Effect of Composted Organic Waste on Miscanthus sinensis Andersson Yield, Morphological Characteristics and Chlorophyll Fluorescence and Content

Abstract

The aim of this research was to determine the impact of composted mushroom substrate and composted municipal waste on the quality and yield of Miscanthus sinensis Andersson biomass. The plant was grown on anthropogenic soil, cultured earth type and hortisol subtype, with a pH of 6.81. Before planting rhizomes, experimental plots were treated with composted mushroom substrate and composted municipal waste, applied separately or in combination, each dose introducing 170 N kg·ha⁻¹ to the soil. During the experiment, observations of plant development and growth were carried out, and the yield was determined. Each growing season’s measurements were taken of the grass height, the number and diameter of stems and the number of leaf blades and of nodes per stem. In order to determine the level of plant stress, relative chlorophyll content and chlorophyll fluorescence parameters were determined. The measurements were carried out in a non-invasive way, using the SPAD-502 chlorophyll meter and OS30p+ plant stress meter. For the research hypothesis, it was assumed that the one-time addition of composted mushroom substrate and composted municipal waste to the soil would increase yields. The experiment also aimed to assess the impact of both types of compost on the yield and morphological characteristics of Miscanthus sinensis. Its yields increased steadily, and, in the third year of cultivation, were higher by 52%. The highest average yields were noted on plots fertilized only with composted mushroom substrate (KPP_100%), with 8.44 Mg·ha⁻¹ DM, and with compost from municipal waste (KOM_100%), with 7.91 Mg·ha⁻¹ DM. The experience presents a solution to the problem of increasing amounts of organic waste and represents an improvement in cultivation techniques to increase crop yields, improve their quality and increase resistance to biotic and abiotic stress. This paper highlights the possibility of applying environmentally friendly organic waste materials to energy crops used as a sustainable energy source.

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.

2. Materials and Methods

2.1. Experiment Description

The field experiment was conducted on experimental plots of the University of Siedlce (52°17′ N 22°28′ E) throughout three growing periods, between 2018 and 2020. It was arranged in three replications in a randomized block design, consisting of 18 experimental plots, each with an area of 2 m². The research material was Chinese silver grass (Miscanthus sinensis Andersson), a perennial species. The experiment was based on two research factors:

• Factor 1—treatment in the form of composted municipal waste (MWC) and composted mushroom substrate (MSC) applied in different combinations, each with the same N dose of 170 kg N ha⁻¹. For example, by applying the combined dose of MWC_75% with MSC_25% (number 3 below), 75% of N was provided by municipal waste compost, and 25% was provided by mushroom substrate compost. Organic fertilizers were applied according to this scheme:
  • Control plot (no treatment);
  • MWC_100% (3.49 kg per plot; 17.450 Mg·ha⁻¹);
  • MWC_75% (2.62 kg per plot; 13.100 Mg·ha⁻¹) + MSC_25% (1.40 kg per plot; 7 Mg·ha⁻¹);
  • MWC_50% (1.74 kg per plot; 8.700 Mg·ha⁻¹) + MSC_50% (2.80 kg per plot; 14 Mg·ha⁻¹);
  • MWC_25% (0.87 kg per plot; 4.350 Mg·ha⁻¹) + MSC_75% (4.20 kg per plot; 21 Mg·ha⁻¹);
  • MSC_100% (5.60 kg per plot; 28 Mg·ha⁻¹).
• Factor 2—experimental years 2018, 2019, and 2020.

Rhizomes of Miscanthus sinensis Andersson were obtained from the Warsaw University of Life Sciences, the Experimental Station of the Institute of Agriculture, and planted on 26 April 2018 after loosening the soil by hand. Then, 9–12 cm long rhizomes, with 6 cm of lignified suckers, were planted manually at a depth of approx. 15–17 cm, six pieces per 2 m², at a spacing of 30 × 30 cm (30,000 rhizomes per ha). The distance between the plots was 1 m. Two weeks later, an even rise of the plants was observed on all experimental plots. Since the beginning of plant growth, no pests or pathogens were observed. Consequently, during the three-year experiment, there was no need to use plant protection products. Once a month, maintenance treatments such as manual weeding were performed. In order to control plant development, morphological features of five randomly selected stems were observed at various times.

2.2. Determination of Soil and Organic Material Properties

Before the experiment, the following physicochemical properties of the soil were determined:

• Granulometric composition by the aerometric method of Bouyoucos Casagrande modified by Prószyński in accordance with the Polish Standard PN-R-04033 [24] and grain size composition according to the Soil Science Society of Poland, 2008 [25];
• PH value in H₂O and in 1 mol/L KCl by the potentiometric method;
• Total hydrolytic acidity and the sum of basic cations (S) by the Kappen method, on the basis of which soil sorption capacity (T) and base saturation were calculated;
• Total C, N and H content by elemental analysis (using the PerkinElmer (U. S. Instrument Division, Norwalk, CT 06859 USA) 2400 Series II CHNS/O Elemental Analyzer with thermal conductivity detector) and the total (in the calculations assumed as total, but in reality, similar to it) content of Ni, Cu, Cr, Zn, Pb, Cd, K and P by the optical emission spectrometry method after wet mineralization of soil samples using aqua regia, at Eurofins OBiKŚ Polska Ltd. in Katowice, Poland, the former Centre for Environmental Research and Control.

Total soil N and C content was 39.40 and 2.85 g·kg⁻¹ DM, with a pH of 6.81. Due to deep tillage and intensive fertilizer treatment of the cultured soil, the amounts of C and N were high. The total content of heavy metals (8.55 Cr mg·kg⁻¹ DM, 0,85 Cd mg·kg⁻¹ DM, Cu 18.85 mg·kg⁻¹ DM and Ni 5.50 mg·kg⁻¹ DM) in the soil before the experiment was several times lower than the permissible limits provided to the Regulation of the Minister of the Environment [26,27]. On the other hand, the content of 149 Zn mg·kg⁻¹ DM and Pb 47.83 mg·kg⁻¹ DM was within the permissible range.

In the representative samples of mushroom substrate and municipal waste composts, the following were determined:

• Dry matter content by drying the sample at 105 °C to obtain a constant weight;
• PH in H₂O and in 1 mol/L KCl by potentiometric method;
• Total N content (Nt) by the modified Kjeldahl method after mineralisation of samples with concentrated sulphuric acid in the presence of a selenium mixture [28];
• Organic C (Corg) by the oxidation–titration method [29];
• Total content of macroelements (P and K) and heavy metals (Co, Pb, Cd, Cr, Zn and Ni) by inductively coupled plasma atomic emission spectrometry (ICP-OES) after soil sample mineralization with aqua regia.

2.3. Determination of Biomass Properties

In August of each year, relative chlorophyll content was measured using the SPAD-502 handheld meter by Minolta. The main parameters of chlorophyll fluorescence were measured using a OS30p+ fluorometer with 30 replications. Fully developed leaf blades of the central part of plants were collected. Then, in the process of preparation for measurements, leaves were kept in darkness for 30 min in order to adapt to non-invasive measurements. The following were determined:

−

Minimum fluorescence (F_o);

−

Maximum fluorescence (F_m).

Each year in October, measurements of the following plant morphological characteristics were taken with six replications:

−

Number of nodes per stem;

−

Diameter of the stem;

−

Number of leaf blades per stem;

−

Length of leaf blades;

−

Width of leaf blades;

−

Number of all stems;

−

Length of the stem.

Plants were harvested mechanically with a brush cutter in January 2019 and 2020 and in February 2021. After cutting them at a height of 10 cm, all of the plants were tied into sheaves and weighed to determine the yield of fresh matter. During each harvest, a representative sample was collected from each plot (five leafy stems) in order to determine dry matter content, morphological characteristics and chlorophyll content and fluorescence. The dry matter yield was determined after drying samples at 105 °C to obtain a constant weight. The biomass yield of Miscanthus sinensis from a plot (kg·plot⁻¹) was converted into a hectare (Mg·ha⁻¹). Additionally, 10 plants from each plot were collected to determine the content of selected heavy metals. At the end of the experiment in February 2021, after the last harvest, rhizome samples (four pieces per plot) were collected from the experimental plots for testing.

2.4. Weather Conditions

On the basis of data provided by the Hydrological and Meteorological Station in Siedlce, Sielianinov’s hydrothermal coefficient (K) was calculated for each month of the growing period. Then, the coefficient values were grouped into classes ranging from extremely dry to extremely wet in order to determine the effect of monthly precipitation and air temperature on Miscanthus sinensis growth and development [30].

2.5. Statistical Analysis

The results were statistically processed using the analysis of variance for a two-factor experiment. The significance of experimental factors on the value of the features was assessed with the Fisher–Snedecor test, and the value of LSD_0.05 for a detailed comparison of means was calculated using Tukey’s test. Statistica StatSoft 13.1 was used for calculations [31].

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. Values of Sielianinov’s hydrothermal coefficient (K) between 2018 and 2020.

Year	Month
	April	May	June	July	August	September	October
2018	1.07 fd	0.50 vd	1.38 o	1.58 o	0.44 vd	0.92 d	1.52 o
2019	0.32 ed	2.83 vw	0.44 vd	0.72 d	1.21 fd	1.01 fd	0.62 vd
2020	0.29 ed	3.24 ew	3.02 ew	0.69 vd	1.09 fd	1.06 fd	2.73 vw

Extremely dry (ed), very dry (vd), dry (d), fairly dry (fd), optimal (o), wet (w), very wet (vw), and extremely wet (ew).

), 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. Selected properties of organic waste materials.

Organic Fertiliser	pH	DM (%)	Corg	C:N	N	P	K
			g.kg−1 DM		g.kg−1 DM
MWC	7.10	68	236	14.93	14.30	17.32	25.4
MSC	6.4	30.00	284	13.59	20.9	8.86	11.21

). 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. Content of selected heavy metals in organic waste materials.

Organic Fertiliser	Co	Pb	Cd	Cr	Zn	Ni
	mg·kg−1 DM
MWC	3.58	78.4	2.08	34.12	623.2	15.6
MSC	0.415	3.98	0.287	3.08	156.9	4.84

) 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 (MWC₂₅+MSC₇₅) 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. Minimum chlorophyll fluorescence (Fo) of Miscanthus sinensis leaves.

Treatment (A)	Years (B)			Mean
	2018	2019	2020
Control plot	295	174	254	241
MWC100	272	217	221	236
MWC75 + MSC25	217	209	178	201
MWC50 + MSC50	284	235	240	253
MWC25 + MSC75	237	182	211	210
MSC100	232	267	254	251
Mean	256	214	226	232

LSD_0.05, A—treatment, A—8.393; B—years, B—4.765; AxB; BxA—interaction, A/B—14.537; B/A—11.671; SE (A) ± 30.82; SE (B) ± 31.81.

). 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. Maximum chlorophyll fluorescence (Fm) of Miscanthus sinensis leaves.

Treatment (A)	Years (B)			Mean
	2018	2019	2020
Control plot	592	502	677	590
MWC100	568	563	607	579
MWC75 + MSC25	565	519	496	526
MWC50 + MSC50	607	788	508	634
MWC25 + MSC75	526	577	573	558
MSC100	369	637	717	574
Mean	538	598	596	577

LSD_0.05, A—treatment, A—21.418; B—years, B—12.160; AxB; BxA—interaction, A/B—37.097; B/A—29.785; SE (A) ± 83.22; SE (B) ± 93.60.

) 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. Relative chlorophyll content (SPAD) of Miscanthus sinensis leaves.

Treatment (A)	Years (B)			Mean
	2018	2019	2020
Control plot	38.77	30.80	38.53	36.03
MWC100	31.27	31.53	38.50	33.77
MWC75 + MSC25	38.07	28.27	40.07	35.47
MWC50 + MSC50	34.90	29.97	40.83	35.23
MWC25 + MSC75	35.67	30.37	40.33	34.46
MSC100	34.43	28.23	36.87	33.18
Mean	35.52	29.86	39.19	34.86

LSD_0.05, A—treatment, A—1.844; B—years, B—1.047; AxB; BxA—interaction, AxB—3.194; BxA—2.565; SE (A) ± 4.97; SE (B) ± 1.84.

) 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].

4. Conclusions

This experiment proved that municipal waste compost could be used in the cultivation of energy crops, meeting the safety criteria with the content of heavy metals within the standards. It is a rich source of potassium (25.4 g·kg⁻¹ DM) and phosphorus (17.32 g·kg⁻¹ DM). With trace amounts of heavy metals, composted mushroom substrate contained 16% more organic carbon and 31% more nitrogen than municipal waste compost. The most effective in increasing the yields of Miscanthus sinensis was composted mushroom substrate applied at a dose of 28 Mg·ha⁻¹, with the highest amount of biomass noted in the experiment of 12.69 Mg·ha⁻¹ DM in the third year of cultivation. Similarly, the highest average yield was on the plot treated with composted mushroom substrate (8.44 Mg·ha⁻¹). Over the years, the highest yield of Miscanthus sinensis biomass were obtained in the third one (10.79 Mg·ha⁻¹ DM, 52% higher than the yield obtained in the first year, with 5.23 Mg·ha⁻¹ DM). Surprisingly, there was no effect of fertilizer treatment on most morphological measurements. Increased maximum values of leaf chlorophyll fluorescence in the second and the third year indicated plant stress caused by unfavourable hydrothermal conditions. That did not result in reduced yield of Miscanthus sinensis and confirmed its resistance to stress caused by adverse weather. Stress factors usually affect plant physiological processes, limiting plant productivity, which can lead to reduced yields. The challenge in obtaining energy materials from biomass is to ensure high and efficient production of crops resistant to stress factors. Further studies are needed to confirm the impact of composted municipal waste on the yields and morphological parameters of Miscanthus sinensis.