RS-substrate for germination, RG-garden soil, EC-electrical conductivity, OM-organic matter
Each stage was completed with a germination phytotest using white mustard (Sinapis alba ). Based on the data from the germination tests and the results of soil substitutes chemical analysis, the share of individual wastes in the consecutively elaborated soil substitutes has been corrected. The pH of soil substitutes and electrolytic conductivity (EC) expressed in the paper permutedly as salinity were found to be the decisive parameters limiting the germination process of white mustard.
On this basis, fly ash from biomass combustion (BFA) and sewage sludge (SWS), with conductivity 39.4 mS·cm-1 and 12.18 mS·cm-1 respectively, were eliminated from the next blending process at the stage II. The lack of these ingredients was supplemented by increasing the share of spent mushroom compost (SMC) and aggregate (AG).
At the stage III of the preparation of soil-substitutes, coal combustion fly ash (CFA), with a conductivity of 3.6 mS·cm-1 was eliminated from the blending process. Instead of CFA, the energetic slag (ES) was introduced as a material with low conductivity/salinity (0.35 mS·cm-1), while improving the structure of elaborated soil substitutes.
The reasons for such blending process was to ensure some basic factors: optimal structure of the blends (loose and lumpy), appropriate chemical parameters (for instance pH, hydrolytic conductivity) along with nutrients (N, P, K, Mg) and particularly organic matter (substances) for ensuring a sustainable reclamation of affected mining areas.
2.4. Seed germination tests
White mustard (Sinapis alba ) was used as the typical plant for seed germination test to assess the suitability of each soil substitute for vegetation development.
Seed germination assays were carried out on plastic sprouting bowls that contain 1 kg of soil substitute (Figure 2). In each sprouting bowl 50 seeds of Sinapis alba were placed in equal distance at the depth of 1 cm. The tests were performed in laboratory conditions under constant temperature (22oC), humidity (30-40%) and controlled light. Each sprouting bowl was watered once a day (50 ml/day) and exposed to white light for 12 hours a day. The sprouting rate was counted after 20 days.
FIGURE 2
2.5. Phytotests with meadow species At the stage III of the experiment, seeds of semi-natural meadow communities were additionally used. They are represented by plant species typical for meadow communities in Central Europe. Taking into consideration habitat condition on waste heaps slopes, the species with low (dry meadow) and middle soil moisture requirements (mesic meadow) were used. Then, 3 g of the mesic and dry meadow seed mixture were sown into four types of soil substitutes (1kg in plastic sprouting bowls). The tests were performed in three repetitions (n=12) from 11 May to 26 June in outdoor conditions without artificial application of water and light. At the end of the tests, the percentage coverage of developed meadow vegetation was evaluated. 2.6. Statistical analysis The normal distribution of data was confirmed using the Shapiro-Wilk test. The relationship between the Sinapis alba germination and physical-chemical parameters of soil substitutes were analysed using Pearson’s linear correlation coefficient with Statistica 12.0 (StatsSoft, Poland). Principal Component Analysis (PCA) was applied in order to determine conditions that influence the development of native meadow vegetation on soil substitutes. The analysis was performed by using the CANOCO package. Variables data were transformed using log(x+1) prior to the analysis response (Lepš & Šmilauer, 2000). 3. RESULTS 3.1. Characteristics of the investigated wastes The wide range of data illustrated in the Table 2 shows the heterogeneity of the wastes and their chemical composition. Among all investigated wastes, only CFA, BFA and AG were extremely low in water content (90-100 % of DM), with SL and ES containing 72.6 % and 77.6 % respectively, and DL 50.8 %. Only SWS and SMC had high moisture content and very low dry matter: 16.6 % and 34.7 %, respectively. Organic matter is the key parameter which decides about the environmental and biological sustainability of soil substitutes. Among all investigated wastes, only three i.e., SL (35.6 %), SMC (63.9 %) and SWS (74.8 %) could be considered as rich in organic matter. The AG, with 15.9 % organic matter, could be involved as a moderate source, but its physical and mineralogical composition may act as a strongly limiting factor. Organic and dry matter have decisive effect on the content of ash in the waste. It is generally observed that materials with high organic matter content may record low ash concentration and vice-versa. Therefore, in the development of soil substitutes, it is strongly recommended to pay special attention to the content of organic matter in the blends. The content of Ca and S in the wastes and their further occurrence in the ready-to-use soil substitutes are of prime importance for the remediation of waste heaps, where the latter and its compounds cause excessive acidity, contrary to calcium, the addition of which significantly increases the pH value. The highest content of Ca i.e., 32 % was observed in DL as compared to SL and AG, containing 0.34 and 0.43 %, respectively. The other wastes were not higher in Ca than 10 %. Three of eight investigated wastes exhibited S content of: 2.39, 2.53 and 3.95 % for SWS, SMC and AG respectively, while the S content in remaining wastes did not exceed 1.0 %. The chemical forms of S in those wastes may affect their utility for incorporating in the development soil substitutes. The research proved that only SWS contained much more N (5.1 %) compared to the other wastes, where the low N content varied in the range 0.15 – 0.4%. It is worth mentioning that natural soil ecosystems are typically poor in nitrogen content, hence the developed soil substitutes should comply with this rule. Nevertheless, a slightly enhanced N level may be expected as a “starter” for boosting plant growth at the anthropogenic (artificial) ground. In terms of P concentrations, only three of the wastes recorded values higher than 1.0 %, two of them typically organic, i.e. SWS (1.25 %) and SMC (1.02 %), and one mineral BFA (1.07 %). It should be observed that, BFA was characterized by the highest K content (5.65 %) while DL and SWS exhibited the lowest content of K, from 0.04 to 0.23 %, respectively. Interestingly, DL contained the highest Mg concentration, reaching even 5.44 %. The patterns of metals distributions among the wastes are quite similar, but disparate depending on the metal types (Table 2). Higher content of Cu was observed at ashes BFA (241 mg·kg-1) and CFA (219 mg·kg-1) as well as for the sewage sludge SWS (129 mg·kg-1). As in the case of Cu and Zn was also found from moderately high concentrations for CFA (219.0 mg·kg-1) and SMC (412.0 mg·kg-1) to very high levels for BFA (911.0) and AG (1055.0 mg·kg-1). Excessive content of Zn was observed only for SWS (1704.0 mg·kg-1). The very high and excessive Zn contents of these wastes may be a potential source of it in soil substitutes. According to the Polish regulations (Minister of Agriculture and Rural Development, 2008), the permissible content of pollutants in organic-mineral fertilizers must not exceed (mg·kg-1 of DM): 100 for Cr, 5 for Cd, 60 for Ni and 140 for Pb. The excessive concentrations of Cd were observed only for SWS (29 mg·kg-1) and BFA (9 mg·kg-1). On the other hand the content of Ni was lower than the Polish limit value only for two of all wastes such as DL (9 mg·kg-1) and SMC (7 mg·kg-1). The excessive values for Pb were noticed for BFA (176 mg·kg-1), AG (213 mg·kg-1) and SWS (300 mg·kg-1). The concentration of Cr in all studied wastes can be considered on acceptable level (< 100 mg·kg-1). 3.2. Impact of pH and electrical conductivity on the Sinapis albagermination
White mustard (Sinapis alba ) is very sensitive to factors limiting growth and hence sprouting also has been selected for verifying the possible potential of the soil substitutes for vegetation support. The pH value and the electrical conductivity were considered as two primary parameters decisive for an optimal seed germination and further plant growth. As reported in Figure 3 and based on the general classification (Bruce & Rayment, 1982; Hazelton & Murphy, 2016), the pH of the soil substitutes showed three basic ranges moderately alkaline (pH 8.0-8.4), strongly alkaline (pH 8.5-9.0) and very strongly alkaline (pH >9.0), which are beyond the highest pH level (i.e. 7.5) potentially tolerable by Sinapis alba . The highest pH values (8.3-9.6) were measured at the stage I, but results of the soil substitutes from stages II and III were in ranges 8.0-8.3 and 8.2-8.8 respectively.
The soil substitutes with low water storage capacity and moderate content of nutrients showed the best sprouting for A3 in stage III (26%) compared with the A2 (12%) and A1 (0%). Soil substitutes with low water retention and low content of nutrients showed good germination for B3 at the stage III (52%). The most promising soil substitute with moderate ability to water retention and low content of nutrients was C3 at stage II (46%). Germination of white mustard in soil substitutes with moderate water retention and moderate content of nutrients showed good results at stages I and II i.e: 58 and 56% for D2, respectively and 66% for D3.
Since white mustard is very sensitive to soil conditions and salinity, the results observed on the Figure 3 supported the hypothesis that salinity could be mostly responsible for the lack or weak sprouting process. Based on the response of white mustard, for EC, the value of 6.50 mS·cm-1 could be considered as the threshold for plants highly sensitive to saline media. Hence, soil substitutes characterized by EC ≤ 6.50 mS·cm-1 should be giving successful germination. Phytotests with white mustard showed that the most promising results were obtained at the stage III. The sprouting in reference soils 90% for RG and 92% for RS confirms that quality seed were used and tests were carried out in proper conditions for white mustard development.
FIGURE 3