Sulfur (S) is a macro element necessary for the growth and proper development of plants, and its concentrations in plant foliage tissues which indicate optimal growth typically range from 0.1–0.5% of a plant’s dry weight [1, 2]. Plants absorb sulfur mostly in the form of SO^2- by their aerial (or, above ground) parts and by their roots in the form of SO₄^2- [2]. Sulfur in plants is responsible for synthesizing the amino acids cysteine, cystine, and methionine, as well as metabolites (e.g., biotin, thiamine, and glutathione)-all of which are contained in S. In case of sulfur deficiency (currently more frequent than excess, especially in agricultural areas) [3-5] plants see a reduction of root growth and leaf area, and decreases in chlorophyll and protein concentration (chlorosis) [2]. However, high/excessive sulfur concentrations also affect the root system (possibly through shifting carbon allocation) and reduce foliage. Excesses also cause leaf stomata disturbances and premature senescence, resulting in the thinning of tree crowns, deformed canopy shapes, limited growth, and a decrease in nutrients [2, 6-9]. Dust from industrial emissions settles on leaves and disturbs the processes of photosynthesis and transpiration [8-10]. Excessive sulfur deposits increase the number of nutrients leaching into the soil. For instance, the displacement of the base cations Ca²⁺ and Mg²⁺ from the sorption complex consequently leads to soil acidification. In acidic soil, trace elements harmful to soil organisms-and which subsequently disturb biogeochemical cycles and vascular plant nutrition increases-are mobilized [11]. The acidification of soil can also lead to the release of toxic Al, thereby detrimentally affecting fine root growth and inhibiting Ca, Mg, and K uptake [8]. Sulfuric pollution can result in nutrient deficiencies and increase the effects of natural stressors, such as physiological drought, frost, pests, and fungal diseases [6].

The most common source of sulfur pollution is sulfur dioxide (SO₂). Not only is SO₂ detrimental to humans, but SO₂ and its products (sulfate, sulfuric acid) affect the environment and agricultural areas on a regional and global scale (e.g., acid deposition, direct and indirect radiation forcing) [12]. SO₂ occurs naturally, predominantly from volcanic activity and biological and photochemical production in oceans’ volatile sulfur gases. It is also released, to a lesser extent, by forest fires, sulfur springs, and sea salt [12, 13]. Although natural SO₂ is assumed to account for approximately one third of total SO₂ emissions, scientists are still developing more accurate measurement methods, such as using spectrometer systems to accurately quantify both high and low SO₂ column densities [13]. Most sulfur emissions come from anthropogenic sources. It is estimated that anthropogenic sources account for more than 70% of global SO₂ emissions, half of which result from fossil-fuel combustion-particularly hard coal and lignite, which typically consist of 1.0–3.0% sulfur in the form of pyrites and marcasites. Additionally, S contamination results from petroleum refining, the smelting of sulfidic ores in the production of heavy metals, and the production of sulfuric acid, paper, and sulfur [6, 12, 14, 15].

SO₂-responsible air pollution may well be one of the most significant factors behind forest decline in the northern hemisphere [8]. Acid deposition results in a high concentration of SO₂ in the air. In 2000, the US Environment Protection Agency observed highly acidic precipitation with a pH of below 4.3. Acid depositions damage forests by acidifying soil (an approximate one-point pH decrease was noted in Germany, Sweden, China) and disturbing the mineral management of trees [16]. In 2000, China (one of the world’s largest emitters of SOx – SO₂, SO₃) estimated that acid deposition and its effects were responsible for a financial loss of 1.77% of the national GDP [17, 18]. In industrial and urbanized areas, sulfur deposition can be sufficient for proper plant yields [2]. However, in the past decade, SO₂ emissions have decreased as a result of EU regulations. The EU introduced SO₂ emissions limits (published in their directives) for sulfur contents in certain liquid fuels [19]. Consequently, energy-related sectors have switched from high-sulfur solid and liquid fuels to low-sulfur fuels, such as natural gas, and have established industrial facilities to implement abatement technologies for flue gas desulfurization. Europe has benefited from these actions. Indeed, from 1990 to 2011 (last published data, actualized in 2015) [20], SOx emissions have decreased by 74% (Figs. 1 and 2) [19]. The EU-28 Member States-as well as Liechtenstein, Norway, and Switzerland-have reduced their national SOx emissions below the level of the 2010 guidelines set by the National Emission Ceilings Directive (NECD) [19].

While still the world’s second largest emitter, China has also achieved a significant reduction since 2007 (from 36.6 to 8.4 megatons as of 2016) [18]. In 1996, China first included the control of SO₂ emissions in its National Five-Year Plan (FYP), which required that SO₂ emissions in 2000 should be under 24.6 Mt [17]. According to the recent Air quality in Europe Report [21], SO₂ concentrations are generally well below the limits for the protection of human health, although daily excesses of the WHO daily mean guidelines persist. Realizing that climate change and environmental degradation pose a threat to Europe and the world at large, the EU is currently furthering its strategy through the so-called “Green Deal.” This strategy assumes that net-zero greenhouse gas emissions will be achieved by 2050 (through the decarbonization of the energy sector), economic growth will not depend on resource use, and hopes for fair action in this context (i.e., no person or region will be left behind).

Fig. (1). Changes in SO2 emission trends in the EEA-33 and EU-28 group of countries. Also shown for the EU-28 is the 2010 NECD and 2020 Gothenburg target. Source: European Environment Agency (EEA)- national emissions reported to the Convention on Long-range Transboundary Air Pollution (LRTAP Convention) [20].

Fig. (2). Changes in SOx emissions by European country between 1990–2011, according to the 2010 NECD and Gothenburg protocol targets (2020 Gothenburg Targets scaled from 2005 base year to show percentage reduction from 1990). Source: EEA-National emissions reported to the LRTAP Convention [20].

Sulfurous and acidic soils occur most frequently in industrialized regions, most especially in the USA, Canada, and southern China [22]. Areas most often associated with abnormally high sulfur concentrations (of above 0.8%, or pH levels of 1.7–3.5) tend to be located in the vicinity of sulfur mines, sour gas processing plants, and former lignite coal mining sites [16, 22-25]. Furthermore, areas affected by lignite mining, connected with pyrites and marcasites, and iron and gold mines, are often connected with Fe, Al, or Fe₂(SO₄)₃ as well as with sulfur contamination [23, 26, 27]. However, areas with high sulfur concentrations are relatively rare-indeed, the reverse trend in agriculture has been observed, but the lack of control over the industry responsible for the introduction of sulfur to soil may well result in significant environmental transformations. In order to effectively restore the environment (or ecosystem), each ecosystem should be examined comprehensively (soil, water, vegetation, and fauna, as well as their components).

Examples of soil heavily contaminated with anthropogenic sulfur (to a globally unprecedented degree) would include areas where sulfur deposits have been mined using the Frasch borehole process (the underground melting of sulfur). This method was used in sulfur mining in, among other areas, the US (until 2000) and Iraq (until 2003). The process is still conducted in southern Poland at the Osiek sulfur mine [25]. The Tarnobrzeg region of southern Poland has one of the largest known sulfur deposits in the world (Piaseczno-Machów-Jeziórko-Jamnica deposit). Sulfur has been mined there since the 1950s, first through the opencast method, and later by the Frasch process [28]. Restoring the (sulfur-contaminated) areas exploited by the Frasch process is fraught with difficulties, yet such areas provide excellent case studies for research on the geochemical changes of mineral sulfur in the ecosystem and the reaction of microorganisms and plants to excess concentrations of this element in the soil.

In this chapter, two important issues for global sulfur emission and its environmental effects, namely natural or industrial SO₂, and sulfur contamination hot-spots (excessive sulfur in soil, and surface and water contamination) where vegetation must function under environmental stress was exemplified. Research into these issues is vital and can provide a unique opportunity to supplement the knowledge concerning ecosystem reaction to environmental stress in the broad context of assessing the dynamics of woodland restoration.

One way to assess air pollution is to measure the accumulation of contaminants in the tree assimilation apparatus of plants [29-31]. Studying the content of a contaminant in plant tissues is known as bioindication. Bioindicators can be species characterized by long life cycles and tend to have specific reactions (both constant and repeatable) to particular changes in the environment. The most positive features of biomonitoring plants are their slow growth, lack of epidermis (e.g., lichens), their status as perennial organisms, and their characteristic ability to accumulate elements exceeding their nutritional requirements [32]. Moreover, their photosynthetic apparatus is sensitive to air pollution, meaning that pollution levels are easy to observe [31, 33]. Plants take in air pollutants through the absorption of particulate matter by the leaves and the deposition of aerosols [33, 34]. Among biomonitoring, the European blueberry (Vaccinium myrtillus L.) is one of the most commonly used in monitoring air quality and its pollution. Mosses and lichens are also commonly used in biomonitoring due to their lack of roots (compared to higher plants), meaning that they collect pollution predominantly from the air [35]. The main advantage of using trees in biomonitoring is their longevity, thereby enabling a systematic repetition of the investigation using standardized sampling and analytical techniques for comparative monitoring [14]. Scots pines (Pinus sylvestris L.) are frequently used in the bioindication of SOx air pollution [36, 37]. The measuring of air pollution (including sulfur and heavy metals concentrations) using bioindication, and more precisely pine needles as bioindicators, was recommended by the United Nations Environmental Program as a standard environmental assessment method [14]. Also, other institutions such as ICP-Forests, State Environmental Monitoring use and recommend plant bioindicator as an alternative to (the more expensive) technical monitoring methods [35]. Pinus sylvestris L. meets the biomonitoring criteria because of its low ecological tolerance to sulfur compounds contained in the atmosphere, as well as its wide geographical range and habitat spectrum [14]. Additionally, Scots pines are evergreens and thus accumulate air pollution over several years. Further to this, their age can be easily determined [37]. The worldwide presence of pines further qualifies them as effective biomonitors as they allow for a global understanding of pollution levels and compare studies conducted on one species [30]. Gaseous pollutants enter the leaf mesophyll, enabling the assessment of the degree of SO₂ air pollution based on the S value concentration in leaves or needles. Pine needles have also widely been used to determine the long-term pollution levels of PAHs, PCBs, OCPS, POPs, trace elements, and sulfur [37].

Based on the sulfur concentration found in pine needles, the current SO₂ air pollution in Poland, as influenced by anthropogenic pressure, has been able to be determined. Pine stands in 295 monitoring sample plots (located in afforested sites in Poland) were measured. On each plot, samples were collected in 3 soil horizons (0–10cm, 10–40cm, 40–100cm), soil organic layers (+5-0 cm), and Scott pine foliage (current-year needles). The map of spatial variability of S in Poland was thus developed based on the S concentration in pine needles. The results showed that the mean S concentration in pine needles was 854.8 mg kg^-1 (ranged from 537.5 to 2,343.2 mg kg^-1) in dry mass. The pine concentration in the studied habitats showed a very poor supply of sulfur, i.e., grade I (from 660–990 mg kg^-1) when using Heinsdorf’s (1999) assessment criteria for the ranges of tree nutrition [38]. It should be remembered that Heinsdorf [38] provided values for the supply of trees in postindustrial sites. Other authors, such as Baciak et al. [36], provided supply levels of Scots pine needles in less industrialized areas (characterized by lower levels of air pollution) as being between 300 to 1,200 mg kg^-1, and from 6,000 to 10,000 mg kg^-1 in highly industrialized areas. ICP-Forest data indicated S content in pine needles in Europe ranges between 790 mg kg^-1 to 1,520 mg kg^-1 in current-year needles, and from 1,050 mg kg^-1 to 1,470 mg kg^-1 in 1-year needles. According to Fober et al. [39], the nutrient supply of trees in natural growth conditions shows normal growth and development (without deficiency symptoms) when S content in photosynthetic apparatus is 300 mg kg^-1 S in dry matter. As such, the levels found in this case study seem to be sufficient and, somewhat crucially, not a cause for alarm. However, based on the study, it is easy to notice that Poland contains areas where there is clearly a higher sulfur concentration in the pine assimilation apparatus. The highest sulfur content in pine needles was noted in western Poland (from 1,300 to 2,300 mg kg^-1 S-sulfur supply in these areas is, according to Heinsdorf, low or sufficient [38]). High concentrations of S in pine needles were recorded mainly in highly urbanized areas-for instance, near Poznań, Wrocław, and Olsztyn (1,321 mg kg^-1 S). This was followed by highly industrialized areas in the vicinity of SO₂ emitters related to the mining, processing, or energy and mining industries. As seen in (Fig. 3), this would include areas in the Upper Silesia region, such as Tarnowskie Góry, Siewierz, and the mining complex at Ostrów Wielkopolski KGZ Żuchlów.

From the above, it was concluded that the level of SO₂ in the air-information on which was obtained through the S concentration in pine needles-is fortunately unalarming. Moreover, based on the research conducted, it would be possible to determine areas with higher S content in pine needles-and therefore higher emissions. Likus-Cieślik et al., (2019) [40] assessed the content of S in pine needles in the context of measuring the improvement of air quality with previous research conducted in the 1980s. They found that the current air quality has improved compared with the study period. There are many possible explanations for this improvement, including EU regulations on fuel transitions, such as shifting to natural gas, and enhancing or developing technology aimed to reduce flue gas desulphurization in industrial facilities.

SO₂ emissions are the most prevalent and widely researched sources of sulfur in the environment, which, thanks to all of the introduced regulations and restrictions imposed on EU countries (and others) have resulted in a reduction of SO₂ air pollution. These observations were confirmed in our studies with the use of Scots pine as a bioindicator, indicating air pollution with this gaseous compound. This was confirmed by the studies we conducted in 2015-2016 (the years when the samples of needles were taken). The results of our studies were compared to the available data from the 1980s - we found that current air quality has improved compared with the previous study period. It can be concluded that the reasons for the improvement of the air were the regulations concerning the emission of gaseous parts, mentioned in the introduction in many countries, and the gradual process of abandoning fossil fuels (limiting coal extraction), but sulfur (S) is still an important element in the environment. In the previous century, S was an element that marked the combustion of fossil fuels, and currently, there is a significant improvement in the quality of the environment in terms of the presence of this element in the air. The highest sulfur concentrations were observed not in areas related to the extraction of even coal, but in highly urbanized places, near large urban agglomerations.

However, on the regional scale, even though these areas are not so significant in the area, the presence of sulfur and its high concentrations are an example of a long-term negative environmental effect of previous exploitation epochs. In spite of great efforts and outlays in the implementation of reclamation treatments, borehole sulfur extraction in the studied area has led to the emergence of hot-spots of very high concentrations of sulfur in the soil (as high as 45,000 mg kg^-1), low pH (<2.0), and EC levels of even 2,080 μS cm^-1. These properties undoubtedly impact aquatic ecosystems, which are characterized by a very high content of sulphates (geological origin-native sulfur ores) and calcium ions (as a result of the use of post-flotation lime/sludge lime in the reclamation process). Despite high concentrations of sulfate ions, acidic reactions (pH below 5) in the waters, possibly due to the aforementioned sludge lime, were observed. The chemical parameters of soil also impacted vegetation-in highly sulfurous or acidic areas, herbaceous vegetation, trees, and shrubs were either very rare or entirely absent. Such surfaces (bare soils) are termed degraded surfaces and tend to be classified as hot-spots. However, while these surfaces occupied only 2% of the scale of the tested object, we found them to significantly (and detrimentally) affect the soil through the undesirable reactions of vegetation. An interesting reaction of plants to excessive sulfur concentration is the strategy of blocking pollutant uptake from the soil (true exclusion or blocking of elements). This strategy is widely adopted by wood-small reeds, which are resistant to environmental stressors and, accordingly, could be used in the reclamation and remediation treatments of such soils typically displaying elevated salinity (or, EC) and sulfur content. The growth of trees in afforested areas indicated that, after careful acidity neutralization, newly introduced herbaceous plants and trees were characterized by relatively positive growth parameters and viability. A significant decrease of sulfur content in highly contaminated environments as a result of rinsing is certainly possible, but this is a lengthy process that cannot be achieved without neutralizing the soil detoxification effect. The controlled experiment and microbiological studies have shown that the soil substrates contaminated with sulfur are almost sterile and that the detoxification process on the most heavily contaminated soils will be extremely time-consuming. It is vital to make a thorough spatial reconnaissance of an area and efficiently plan activities before reclamation is undertaken. Moreover, one should also consider possible difficulties and obstacles related to further management of the area once reclamation has been completed. The extremely polluted by S area are a variety of ecosystems, and the effects of sulfur (the load that enters the environment) are difficult to predict, which makes such ecosystems very interesting, offering wide opportunities for researchers. At the moment, we know that the presence of sulfur strongly depletes these areas, affecting the vegetation, groundwater, and surface waters of the area. It would also remember that soil-particular its protection-should be treated as a matter of priority because soil, apart from its important role within an ecosystem (the environment of micro- and macro-organisms, a reservoir of many nutrients, etc.), is a protective filter against water contamination and the easy release of volatile elements into the air. Hence, heavily transformed areas should be subject to continuous monitoring, even though the restoration of a given area is considered completed.