more calcium, magnesium, nitrogen into the soil that further is subjected to accelerated mineralization [
66]. As a result, the trace elements can be theoretically activated and able to percolate into the lowermost soil horizons [
48,
57].
Moreover, the highest content of trace elements was observed in the organic horizon (
Table 4) regardless of the plant habitat [
63,
64,
67]. This is not unexpected, since trace elements are prone to strong accumulation in litter [
67]. In this study, in the case of
Vaccinium myrtillus, it can be seen that with increasing altitude, the content of trace elements in O horizons was also higher, see, e.g., Zn, which may be influenced by the deposition from external factors and have at least traces of some anthropogenic activity [
30,
67]. Similarly, the content of Cd in Olf horizon of P10 also might suggest anthropogenic input, since very high values were noted (4.28 mg∙kg
−1,
Table 4). The anthropogenic origin of Cd was also visible on PCA diagram (
Figure 4). It was noticeable that Cd arranged in a completely different part of the chart, what is in good agreement with the assumption that it has a different origin than another studied trace elements (
Figure 4). Considering such high values, the wet deposition of trace elements from long-distance transport seems reasonable [
20,
68].
Many authors have noted that mountain areas are susceptible to retaining pollution, often containing trace elements [
16,
19,
69,
70]. The Tatra Mts. are exposed to airborne pollution from the local and the distant sources, thus it was assumed that the studied soils could hold higher contamination levels [
17,
18,
71]. For example, studies of Miechówka et al. [
18] showed an excessive concentration of Cd, Pb, and also Cr and Zn in non-forest soils from TPN. Similarly, Grodzińska et al. [
72] also reported that the TPN had one of the highest levels of pollution with Cd, Cr, Ni, Pb, Cu, and Zn among the Polish National Parks.
However, when analyzing the content of trace elements, pollution cannot be clearly demonstrated. The values of trace elements were rather low when considering potential contamination and only showed differences between individual soil horizons (
Table 4). Based on that, it seems that Tatra Mts. is unaffected by pollutants compared to other selected areas of mountainous and upland National Parks in Poland, e.g., Ojców National Park, Babia Góra National Park, Pieniny National Park, Karkonosze National Park, Roztocze National Park [
20,
30,
72,
73]. Furthermore, also analyzing other data concerning the mountainous areas, from, e.g., Ghazaryan et al. [
74], the studied soils indicated very natural ranges of values.
To analyze the potential pollution of studied soils more comprehensively, a set of pollution indices was calculated, which could indicate even the smallest potential ecological risk and allow the determination of the cause of possible enrichment with trace elements [
27,
28]. The indicators, except a few horizons marked as yellow, orange, and/or red (
Figure 5a,b), confirmed that the studied soils were not contaminated (
Figure 5a,b).
Based on I
geo and PLI, only soils P8 and P9 showed pollution, however locally (
Figure 5a), especially with Cu (P8 and P9) and Zn (P9), but it is the result of inheritance of trace elements from parent material and cannot be strictly connected to anthropogenic activity [
28]. The RI did not indicate any potential ecological risk, whereas only the CSI values suggested low severity (
Figure 5b) [
28]. Quite similar results were shown by Stobiński and Kubica [
71], who noted that the soils of five major valleys of the Polish part of the Tatra Mts. (Chochołowska, Kościeliska, Suchej Wody, Rybiego Potoku, and Bystra Valleys) indicated the incidental occurrence of high concentrations of trace elements and stated mainly a natural origin for any enrichment. However, the obtained data are in contradiction with Mazurek et al. [
20,
30], Pacyna and Pacyna [
75], and Steinnes et al. [
76], who reported that even soils located far from direct emission sources can be strongly contaminated as the result of anthropogenic activity. Nonetheless, such low values of trace elements in the studied soils may be an effect of research site localizations. Perhaps the soil profiles were located within an topographic (mountainous) barrier, below the main ridge in an isolated position on the slope, that protects the studied area, and trace elements–rich pollution could not reach and accumulate on the surface of the studied soils [
19,
30].
Contrariwise, the pollution indices themselves may not be suitable for Podzols, where the translocation of trace elements via podzolization into deeper soil horizons additionally raises trace element content. This could be an impediment both for geochemical background calculation but also for pollution indexes estimation. Therefore, the calculated pollution indices could not always give the appropriate values. By the fact that the expected deep storage in the studied soils occurs very locally (
Figure 3a,e), a significant role in the heterogeneous distribution of trace elements is played rather by the differentiation of the parent substrate and the anomalies discussed above—abrupt decreases and increases of trace elements contents, not related to the prevailing soil process (
Figure 3e). It should be noted that Podzols require a specific approach and, above all, specific diagnosis on potential trace elements contamination. However, there are no other reports in the literature indicating that in case of soils under podzolization, the application of pollution indices may misinterpret the degree of pollution. Moreover, based on the obtained total content of trace elements, as their values are rather low and did not suggest pollution, the values of the calculated pollution indices (
Figure 5a,b) seemed to be reasonable.
Yet, pollution indices did not show variation in trace elements content in soil under different vegetation cover. No trend was visible, suggesting that the contamination degree could be potentially higher under a specific vegetation cover (
Figure 5a,b). The obtained pollution indices data mostly suggested a similar degree of pollution. Only soil P1 under
Plagiothecio-Piceetum (tatricum) and P9 under
Hieracio (vulgati)–Nardetum were distinguished from the rest of the studied Podzols (
Figure 5a,b); nonetheless, this did not uniquely determine any relationship between indices of pollution and different type of vegetation cover.