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