4. Discussion
Soil organic matter plays a major role in terrestrial ecosystems. The concentration of organic matter in developed soil substitutes is within the range 20.3-27.9 %, and these levels are highly satisfactory, particularly if we compare to natural soils with about 1 to 5 % of total organic matter (Garratt et al., 2018). Organic materials maintains the soil structure, improves water infiltration, increases the water holding capacity and reduces the risk of soil erosion. In addition, its decomposition provides nutrients for plants (Krull et al. , 2009). The major components of soil organic matter are humic substances and fulvic acids. The results of (Asik et al. , 2009) and (Çeliket al. , 2010) studies have shown positive effects of the humic substances on seed germination, seedling growth, root initiation, root growth, shoot development and the uptake of macro- and microelements. Soil humic substances may also mitigate abiotic stress conditions caused by unfavorable pH, and high salinity. According to (Hazelton & Murphy, 2016) the optimal pH range for a variety of plants is from 5 to 8. Obtained data pf pH for soil substitutes was in range from 5.1 to 11.7. The results of EC varied within very large range, from 4.7 to 11.7 mS·cm-1, which corresponds to two classifications (Miller & Donahue, 1995): moderately saline in the range of 4 to 8 mS·cm-1 (sprouting, biomass and yields of many plants are restricted) and strongly saline from 8 to 16 mS·cm-1 (only tolerant plants develop satisfactory biomass and yields).
The primary macronutrients for plant growth and developments are N, P, K while the secondary are Ca, Mg and S. Nitrogen is responsible for biomass build up (Heaton et al. , 2004; Lee et al. , 2017), P for good development of plant root systems (Shen et al. , 2018; Wissuwa et al. , 2005), K for internal water management in plants (Grzebisz et al. , 2013) and Mg for photosynthesic activity (Guoet al. , 2016; Hermans & Verbruggen, 2005).
The organic matter content also increases P sorption capacity of the soil (Kang et al. , 2009), which could cause decreasing phosphorus availability for plants in the soil substitutes with higher organic matter contents. Phosphorus constitutes an essential element for seed germination, seedling establishment, and plant growth (John et al. , 2016; Malhotra et al. , 2018; Neitzke, 2002; Venterink & Güsewell, 2010) at the same time affecting the variety of meadow species (van Dobben et al. , 2017; Venterink & Güsewell, 2010; Weigeltet al. , 2005). The species characteristic for mesic meadows have shown increasing coverage with higher P concentration, which proved its role in soil fertility and plant growth.
The pH of soil substitutes has shown significant influence on developing both semi-natural meadow communities, which renders it the major factor affecting habitat conditions for many meadow species (Venterink & Güsewell, 2010). Soil substitutes for the establishment of semi-natural meadow communities were characterized by alkaline conditions (pH 8.16-8.78) and content of Ca between 5.23 to 8.23%. Such calcareous conditions often exhibit a high concentration of bicarbonate in the soil solution and induce low availability of Fe and Zn (George et al. , 2011). On the other hand, high pH has shown a negative effect on developing dry meadow communities, contrarily to Ca manifesting positive effect on mesic meadow communities, and it could be associated with a decrease of negative influence of S. The habitats with high content of Ca are characteristic for the type of meadow communities that are considered as the richest and most endangered ecosystems of the natural environment of Europe (Boroń et al. , 2019). Generally over the past few decades, a significant decrease in area of semi-natural meadow has been observed across the world (Tokarczyk, 2017). The semi-natural meadow vegetation are ecosystems with the ability to deliver ecosystem services such as: pollination, herbs for traditional medicinal use, nutrient cycling, nutrient and water retention, biomass production, recreation and climate regulation (Lamarque et al. , 2011; Villoslada Peciña et al. , 2019). Data from the current preliminary study have shown that using waste materials for the development of soil substitutes, which are eco-friendly and suitable for semi-natural meadow communities is possible. Further research is needed to evaluate long-term development of semi-natural meadow communities on such type of reclaimed coal-mine affected areas. The mining waste heaps seem to offer suitable area for these types of semi-natural communities development.
5. Conclusions
Using coal combustion by-products along with sewage sludge and spent mushroom compost allowed the elaboration of soil substitutes addressing land rehabilitation of coal mine affected areas. Sewage sludge and spent mushroom compost enrich soil substitutes with organic matter and valuable nutrients such as N, P and K.
The application of phytotests – white mustard, mesic meadow and dry meadow species – for evaluating the potential vegetation support, have revealed three key factors which controlled the eco-usefulness of the soil substitutes: pH, electrical conductivity and organic matter content.
Stabilizing the pH to values around 8.0 was critical for neutralizing the excess of acidity at the reclamation site. Ground phytotests showed that soil substitutes with EC ≤ 6.50 mS·cm-1 were the most promising for plant growth. This pattern should be complemented with 23 % of OM as it is the most suitable for developing meadow vegetation. A higher concentration of OM (as ascertained by PCA) could negatively affect cover meadow vegetation.
Experimental data presented in the paper shows that eco-friendly characteristics of soil substitutes were reached. Ca and N levels varied respectively from 5.32 to 8.23 % and 0.44 to 0.60 %, whereas K: 1.63-1.98 %, Mg: 1.01-1.38 % and P: 0.13-0.21 % exhibited values indicating sufficiency for supporting plant growth and further green biomass production.
The environmentally oriented recovery of industrial wastes as proceeded at the current study met the circular economy objectives.
Acknowledgments: This research was funded by the Research Fund for Coal and Steel (Grant Agreement No. 847205-RECOVERY-RFCS-2018 07/2019-06/2023) and the Polish Ministry of Science and Higher Education (Contract No. 847205 z 17.04.2019 5036/FBWiS/2019/2 z 17.12.2019) under the project RECOVERY “Recovery of degraded and transformed ecosystems in coal mining-affected areas.