4 Discussion
4.1 Influence of Mg-rich alkaline dust deposition on chemical properties of affected soil
Calcium and magnesium are important macronutrients necessary for all living organisms. However, problems might arise due to not only their shortage, but also their excess. Excess of both macronutrients has a negative effect on plants through increased pH, reduced availability of many micronutrients, and also heavy metals (Balakrishnan et al., 2000; Guo et al., 2016).
The natural regional background content of total Mg in topsoils unaffected by alkaline deposition occurs within a range of 9.1–15.2 g kg-1 (Čurlík & Šefčík, 1999). Only 3 out of 14 sampling sites (sites 3, 5, and 14) corresponded to this range, while the others contained high to extremely high total Mg concentrations as result of anthropogenic enrichment.
Available Mg far exceeded very high content for texturally medium soils (> 0.255 g kg-1) at all sampling sites, even at sites 13 and 14, which according to the data referred by Turčan Consulting (1992) were minimally affected by Mg-rich, alkaline dust deposition. However, sampling sites 13 and 14 are located in the direction of predominant winds, behind the magnesite processing factories.
Results achieved in this study indicated that Mg-rich, alkaline dust caused long-lasting soil degradation. The evidence is the relationship of our results with the findings of Turčan Consulting (1992), who during 10 years (1980–1990) measured the deposition of alkaline dust in the affected area. Locations assigned by Turčan Consulting (1992) as having the highest dust deposition (> 41–25 g m-2 30 days-1), corresponded to sampling sites 6–9, where still nowadays (after 40 years) we found the highest Mg total but mainly Mg available (Figures 1 and 2). These sampling sites also had the highest pH values and carbonate content. Unfortunately, since 1990 no detailed spatial research of alkaline dust deposition in the affected locality has been carried out.
According to Hronec (1992), natural leaching in the soil-climatic conditions of Slovakia can reduce total Mg content in soil on a yearly basis by 26–34 kg ha-1, provided that additional Mg-rich alkaline dust does not enter the soil. However, Brozmanová (2018) stated that there are still up to 20 tons of particulate matter yearly emitted into the environment from magnesite processing factories every year. However, this quantity represents only 0.25% compared to the situation in 1970, when 7,846 t year-1 were emitted. These values have proven that adopted dust reduction measures are more effective. Conversely, Bobro & Hančulák (1997) stated that although there is no longer a massive supply of magnesium to the soil, the supply is still active and it is likely that soils will not be able to get rid of the excess of this element through natural processes.
Before the intensification of production in magnesite processing factories, the initial pH of local topsoil was 5.5–6.5 (Hronec et al., 1992). At present, in deteriorated areas, neutral to alkaline soil pH prevails (Figure 2d). Since increased pH and carbonate content more or less copied the localities heavily loaded by alkaline dust deposition and correlated with the Mg content in the soil, it can be concluded that in addition to MgO, Mg(OH)2, 4MgCO3.(MgOH)2.4H2O, soil degradation is also dominated by MgCO3, i.e. magnesite. Our assumption was confirmed by data published by Baluchová et al. (2011), who investigated the mineralogical composition of dust fallout from 2006–2008 in the Jelšava region. They identified magnesite as the dominant mineral (>60%), while periclase had variable content, dolomite presented <10%, and calcite <5%. Furthermore, Baluchová et al. (2011) stated that beside the magnesite processing plant, an important source of magnesite in alkaline dust could be mining, as well as abandoned surface mines. Conversely, the chemical composition of alkaline dust fallout reported by Šály and Minďáš (1995) showed a 35%–50% dominance of amorphous MgO, and 10%–20% of other minerals (periclase, dolomite, and calcite). This information showed that the chemical composition of alkaline dust has changed over time, as confirmed Baluchová et al. (2011). They reported that a decreasing proportion of periclase and an increasing proportion of magnesite in dust particles indicate that dust-reduction measures in Jelšava and Lubeník are effective.
Considerable spatial differences in the concentration of Zn, Cu, Pb, and Ni at studied sites (Figure 4) might be due to the changing atmospheric pressure and other meteorological factors during the deposition of alkaline dust in the soil. The quantity of total and available forms of analyzed heavy metals was lower than limit values reported by the U.S. Environmental Protection Agency (1993). Therefore assayed soil was classified as unpolluted.
In studied locality, no significant linkage between Mg and monitored heavy metals were found. However, during the processing of magnesite, trace amounts of some elements (Cu, Ni, As) were emitted together with Mg emissions into the atmosphere (Hronec et al., 1992). Potentially toxic elements (Zn, Cu, Cr and especially Mn) are directly bound to the emitted dust and pollute soil and other components of the environment (Fazekašová et al., 2017). Hančuľák & Bobro (2004) reported that in 1999, the alkaline dust in Jelšava contained 394,500 ppm Mg, 13,100 ppm Ca, >1 ppm Cd, 75 ppm Cu, 5 ppm Ni, >1 ppm Pb and 400 ppm Zn. Increased concentrations of Zn, Cu, Cd, Ni can be attributed mainly to alkaline dust fallout, but also to fuel oil used in the past.
The mobility and availability of metals are influenced by number soil properties and processes. Despite not all of them are equally important for each metal, but some properties are of greater importance than others, in particular: the quantity and quality of organic matter, soil texture, pH, sorption capacity, the forms in which cations occur, oxidation–reduction potential, and the activity of microorganisms and concentrations of macro- and micronutrients (Ashworth & Alloway 2004; Chojnacka et al. 2005).
4.2 Influence of Mg-rich alkaline dust deposition on soil organic matter and enzymatic activity
In soils affected by high amount of Mg-rich, alkaline dust deposition, the microbial activity, biomass production is limited, original vegetation is replaced by vegetation resistant to high alkalinity, Mg concentration, unfavourable and macro- and micro-nutrients ratio (Kautz et al., 2001; Blanár et al., 2019). Results in this study also demonstrated the disruption of soil biological properties as well as soil organic matter quantity and quality. The lowest quantities of total and labile organic carbon were found in sampling sites the most loaded by total, but mainly available Mg (Figures 2a, b and 5a). Despite the CT content was lower in sites the most polluted with alkaline deposition, the relationship between CT and Mg was not significant (Table 1). Similar relationships were confirmed also by Fu et al. (2011) and Yang et al. (2012). On the other side, significant negative correlation between the CL and the available Mg suggest, that in localities containing high excess of available Mg, lower stock of newly formed organic matter prevailed. Since plants are the main source of fresh organic matter, their shortage resulted in low stock of labile soil organic matter, mainly in the areas the most affected by excess of available Mg as well as high alkalinity.
The quantity of labile organic carbon was significantly related with soil microbial activity. According to Lemanowicz (2019), dehydrogenases together with catalase activities provide information regarding microbial activity in soil. Alkaline and acid phosphatases catalyse the hydrolysis of organic phosphorus compounds and their transformation to inorganic phosphorus (Nannipieri et al., 2011). The activity of all studied enzymes significantly decreased with higher content of available Mg what proved that soil microbial activity was negatively influenced by excess of available Mg. Decline in acid phosphatase activity with increased alkalinity was in agreement with research of Dick et al. (2000) who stated that the optimum pH of soil for the activity of acid phosphatase is 4.0–6.5, while for alkaline phosphatase it is 9.0–11.0. Błońska et al. (2016) consider pH as dominant factor affecting the total microbial abundance and activity of enzymes.
Accordingly with our research, Yang et al. (2012) observed significant decrease in microbial biomass carbon and nitrogen, and potential net N mineralization rate with increased soluble Mg content and pH values. Bartkowiak et al. (2017) and Lemanowicz (2018) highlight the role of enzymatic activity as an early indicator of changes in the intensity of microbial processes as response on soil degradation. Sufficient content and quality of soil organic matter is important for intensive microbial activity. Moreover, organic matter protects and immobilizes enzymes. It stabilizes the protein structure of enzymes, decreases their sensitivity to negative changes caused by environmental factors (Zhang et al., 2015).
A significant rise in enzymatic activity was associated with an increase in both total and labile soil organic carbon content (Table 1). Thus, in addition to filters that effectively capture alkaline emissions, one of the most important measures for enhancing the enzymatic activity of soil degraded by alkaline dust deposition is the enrichment of soil with organic matter, as was confirmed by our results.
4.3 Reclamation and land use possibilities around magnesite processing plants
Reclamation methods of land degraded by Mg-rich, alkaline dust deposition from magnesite processing plants have already been suggested (Holobradý, 1981; Hronec et al., 1992). However, their implementation only seems to be more effective currently, as alkaline emissions have decreased by 99.75% compared to 1970 (that is, to 20 tons of particulate matter per year) (Brozmanová, 2018). Therefore, an effective revitalisation of the affected area could be started by procedures already known from the past.
Classical methods suggest that from the most affected areas the impermeable Mg-rich crust should be mechanically removed, milled, and provided not containing excess concentration of heavy metals or other pollutants it can be used as a good magnesium fertilizer on acidic or sandy soils. Holobradý (1981) suggested use chemical reclamation at each locality where the available Mg exceeded 2,000 mg kg-1. The dose of ameliorative matter should be calculated based on the concentration of available Mg in the soil. In practise, the reclamation was based on a mechanical loosening of the soil with concurrent incorporation: 10–50 t ha-1 of gypsum, or 10–50 t ha-1 of citric-gypsum (waste from citric acid production), or 2,000 L ha-1 of sulphite leaches (pulp waste containing Ca(HSO3)2), or ground sulphur. After the above-mentioned chemical melioration, soluble magnesium sulphate is formed and gradually leached out of the soil by rainwater. To increase soil microbial diversity and biological activity, it is recommended to incorporate 40–50 t ha-1 of farmyard manure (FYM) every 3–4 years. In case of shortage the FYM, it is possible to use composts, or recently recommended manure-biochar, compost-biochar composite composts (El-Naggar et al., 2015). The effect of biochar on reclamation of soils degraded by excess of magnesium was not studied yet. However, enhancing the soil with biochar can improve chemical and physical soil properties and hence stimulate biological activity (Beesley et al., 2011). Therefore, it is important to study the effect of biochar on soil degraded by alkaline dust deposition. On the other side, biochar enriched by Mg hydr(oxid) was stated as having the potential to prevent phosphorus leaching from organic soils (Riddle et al., 2019).
Similar problem with excess of Mg, but coming from irrigation water was solved by Vyshpolsky et al. (2008; 2010). They highlighted positive effect of phosphogypsum application (by-product of the phosphate fertilizer industry) at a dose of 4.5 t ha-1, before the snowfall, every 4–5 years for optimizing the ionic balance of soil with heavily exceeded levels of Mg2+ in Southern Kazakhstan. Wang et al. (2015b) successfully decreased Mg content in soil samples using anionic polyacrylamide and calcium dihydrogen phosphate and controlled leaching of soil columns.
More recent methods include biological reclamation, which involves the growing of Mg hyper-accumulating plants that, after composting, could be used as an organic fertilizer naturally enriched with Mg. This method can be used at localities with the content of Mg less than 2,000 mg kg-1. Markert (1992) in Parzych & Astel (2018) stated that in general, the natural Mg content in the dry plant biomass is 1,000–3,000 mg kg-1. Despite the plants with higher Mg accumulation that have been identified, they did not grow in the soil with excessive Mg content: Stellaria nemorum (L.) 5,716±746 mg kg-1, Urtica dioica (L.) 5,127±581 mg kg-1, Caltha palustris (L.) 4,965±602 mg kg-1 (Parzych et al., 2018). Higher Mg accumulation was identified in plants growing in affected area and forming large monocultures: Elytrigia repens (L.) 21,208 mg kg-1, Phragmites australis (Cav.) Trin. 6,860 mg kg-1 and Agrostis stolonifera (L.) 5,419 mg kg-1 (Fazekaš et al., 2018). Of these plants, onlyPhragmites australis was characterised by high biomass production, that is 12.7 t ha-1 of dry matter (Demko et al., 2017). Therefore, its use as a source of biomass bio-forticated by Mg for compost production can be considered. Effective phytomeliorative removing of excess Mg2+ from lightly Mg-contaminated soil was demonstrated by Wang et al. (2014) usingAneurolepidium chinense (Trin.) and Puccinellia distans(Jacq.) Parl. with the application of Ca(H2PO4)2 . H2O. They stated that planting A. chinense andElymus dahuricus (L.) with the application of Ca(H2PO4)2 . H2O could accelerate the vegetation restoration in moderately and severely Mg-contaminated soil.
Hronec et al. (1992) suggested that land containing <1,000 mg kg-1 of available Mg could be converted gradually into arable land. As already mentioned, in the past, the land near factories was used for agricultural purposes. However, after soil contamination with alkaline dust, especially during the period 1958–1984, these soils were excluded from agricultural use. Based on the available Mg content (Figure 2b), we highlight the possibility of reusing the land at sampling sites 4, 5, 13, and 14 (Figure 1) for agricultural production. To accelerate the removal of excessive Mg, we recommend use the phytoremediation.
At sites with content of available Mg higher than 1,000 mg kg-1 (where only limited species of Mg tolerant vegetation grow), care must be taken to maintain the vegetation covering the soil (sites 1, 2, 3, and 6–11). According to the Shannon Index, plant diversity on the studied sited was extremely low (0.0) to middle low (1.5) (Fazekaš et al., 2018). It is necessary to maintain a favourable state of natural vegetation, for example by mulching of meadows, thereby limiting the spread of invasive plants, as well as avoiding the removal of aboveground plant biomass. Sufficient plant biomass is necessary to increase the storage of soil organic matter, which is an important factor in increasing the biological and enzymatic activity (Mganga et al., 2019; Nyawade et al., 2019; Wei et al., 2019), even in soils deteriorated by alkaline dust deposition (Table 1). Feeding cattle with biomass produced on deteriorated areas is not appropriate due to the plants dusting. Consequently, many diseases that threaten animals occur (nervous, respiratory and digestive disorders, diarrhoeas, weight loss, disruption of the sexual cycle, miscarriages) (Hronec et al., 1992; Machín & Navas 2000). In addition, biomass is of low nutritional value, as in the most affected areas dominate plants:Elytrigia repens , Agrostis stolonifera , Puccinellia distans , Chenopodium glaucum (L.), invasive Solidago canadensis (L.), and recently also Phragmites australis , known as invasive in some alkaline sites (Bart et al., 2006).
An interesting use of deteriorated area could be the growing of plants for energy purposes. A prospective plant is Phragmites australis , which is abundant in humid locations with pH above 9 (Huttmanová et al., 2015) and has spontaneously appeared in the locality only recently. Natural production in Slovakia is 12.7 t ha-1 of dry matter with high-energy storage of 221.622 GJ ha-1(Demko et al., 2017). Therefore, it is more profitable to usePhragmites australis for direct biomass combustion, or production of biofuel pellets, than for the production of biogas and methane. Alternatively, Suhai et al. (2016) stated that this plant species is a sustainable and renewable resource for the production of bioethanol.
At present, when the presence of Mg-rich, alkaline dust in the soil has been significantly reduced, the application of these measures can offer a more lasting positive result compared to the previous period, when the high fallout of alkaline dust had not allowed successful land reclamation in the vicinity of magnesite processing plants. Subsequently, gradually returning the soil and landscape in the affected area to a more productive state will be possible.