3 Results
3.1 Influence of alkaline dust fallout on basic chemical characteristics and quantity of heavy metals in soil
Alkaline dust generated during magnesite processing contains beside magnesium also the calcium, therefore Figure 2a, b and c presents the total and available contents of Mg and Ca determined at all 14 investigated sites. Results showed that sample sites close to factories 3–4 folds exceeded the natural regional background content of total Mg in topsoils (9.1–15.2 g kg-1).
Consequently, the available Mg 3–68 fold exceeded very high content for texturally medium soils (>0.255 g kg-1) at all grassland sampling sites, even at distance of 10 km from factories. The dynamics of changes in total as well as available Mg concentration in soil (depending on the direction of prevailing winds and therefore alkaline emissions spreading) are clearly documented in the quadratic polynomial trend (Figure 3a and b). While both forms of Ca showed no trends (Figure 3c and d), the highest contents of total Mg were found in the sampling sites near both magnesite processing plants, particularly in Jelšava. The contents of available Mg gradually increased in the direction of predominant winds, beginning near the factory in Lubeník, and the highest concentration was determined near the factory in Jelšava. The same sampling sites also had the highest pH values and carbonate content (Figure 3e and f).
Sites situated in close proximity to both sources of pollution recorded the highest values of soil pHH2O 7.60–9.39. With increased distance from the factories, pH values, together with Mg content, demonstrated a decreasing tendency (pHH2O7.04–7.98). Soil pH was highly significantly affected by the content of total and available magnesium (r = 0.788; r = 0.894; P<0.001) respectively, while calcium did not result in significant increase of soil pH. Accordingly, no significant relationship was found between total Mg and Ca and also between available Mg and Ca (Table 1). Increased carbonate concentration (CO32-) more or less followed the localities heavily loaded by Mg-rich dust fallout and correlated with the available and total Mg content in the soil (Table 1, Figure 2). Conversely, there was no linkage between carbonate content and total or available calcium.
Figure 4 displays the concentrations of total and available zinc, copper, lead, and nickel on studied soils. The contents of analysed metals varied by sampling sites. This was also confirmed by analysis of variance. Increase of available Ni content was associated with an increase in the total Ca concentration (r = 0.663; P < 0.01) not Mg (r = 0.335; P > 0.05). In addition, the concentration of available Ni (r = 0.811; P < 0.001) and Cu (r = 0.566; P < 0.05) significantly correlated with the content of available Ca. We found no statistically significant relationship between total or available Mg and total or available forms of monitored heavy metals (Zn, Cu, Pb, and Ni).
3.2 Influence of alkaline dust deposition on the storage of soil organic matter and enzymatic activity
Soil degraded by high amount of Mg-rich, alkaline dust fallout, especially in localities where a solid Mg-rich crust has been formed on the surface, is characterised by low stock and altered quality of soil organic matter. Total organic carbon (CT) was in range 5.4–24.3 g kg-1, i.e. low content predominated (Figure 5a).
The dynamics of changes in total as well as labile carbon (CL) content depending on the direction of alkaline emissions spreading is documented in the quadratic polynomial trend (Figure 6). The lowest contents of CT and especially CL were found in sampling sites 6–9 close to the factory in Jelšava. The same sampling sites contained the highest quantity of total but mainly available Mg (Figure 2a and b). Although the CT content was lower in the localities most polluted with alkaline dust fallout (Figure 5a), there was no significant relationship between CT and total and available Mg (Table 1). Conversely, there was a significant negative correlation between the labile fraction of organic matter (CL) and the available Mg (r = -0.617; P < 0.05), suggesting that in the most affected areas, limited formation prevails and therefore a low quantity of new, labile organic matter. Formation of new, labile organic compounds was significantly impeded also by high pHH2O values (r = -0.602; P < 0.05), as shown in Table 1.
ANOVA test disclosed significant differences in enzymatic activities on the sampling sites. The highest activity of dehydrogenases, catalase, alkaline as well as acid phosphatases were reported in the soil collected from sites 11 and 13, which also contained the highest quantity of CL (Figure 5).
We determined a significant decrease in soil enzymatic activity because of increased Mg content (Table 1). In particular, depending on the available Mg content, the alkaline phosphatase, acid phosphatase, dehydrogenase and catalase activities significantly decreased (r = -0.613; -0.640; -0.574; -0.610; P < 0.05). Moreover, the activity of acid phosphatase was negatively influenced by increased pHH2O (r = -0.608; P < 0.05). Conversely, alkaline phosphatase activity increased in accordance with the content of available Ca (r = 0.538; P < 0.05). Thus, in the affected area, the excess of available Mg, as well as increased pH values, and decreased content of labile soil organic matter were associated with Mg-rich, alkaline dust deposition and caused a significant decrease in soil enzymatic activity.