3 Results
3.1 Influence of alkaline dust fallout on basic chemical properties and content of heavy metals in soil
Alkaline dust generated during magnesite processing contains beside magnesium also the calcium, therefore Table 1 presents the total and available contents of Ca and Mg 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 content in soil (depending on the direction of prevailing winds and therefore alkaline emissions spreading) are clearly documented in the quadratic polynomial trend (Figure 2c and d). While both forms of Ca showed no trends (Figure 2a and b), 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 the prevailing 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, carbonate content, and the lowest hydrolytic acidity (Figure 2e 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 cause statistically significant increase of soil pH. Accordingly, no significant relationship was found between total Mg and Ca and also between available Mg and Ca (Table 2). As expected, the lowest hydrolytic acidity (H) was observed in areas most affected by alkaline dust deposition (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 (Tables 1 and 2). Conversely, there was no linkage between carbonate content and total or available calcium. Both total and available Mg (as well as carbonates) contributed to the significant increase in soil conductivity (r = 0.787; 0.870 and 0.781; P < 0.001).
Table 3 displays the contents of total and available zinc, copper, lead and nickel on studied sites. The contents of analysed metals varied by sampling sites. This was also confirmed by analysis of variance. Table 4 clearly demonstrates that the increase of available Ni content was associated with an increase in the total Ca content (r = 0.663; P< 0.01) not Mg (r = 0.335; P > 0.05). In addition, the content 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 content 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 content 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 (Table 5).
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 3). 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 (Table 1; Figure 2). Although the CT content was lower in the localities most polluted with alkaline dust fallout (Table 5), there was no significant relationship between CT and total and available Mg (Table 2). 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 2.
The results of the ANOVA test (Table 5) showed a significant effect on the changes in activity of study enzymes for the sampling sites. The highest activity of DEH (0.520 mg TPF kg-124h-1), catalase (0.327 mg H2O2 kg-1h-1), AlP (2.087 mMpNP kg-1h-1) and AcP (2.819 mMpNP kg-1h-1) were reported in the soil collected from sites 11 and 8, which also contained the highest quantity of CL.
We determined a significant decrease in soil enzymatic activity because of increased Mg content (Table 2). In particular, depending on the available Mg content, the alkaline phosphatase, acid phosphatase, dehydrogenase and catalase activities significantly decreased (r = -0.613; r = -0.640; r = -0.574; r = -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.