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 enhancement.
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 the prevailing 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 (Table 1). These sampling sites
also had the highest pH values, carbonate content, and the lowest
hydrolytic acidity. 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 (Table 1). 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 content of Zn, Cu, Ni and Pb at
studied sites (Table 3) might be due to the changing atmospheric
pressure and other meteorological factors during the deposition of
alkaline dust in the soil. The values of the heavy metals under study
classify the analysed soils as unpolluted. All soil samples had lower
values of total and available heavy metal content than limits reported
by the U.S. Environmental Protection Agency (1993).
In studied locality, no significant linkage between Mg and monitored
heavy metals were found (Table 4). 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.
Mobility and availability of metals are controlled by many chemical and
biochemical processes in soils. Not all are of the same importance for
each metal, and this largely depends on many soil properties such as:
organic matter content; the content of clay fraction; 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). Disruption of soil biological
properties as well as soil organic matter content and quality
demonstrated also results in this study. The lowest contents of total
and labile organic carbon were found in sampling sites the most loaded
by total, but mainly available Mg (Tables 1, 5). Despite the
CT content was lower in sites the most polluted with
alkaline deposition, the relationship between CT and Mg
was not significant (Table 2). 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 content of labile soil organic matter significantly related with
soil microbial activity. According to Lemanowicz (2019), activity of the
dehydrogenases could be considered a good indicator of the oxidative
metabolism in soils, and together with catalase are used to give
information on the microbial activities 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 and for alkaline
phosphatase is 9.0–11.0. According to Błońska et al. (2016), pH is a
major factor affecting the total microbial count 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. The
enzymatic activity is an early indicator of changes in the level of
intensity of biological processes and the level of soil degradation, and
it is usually correlated with its physical and chemical properties
(Bartkowiak et al., 2017; Lemanowicz, 2018). Organic matter offers a
protective function towards enzymes, which are thus immobilised. It has
a positive effect on the stability of protein structure, decreasing the
sensitivity to negative changes triggered by environmental factors
(Zhang et al., 2015).
A significant increase in enzymatic activity was associated with an
increase in both total and labile soil organic carbon content (Table 2).
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
used as a good magnesium fertilizer on acidic soils. Holobradý (1981)
suggested use chemical reclamation at each locality where the available
Mg exceeded 2,000 mg kg-1. Ameliorative matter dose
should be calculated based on the available Mg content 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). After the above-mentioned
chemical melioration, soluble magnesium sulphate is formed and gradually
leached out of the soil by rainwater. To increase soil microbial
biodiversity and biological activity, it is recommended to incorporate
40–50 t ha-1 of farmyard manure every 3–4 years.
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 phosphorous
fertilizer industry) at a dose of 4.5 t ha-1, before
the snowfall, every 4–5 years to optimize 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
(Table 1), 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 investigated sites 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 content 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 (Tables 1 and 5).
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.