5 Conclusions
Mg-rich, alkaline dust causes long-lasting soil degradation. The evidence is the relationship of findings reached in this study with the results of dust deposition measured during 10 years, 40 years ago.
Sites close to factories 3–4 folds exceeded the natural regional background content of total Mg in topsoils. 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, along the direction of the alkaline emissions spreading. Areas close to factory contained up to 14.4–17.4 g kg-1 of available Mg. Studied heavy metals (Zn, Cu, Pb and Ni) did not exceeded limit values and their concentration was not in linkage with the content of Mg, as well as enzymatic activity.
Higher excess of available Mg caused significant increase of soil pH (up to 9.39) and worsened conditions for the growth of vegetation. Since plants are the main source of fresh organic matter, their shortage resulted in low stock of labile soil organic carbon (0.50–0.96 g kg-1) and consequently weaker enzymatic activity. Thus, in addition to effective, alkaline emissions capturing filters, an important measure supporting the microbial activity of affected soil is enrichment by organic matter.
At present, when the entry of Mg-rich, alkaline dust into the soil has been significantly reduced (by 99.75% compared to year 1970), the application of measures can bring more lasting positive results than in the past.
Nevertheless, classical methods are still the most effective for reclaiming 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 magnesium fertilizer. Soil with available Mg exceeded 2,000 mg kg-1 has to be treated chemically, by incorporating gypsum compounds, sulphur, as well as high doses of farmyard manure, or composts, manure-biochar, compost-biochar composite composts.
Recent methods are applicable to less affected areas. These involve the growing of Mg hyper-accumulating plants that, after composting, can be used as an organic fertilizer enriched with Mg, or growing plants that can be used for energy purposes. In the studied area, among naturally occurring vegetation, only Phragmites australis is characterised by high biomass production.
Sampling sites, where the available Mg decreased under the critical level of 1,000 mg kg-1, can be reused for agricultural production. Over the whole affected area, care must be taken to maintain a favourable state of natural vegetation. Consistent application of measures will enable a gradual return of the soil and landscape to a more productive state.