1. Introduction
Continuous urbanization and industrialization mean increased energy demand and high levels of municipal waste causing environmental issues [
1]. This creates a global need for new methods of waste disposal and for alternatives to fossil energy sources to maintain sustainable development. The use of biomass as a renewable energy source may reduce the amount of generated waste and greenhouse gas emissions [
2]. Additionally, the introduction of circular management might change the way organic waste is utilized, with materials rich in organic carbon treated as a resource [
3]. The use of byproducts or waste materials as a source of organic matter creates an opportunity to promote sustainable development in agriculture [
4]. The basic condition for restoring productivity in degraded areas is the use of materials rich in organic matter, combined with adequate crop cultivation and agrotechnical treatments [
5]. Marginal land consists mainly of degraded areas polluted with heavy metals or pesticides, with low productivity, and is often used for the disposal of municipal or industrial waste [
6]. Due to the impossibility of cultivating plants for food purposes, energy crops can be grown on marginal land in order to obtain material for biofuel or electricity. Perennial grasses are characterized by higher nutrient absorption capacity, due to which their development is possible on soils with low edaphic parameters [
7].
As a natural source of energy, biomass is a substitute for fossil fuel [
8].
Miscanthus sinensis Andersson is a promising energy plant with high yields and is different from other species because of its high resistance to environmental stress and to low temperatures [
9]. It is also known as a plant with rapid growth and an ability to adapt to adverse environmental conditions [
10]. According to many authors,
Miscanthus sinensis is an energy crop from which it is possible to obtain, among others, biofuel and thermal energy [
11]. It does not compete with food crops, as it may be grown on marginal and degraded land, particularly in highly urbanized areas [
12]. To increase the fertility of poor-quality soil, it is necessary to provide it with necessary nutrients and to improve its structure. Lower yields can be increased on marginal or nutrient-poor land by adequate fertilizer treatment [
13].
The amount of fuel produced from energy crops is dependent on their yield. In turn, plant growth and development are conditioned, among others, by environmental and genetic factors, which are subject to constant improvements. Progress in plant growing techniques and research methods is a way to increase crop yields and quality. Physiological activities of plants and their resistance to biotic and abiotic stress can be increased by appropriate fertilizer treatment. Organic fertilizers are a safe alternative to chemical ones [
14]. They increase the fertility of soil and its content of humic acids [
15], which are a breeding ground for microorganisms positively affecting soil structure [
16]. The treatment of plants with composted organic waste is a way to increase crop yields by maintaining the soil content of organic matter and organic N for a longer time [
17].
Organic materials can be used in organic farming, and they contribute to reducing the amount of mineral fertilizers in conventional agriculture [
18]. The processes of mineralization and conversion of organic matter occurring during composting of organic waste leads to obtaining a stable product with a high fertilizing potential [
19]. Composts from municipal waste increase soil fertility and improve plant growth and productivity [
14,
17]. The results obtained by Dębicka et al. clearly show that compost from municipal waste does not pose a threat to the environment and, due to its phosphorus content, contributes to reducing the demand for mineral fertilizers [
19]. Compost from municipal waste also improves the efficient use of water by plants and contributes to the regeneration of degraded soil. However, its use in agriculture depends on the quality of the waste material and, above all, on its content of potentially toxic elements [
20].
Composted mushroom substrate is characterized by a high content of organic matter, which can reduce the use of other organic fertilizers [
21]. It improves soil structure, provides nutrients for plants and increases their availability. It also contributes to increasing the microbial population in the soil, strengthens plant root structure, regulates soil pH level and improves soil water retention. Due to its ability to generate salts and enzymes, composted mushroom substrate increases plant resistance to diseases, thereby reducing the use of fungicides and inorganic fertilizers [
22]. Both composted mushroom substrate and composted municipal waste are unused products of human economic and livelihood activities, treated as problematic waste. Little attention has been paid to research on
Miscanthus sinensis as one of few energy plants with remedial abilities [
23].
In order to obtain high-quality yields of energy crops grown on marginal land with low productivity, it is important to increase soil fertility by introducing high-quality fertilizing material. A pragmatic solution is to use municipal and industrial waste rich in organic matter in the cultivation of energy crops, with better waste management and increased crop yields due to increased soil fertility. This present field experiment underlines the importance of using organic waste materials as a valuable fertilizer, which not only contributes to optimizing yields but can also replace mineral fertilizers. This experiment aims to identify the best combinations of organic fertilizers, increasing yields of Miscanthus sinensis. It will contribute to the efficient production of renewable energy maintaining environmental sustainability.
3. Results
According to the values of Sielianinov’s coefficient (K), more favourable weather conditions for
Miscanthus sinensis Andersson growth were in 2018 (
Table 1), and less favourable conditions were in 2019 and 2020. In 2018, April and May were fairly dry and very dry, but the following months of June, July and October were optimal for the growth and development of Chinese silver grass.
Organic fertilizers used in the experiment differed in their dry matter content. Its percentage in mushroom substrate compost was 30%, while in municipal waste compost, it was 68% (
Table 2). The pH value of the former fertilizer was 6.41, close to neutral, and 7.10 of the latter. According to Vitti et al., [
32] the pH of high-quality compost should range from 6.0 to 8.0.
Madej et al. [
33] reported that compost from waste plant raw materials contained high amounts of dry matter, up to 450 g·kg
−1, with N ranging from 13 to 15 g·kg
−1 DM, P ranging from 2.19 to 4.81 g·kg
−1 DM and K at 4.98 g·kg
−1 DM. The content of micronutrients in mushroom compost and municipal waste compost was as follows: N—20.9 g∙kg
−1 and 14.30 g∙kg
−1; P—8.86 g∙kg
−1 and 17.32 g∙kg
−1; K –11.21 g∙kg
−1 and 25.4 g∙kg
−1. Thus, the content of N was particularly high in mushroom substrate compost (20.9 g·kg
−1), with a C:N ratio of 13.59.
The content of heavy metals in mushroom compost (
Table 3) was at a relatively low level and did not exceed their permissible limits provided by the BN-89/9103-09 Polish Standard [
34]. In turn, zinc content was 156.9 mg·kg
−1 DM, with low amounts of cadmium (0.287 mg·kg
−1 DM) and cobalt (0.415 mg·kg
−1 DM). Both municipal waste compost and mushroom substrate compost were of good quality, with a pH close to neutral, relatively high nutrient content and low heavy metal content. The values of those parameters were similar to those provided by Mladenov [
35].
Yields depend on the genetics of the plant, its adaptation to environmental conditions, soil quality, water availability and fertilizer treatment [
33]. Stewart et al. [
7] recorded the dry matter yield of
Miscanthus sinensis at a level of 4–4.6 Mg·ha
−1. However, in the present experiment, higher values were obtained (
Figure 1), with an experimental average of 13.10 Mg of fresh matter per ha. Yields of dry matter varied significantly across fertilizer combinations and years of the experiment. A significant interaction between them was noted as well. The highest value was recorded in the third year (10.79 Mg·ha
−1 DM), and the lowest was recorded in the first (5.23 Mg·ha
−1 DM). In the third year, the average dry matter yield (10.79 Mg·ha
−1) was more than twice as high as in the first. A similar relationship was observed by Filipek-Mazurek and Gondek [
36], who argued that the yields in the first year were lower because the mineralization of nutrients supplied to the soil had not fully started yet. Gubiśová et al. [
37] also observed that the highest yield of Miscanthus was in the third year of cultivation.
As a three-year average, the lowest amounts of fresh (11.41 Mg·ha−1) and dry matter (6.93 Mg·ha−1) were noted on the unfertilized control plot. Compared to the control plot, organic fertilizers significantly affected the biomass yield of Miscanthus sinensis. The highest yield (12.69 Mg·ha−1 DM) was noted on the plot with the highest dose of mushroom substrate compost in the third year. This organic fertilizer, with its high content of organic C and N, had a positive effect on the growth of plants. Applied on its own (MSC100%), it contributed to the highest three-year average yield, with 8.44 Mg·ha−1 DM.
In the third year of Chinese silver grass cultivation, Clifton-Brown et al. [
38] recorded a yield ranging from 4.6 to 17.3 Mg·ha
−1. Dradrach et al. [
39] noted a yield of
Miscanthus sinensis at 8.39 Mg·ha
−1 DM. Lim et al. observed that the length of the growing period had an impact on its quality and yield [
40].
The height of the plants and the number and area of leaf blades determine the biomass yield of
Miscanthus sinensis [
40]. With an average of 198.57 cm, the length of
Miscanthus sinensis stems significantly varied throughout the experiment (
Figure 2). The largest statistically significant difference was between the first and third year. However, the growth rate of
Miscanthus sinensis did not differ significantly between the first and second years. In the second year, it was lower than expected because weather conditions during the growing period were unfavourable, ranging from extremely dry to fairly dry. Thus, water shortages might have contributed to plant stress, resulting in the low growth of the plants. Plant growth is affected not only by water availability but also by many other factors such as soil conditions, temperature, the date of planting and the quality of planting material [
38].
Naturally occurring species of
Miscanthus sinensis reach an average height of 1–2 m [
7]. Clifton-Brown et al. reported that the height of cultivated
Miscanthus sinensis was 1.0–2.3 m [
38], and in the present experiment, the values were similar. However, no significant effect of mushroom substrate compost or municipal waste compost treatment on the length of
Miscanthus sinensis stems was observed. Stewart et al. reported that the treatment of
Miscanthus sinensis with N, P and K macronutrients (170:250:140 kg.ha
−1) affected the yield in a statistically significant way, and fertilized plants were on average 16–33% higher than those on control plots [
7]. The length of
Miscanthus stems is affected by the latitude of the place of cultivation. Lim et al. reported that the length of stems on higher latitudes (China, Russia) was on average 178.2 cm, while on lower latitudes (Korea, Japan), it was more favourable, with an average of 237.2 cm [
40].
The number of stems per plot was affected by treatment, but it also varied over the years of research. It increased by 50% each year, and a three-year average was 98 stems per plot (
Figure 3). Plants produced 126 stems per plot more in the third year than in the first, and this was the largest statistical difference. The most
Miscanthus sinensis stems (220) were recorded on the plot with mushroom substrate compost (MSC100%) in the third year, but its combined application with municipal waste compost (MWC
25+MSC
75) lowered the number to 182. Scorida et al. recorded 12 to 208
Miscanthus siensnsis Andersson stems per plot [
41]. In the second year of the experiment, weather conditions were unfavourable for plant growth, but this did not reduce the number of stems. According to Malinowska et al., stress caused by periodic water deficit does not inhibit the development of
Miscanthus sinensis [
42].
The number of nodes per stem significantly varied across years (
Figure 4). The lowest number was recorded in the first year (5.88 nodes per stem), and the highest was recorded in the third (10.74). No significant effect of treatment on the number of nodes was observed. As an average across fertilizer treatments and years of research,
Miscanthus sinensis produced 8.73 nodes per stem.
As an average across years of research and treatment combinations, the diameter of the Miscanthus sinensis stem was 6.46 mm. The research factors did not significantly affect the value of this feature.
The number of leaf blades per
Miscanthus sinensis stem (
Figure 5) significantly varied across years of research (2018, 2029, 2020). The highest was recorded in the third year, with nearly twice as many as in the first. The number of leaf blades increased over the years of research, which was related to increasingly faster development of plants. No significant effect of organic fertilizer treatment on this feature was observed.
Treatment combinations did not affect the length of the leaf blade or the diameter of
Miscanthus sinensis stems in a statistically significant way (
Figure 6). They did not vary across years of research either. As an experimental average, the length of the leaf blade was 69 cm and was within the range of 12–100 cm reported by Robson et al. [
43]. Stewart et al. treated
Miscanthus sinensis with N, P and K fertilizers (170:250:140 kg·ha
−1), and the yield increased in a statistically significant way, with leaf blades being 8–23% longer than those on the control plot [
7]. However, no statistically significant effect of organic fertilizer treatment was noted in the present experiment.
The width of leaf blades, as well as their number per stem, varied across years of research (
Figure 7). It did not increase significantly during the second year of cultivation, but the difference between the first and third year was 21%. As an experimental average, the width of the leaf blade was 14.52 mm. This value was within the range of 2–36 mm recorded by Robson et al. [
43].
The value of the minimum chlorophyll fluorescence of Chinese silver grass leaves varied significantly depending on the organic waste and years of research (
Table 4). As an average of treatment combinations, the highest minimum chlorophyll fluorescence (256), indicating the low efficiency of excitation energy transfer by chlorophyll molecules (energy losses), was in the first year. The most favourable values of the index were in the second year of cultivation and amounted to 214.
Compared to the plot with no fertilizer treatment, the organic materials used in the experiment had a significant effect on the minimum chlorophyll fluorescence of Chinese silver grass, except for plants treated with 100% municipal waste compost. The highest value of minimum fluorescence (253) was on the plot where municipal waste compost and mushroom substrate compost were used in equal doses in terms of the amount of N. As a three-year average, the lowest minimum chlorophyll fluorescence of Miscanthus sinensis leaves (201) was noted on the plot with 75% municipal waste compost and 25% mushroom substrate compost.
On the basis of the results, it could be assumed that unfavourable weather conditions in 2019 and 2020 caused plant stress and changes in photochemical processes competing with the PSII reaction centres for excitation energy. This was confirmed by the significantly higher values of maximum chlorophyll fluorescence of the leaves noted in those years (
Table 5) than in the first year. The values of Fm did not affect the yield of
Miscanthus sinensis. Malinowska et al. observed that stress caused by water shortage did not significantly affect the biomass yield of
Miscanthus sinensis or the height of the plants. Among similar species,
Miscanthus sinensis Anderson has the highest resistance to drought stress indicated by changes in stomatal conductance [
42].
Application of different proportions of composts resulted in significant differences in the value of maximum chlorophyll fluorescence. The plant reacted strongly to the combination of mushroom substrate compost and municipal waste compost applied in equal proportions. A significant interaction between years and treatment combinations was also noted. As a three-year average, the lowest maximum chlorophyll fluorescence value (526) was recorded on the plot with 75% of municipal waste compost and 25% of mushroom substrate compost.
As an average effect of the research factors, the relative chlorophyll content index of
Miscanthus sinensis (
Table 6) was 34.86, ranging from 33.18 to 36.03. It varied considerably throughout the experiment. The highest was noted in 2020, amounting to 39.9, and the lowest, with 29.86, was noted in the second year (2019), when weather conditions, according to Sielianinov’s coefficient, ranged from extremely dry to fairly dry. The plants responded to that stress factor with a low SPAD value of 29.86. The differences between relative chlorophyll content in the leaves of
Miscanthus sinensis were statistically significant. Across treatment combinations, the most favourable value, statistically different from the others, was noted in plants treated with municipal waste compost on its own and mushroom substrate compost on its own.
In the first year, statistically significant highest relative chlorophyll content (SPAD) of 38.77 was noted on the control plot and on that with 75% municipal waste compost and 25% mushroom substrate compost. In the second year (2019), the highest value of 31.53 was noted for plants treated with municipal waste compost on its own. This value differed statistically only from two plots (one with 100% MSC and the other with75% MWC in combination with 25% MSC). In the third year, the highest relative chlorophyll content was noted for plants treated with both composts applied in equal proportions. That value was statistically different from those on other plots. Weng et al. reported that
Miscanthus sinensis was not only resistant to drought stress, but it also showed regenerative abilities after water scarcity damage [
44].