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.