The Effects of Different Doses of Organic Waste on Prairie Cordgrass (Spartina Pectinata L.) Yield and Selected Energy Parameters

Abstract

Increasingly cultivated all over the world, energy crops, with their large biomass production, can be used to produce liquid biofuel and biogas. The aim of the research was to evaluate the yield and selected energy parameters of prairie cordgrass (Spartina pectinata L.) treated with different doses of municipal sewage sludge and mushroom substrate. Heat of combustion, calorific values and ash content were investigated in the first three years of its cultivation. Carried out between 2018 and 2020, the research was based on a field experiment established at an experimental facility in Central-Eastern Poland. Organic waste doses, each of them introducing 170 kg N ha⁻¹, were applied once in spring 2018 before planting pieces of Spartina pectinata rhizomes. The experimental factors (organic waste doses and years of research) significantly affected the yield of prairie cordgrass. Significantly, the highest yield of its biomass was produced in response to municipal sewage sludge applied together with mushroom substrate (O₂₅ + PP₇₅) and in response to mushroom substrate applied on its own (SMS). Those values, averaged over three years of research, were, respectively, 4.23 and 4.18 Mg ha⁻¹. Organic waste treatment had a significant impact on ash content in dry matter. On average, the highest ash content in dry matter was recorded in response to mushroom substrate (5.73%) and the lowest (4.98%) in plants treated with the highest dose of sewage sludge together with the lowest dose of mushroom substrate (O₇₅ + PP₂₅). The higher dry matter content in plant biomass was, the better the energy parameters were.

1. Introduction

The world’s energy needs are constantly increasing. In 2021, global demand for primary energy increased by almost 6%. This was due to, among other factors, the economic recovery after the COVID-19 pandemic, the development of global economy as well as global warming. As a result, energy needs for heating and cooling in the world have increased. The ever-increasing demand for energy also caused an increase in CO₂ emission, which in 2021 reached a historic level of 36.3 gigatons [1,2]. Energy from plant biomass is considered environmentally friendly because the amount of carbon dioxide produced in the combustion process is compensated by the photosynthesis of energy crops. Thus, the production of energy from plant biomass contributes to the reduction in CO₂ emission, thereby lowering the greenhouse effect [3]. Energy crops can be divided into four groups. Annual plants such as cereals, rapeseed, corn and sugar cane belong to the first one. The second group comprises woody plants of short rotation, i.e., willow, poplar and aspen. The third is constituted by fast-growing perennial grasses like giant miscanthus, reed canary grass, prairie cordgrass and switchgrass [4]. The last of the abovementioned groups of energy crops consists of perennials, e.g., Jerusalem artichoke. Plant biomass varies greatly in terms of dry matter content and chemical composition, which are both important from an energy point of view. The yield and calorific value, but also the content of ash, which remains as waste after combustion, are also important. Ash left after the combustion of plant biomass can be used as mineral fertilizer [5]. Organic waste containing plant nutrients can be successfully applied to energy crops. Materials used this way are sewage sludge, a byproduct of wastewater treatment; mushroom substrate [6,7,8,9], a waste product in the cultivation of white two-spore mushrooms (Agaricus bisporus) and digestate from biogas plants [10,11]. The natural management of organic waste is important both from environmental and economic perspectives [12,13,14]. Agricultural use of waste organic materials contributes to increasing plant yield and improving soil physicochemical properties [15,16].

The aim of the experiment was to evaluate yield and selected energy parameters of prairie cordgrass (Spartina pectinata L.) in response to different doses of municipal sewage sludge and mushroom substrate. The following research hypothesis was formulated: the application of organic waste of fertilizing character increases the yield of Spartina pectinata L. and affects selected energy parameters.

2. Methodology

2.1. Description of the Experiment

The field experiment was conducted on the premises of the experimental facility of the University of Natural Sciences and Humanities in Siedlce (52°17′ N, 22°28′ E) on soil classified as anthropogenic (A) culture-earth type (AK) and hortisol subtype (AKho) [17]. The soil had developed from glaciofluvial sand, and it had a heavy sandy loam texture with fine sandy loam in the subsoil. The plant used in the experiment was prairie cordgrass (Spartina pectinata L.). The experiment was conducted in a randomized block design, with three replications, according to the following scheme:

• Control (no fertilizer treatment);
• Municipal sewage sludge, introducing 170 kg N ha⁻¹ (0.907 kg of sludge per plot, i.e., 4.54 Mg ha⁻¹), (SS);
• Municipal sewage sludge + mushroom substrate, introducing 170 kg N ha⁻¹, 75% was sewage sludge and 25% mushroom substrate (0.683 kg + 1.40 kg per plot, i.e., 3.38 Mg ha⁻¹ + 7 Mg ha⁻¹), (SS₇₅ + SMS₂₅);
• Municipal sewage sludge + mushroom substrate, introducing 170 kg N ha⁻¹, 50% was sewage sludge and 50% mushroom substrate (0.454 kg + 2.80 kg per plot, i.e., 2.25 Mg ha⁻¹ + 14 Mg ha⁻¹), (SS₅₀ + SMS₅₀);
• Municipal sewage sludge + mushroom substrate, introducing 170 kg N ha⁻¹, 25% was sewage sludge and 75% mushroom substrate (0.227 kg + 4.20 kg per plot, i.e., 1.13 Mg ha⁻¹ + 21 Mg ha⁻¹), (SS₂₅ + SMS₂₅);
• Mushroom substrate, introducing 170 kg N ha⁻¹ (5.60 kg per plot, i.e., 28 Mg ha⁻¹), (SMS).

In the spring of 2018, microplots with an area of 2 m² were arranged. Grass rhizomes were obtained by cutting the root system into pieces and plating them by hand at a depth of 15–20 cm, with three rhizomes per 1 m². Waste organic materials were applied once, in the spring, before planting prairie cordgrass rhizomes. The choice of the abovementioned organic materials to be applied to the plant was dictated by a number of factors. The cultivation of bispore mushroom (Agaricus bisporus) is a dynamically developing branch of production in Poland, and in the Siedlce region it had developed intensively compared to other parts of the country. The research on the agricultural use of mushroom substrate in this area was extremely important because of the need for its disposal. Municipal sewage sludge was obtained from the sewage treatment plant in Siedlce. The capacity of the treatment plant was 24,000 m³ day⁻¹ with 1897 Mg of sewage sludge produced a year, an average of 5.2 Mg day⁻¹. In each year, mechanical weed control was used on the plots since herbicides would have damaged young shoots of plants.

2.2. Soil and Organic Materials Analysis

Before the experiment was established, a soil air-dry representative sample was collected from the arable layer (20 cm), and the following were determined:

• Granulometric composition, using the Bouyoucosa–Casagrande aerometric method in the Prószyński modification in accordance with the Polish Standard PN-R-04033 [18] and with the Classification System of Soil Texture and Mineral Materials [19];
• pH value in H₂O and in 1 mol KCl dm⁻³, using the potentiometric method;
• Total nitrogen (TN), carbon (TC) and hydrogen content, by elemental analysis using the PerkinElmer Series II 2400 CHNS/O Analyzer with the thermal conductivity detector (TCD);
• The total content of P, K, Cd, Pb, Zn, Cr, Cu and Ni, by mineralizing soil samples with aqua regia and using the inductively coupled plasma–optical emission spectrometry (ICP–OES) method at Eurofins OBiKŚ Polska Ltd. in Katowice, formerly the Centre for Environmental Research and Control.

Soil pH value in H₂O was 6.93, but in 1 mol KCl dm⁻³ it was 6.60. Total N and C content in the soil (gkg⁻¹) was 2.85 and 40.50, respectively. Soil content of the majority of heavy metals (Cr, Cd, Cu and Ni) before the experiment was several times lower than the amounts provided in the Regulation of the Ministry of the Environment [20] for light soils when applying municipal sewage sludge plants not intended for consumption or feed production. On the other hand, the content of Zn and Pb was within the standards. The content of heavy metals in the soil was (mgkg⁻¹) Pb—48.89, Cd—0.959, Cr—8.95, Cu—18.85, Zn—149.7, Ni—5.53.

In representative samples of municipal sewage sludge and mushroom substrate the following were determined:

• Dry matter, by drying the sample at 105 °C until constant weight was obtained;
• pH value in H₂O and 1 mol KCl dm⁻³, by the potentiometric method;
• Total nitrogen (TN), by the modified Kjeldahl method, mineralizing samples in concentrated sulfuric acid (VI) in the presence of selenium mixture [21];
• Organic carbon (C_org), by the oxidation–titration method [22];
• Total content of macroelements (P and K) and heavy metals (Co, Pb, Cd, Cr, Cu, Zn and Ni), by mineralizing soil samples with aqua regia and using the inductively coupled plasma–optical emission spectrometry (ICP–OES) method.

2.3. Biomass

Prairie cordgrass biomass was harvested in January 2018 and 2019 and in February 2020, with the yield of fresh matter determined using the weight method. During the harvest, a representative sample of five shoots with leaves was collected from each plot to determine dry matter content and to perform chemical analyses. Dry matter yield was determined after drying plant material at 105 °C to obtain a constant weight. In representative samples of plants, after prior shredding and grinding, ash content (PN-ISO 117:2002), heat of combustion (PN-G-04513:1981) and calorific value in the natural and dry states were determined (PN-91/G-04510) in the Energy Company in Siedlce. Heat of combustion was determined after complete combustion of the samples in a pressurized oxygen atmosphere in the bomb calorimeter (constant volume), and the measurement of temperature rise was made in the vessel calorimeter.

2.4. Meteorological Conditions

Meteorological conditions from 2018–2020 were assessed on the basis of data made available by the Institute of Meteorology and Water Management, National Research Institute (PIB) in Warsaw. In order to determine temporal variability and the impact of precipitation and air temperature on the growth and development of plants, Sielianinov’s hydrothermal coefficient (K) was determined according to the following formula:

$$K=\frac{P}{0.1\mathsf{\Sigma }\mathrm{t}}$$

(1)

where:

P—monthly rainfall,

Σt—the sum of daily air temperature values in a given month [23].

Then, ten classes of hydrothermal conditions were established using Sielianinov’s coefficient (K):

K ≤ 0.4 extreme drought (ed),

0.4 < K ≤ 0.7 severe drought (sd),

0.7 < K ≤ 1.0 drought (d),

1.0 < K ≤ 1.3 moderate drought (md),

1.3 < K ≤ 1.6 optimal (o),

1.6 < K ≤ 2.0 moderately wet (mw),

2.0 < K ≤ 2.5 wet (w),

2.5 < K ≤ 3.0 severely wet (sw),

K > 3.0 extremely wet (ew) [24].

2.5. Statistical Analysis

The results were subjected to statistical processing using analysis of variance for a two-factor experiment. The significance of the effect of experimental factors on the value of the examined parameters was assessed on the basis of the Fisher–Snedecor F test. The value of LSD_0.05 (for a detailed comparison of means) was calculated with Tukey’s test. The Statistica StatSoft 13.1 [25] program was used for the calculations. Pearson linear correlation coefficients between the calorific value of the biomass and the selected energy parameters for p ≤ 0.05 were calculated.

3. Results and Discussion

Sielianinov’s coefficient (K) indicated that optimal hydrothermal conditions for the growth and development of prairie cordgrass were only in June, July and October in the first year of research (

Table 1. Value of Sielianinov’s hydrothermal coefficient (K) in individual months of in 2018–2020.

Year	Month
	April	May	June	July	August	September	October
Year 1 (2018)	1.07 (md)	0.50 (sd)	1.38 (o)	1.58 (o)	0.44 (sd)	0.92 (d)	1.52 (o)
Year 2 (2019)	0.32 (ed)	2.83 (sw)	0.44 (sd)	1.72 (d)	1.21 (md)	1.01 (md)	0.62 (sd)
Year 3 (2020)	0.29 (ed)	3.24 (ew)	3.02 (ew)	0.69 (sd)	1.09 (md)	1.06 (md)	2.73 (sw)

md—moderate drought, ed—extreme drought, sd—severe drought, sw—severely wet, ew—extremely wet, o—optimal, d—drought, md—moderate drought.

). The first year was the only one during the experiment in which optimal weather prevailed for a few months. The most difficult conditions for the growth and development of plants were in the second year, when, with the exception of May, extremely dry to quite dry months prevailed. In the third year of research, hydrothermal conditions were not favorable either. The month of April was extremely dry, while May and June were extremely wet. July was very dry, and August and September were quite dry. For many years, not only in Poland, problems related to long-term periods of drought have been observed [26,27].

Municipal sewage sludge applied to prairie cordgrass contained high amounts of dry matter (93%) and selected macroelements (

Table 2. Chemical composition of organic materials used in the experiment.

Organic Waste	pH	DM (%)	Corg (g kg−1)	C/N	N (g kg−1)	P (g kg−1)	K (g kg−1)
municipal sewage sludge (SS)	6.40	93.0	348	7.80	40.5	19.8	2.56
mushroom substrate (SMS)	6.41	30.0	284	13.6	20.9	8.86	11.2

). Its C/N ratio of 7.8 was beneficial for soil microorganisms and for plants. Its content of N, P and K was similar to that reported by other authors [28]. According to the literature [29], potassium content in municipal sewage sludge is usually low. Similarly, in the present experiment, its content was low, with 2.56 g kg⁻¹. Sewage sludge usually contains high amounts of nutrients having a positive effect on the soil, i.e., on its chemical, biological and physical properties, and when applied to various plants, it may be more effective than mineral fertilizers [30,31]. The content of heavy metals in the sludge was relatively low (

Table 3. Content of selected heavy metals in organic materials used in the experiment.

Type of Organic Waste	Heavy Metal Content (mg kg−1 DM)
	Co	Pb	Cd	Cr	Zn	Ni
municipal sewage sludge (SS)	3.58	36.12	1.81	15.44	987.2	44.23
mushroom substrate (SMS)	0.415	3.98	0.287	3.08	156.9	4.84

). Acceptable standards for their content are provided in the Regulation of the Ministry of the Environment [20]. The organic waste did not exceed them, which allowed its use in the cultivation of Spartina pectinata.

The 30% dry matter content of mushroom substrate was relatively high (Table 2 and Table 3), which was confirmed by many authors [32,33]. Organic carbon content, an indicator of soil biological activity, was 284 g kg⁻¹. Mushroom substrate contained high amounts of nitrogen, with 20.9 g kg⁻¹, and the C/N ratio was 13.6. Its reaction was close to neutral (pH 6.5). Its chemical composition determined in the experiment was confirmed by many reports in the literature on the nutritional value of mushroom substrate left after the cultivation of white mushrooms [34,35,36].

Years of research and doses of organic materials significantly affected the yield of fresh and dry biomass (

Table 4. Biomass yield of Spartina pectinata (Mg ha⁻¹) in the first three years of cultivation.

Year of Research (B)	Experimental Plot (A)						Mean
	Control Plot	SS	SS75 + SMS25	SS50 + SMS50	SS25 + SMS75	SMS
	Fresh Matter
Year 1	1.20	1.51	1.63	1.47	1.89	1.78	1.58
Year 2	2.31	3.89	4.60	3.47	3.68	4.10	3.68
Year 3	6.89	9.54	8.54	9.14	10.12	9.74	8.99
Mean	3.47	4.98	4.92	4.69	5.23	5.21	4.75
LSD0.05      A-0.980; B-0.563; A/B-1.70; B/A-1.38
dry matter
Year 1	0.96	1.22	1.30	1.21	1.47	1.42	1.26
Year 2	1.96	3.07	3.77	2.78	2.91	3.24	2.96
Year 3	5.37	7.35	6.75	7.43	8.30	7.89	7.18
Mean	2.76	3.88	3.94	3.81	4.23	4.18	3.80
LSD0.05         A-0.333; B-0.191; A/B-0.469; B/A-0.341

SS—municipal sewage sludge dose introducing 170 kg N ha⁻¹; SMS—mushroom substrate dose introducing 170 kg N ha⁻¹; SS₇₅ + SMS₂₅; SS₅₀ +SMS₅₀; SS₂₅ + SMS₇₅—municipal sewage sludge used together with mushroom substrate in various proportions, each dose introducing 170 kg N ha⁻¹.

). The lowest value was recorded in the first year of research, but with subsequent years it increased; a similar tendency was noted in the research of Kowalczyk-Juśko [37]. In the third year, the yield of dry matter was more than five times higher than in the first one and amounted on average to 7.18 Mg ha⁻¹ DM. According to Kowalczyk-Juśko [37], the yield of prairie cordgrass ranged from 0.78 Mg ha⁻¹ in the first year to 10.95 Mg ha⁻¹ in the third. Studies conducted in Ireland and England by El Bassam [38] also indicated very low yield of the grass in the first year after planting. In the present experiment, the highest yield was noted in response to the combination of municipal sewage sludge applied with mushroom substrate at a dose of O₂₅ + PP₇₅ and in response to mushroom substrate applied on its own (SMS). Dry matter yield in those plots was 4.23 and 4.18 Mg ha⁻¹, respectively, with 2.76 Mg ha⁻¹ in the control plot.

Figure 1 presents the hygroscopic moisture of Spartina pectinata biomass in the natural state by year. It ranged from 3.78% to 5.76%, with the average value of 4.74% in the first year of research, 4.25% in the second, the lowest of all, and 5.58% in the third. According to Kowalczyk-Juśko [5], natural moisture content varied across different plant species, and for thornless rose it was 14.6%, for tuberous sunflower 9.7%, for Pennsylvania mallow 8.3% and 13.5% for prairie cordgrass. Excessive moisture can significantly reduce the calorific value of plant biomass.

Average ash content was lower in the Spartina pectinata natural state (5.03%) than in the dry state (5.30%) (

Table 5. Ash content (%) in the Spartina pectinata biomass in the natural and dry states.

Year of Research (B)	Experimental Plot (A)						Mean
	Control Plot	SS	SS75 + SMS25	SS50 + SMS50	SS25 + SMS75	SMS
	Biomass in the Natural State
Year 1	5.17	5.33	5.11	5.61	5.31	5.20	5.29
Year 2	5.31	5.09	5.69	5.37	5.59	5.82	5.48
Year 3	5.08	4.13	3.41	3.57	4.68	4.98	4.31
Mean	5.19	4.85	4.74	4.85	5.19	5.33	5.03
LSD0.05    A-NS        B-0.449       A/B-NS        B/A-NS
Biomass in the dry state
Year 1	5.39	5.60	5.38	5.86	5.58	5.51	5.55
Year 2	5.54	5.31	5.96	5.61	5.83	6.08	5.72
Year 3	5.39	4.38	3.61	3.72	4.95	5.61	4.61
Mean	5.44	5.10	4.98	5.06	5.45	5.73	5.30
LSD0.05 A-0.721            B-0.414      A/B-1.25       B/A-1.01

SS—municipal sewage sludge dose introducing 170 kg N ha⁻¹; SMS—mushroom substrate dose introducing 170 kg N ha⁻¹; SS₇₅ + SMS₂₅; SS₅₀ +SMS₅₀; SS₂₅ + SMS₇₅—municipal sewage sludge used together with mushroom substrate in various proportions, each dose introducing 170 kg N ha⁻¹, NS—not significant difference.

). Treatment combinations significantly affected grass ash content in the dry state. According to Wilk [39], ash content in woody biomass ranged from 0.3 to 7.4% and from 4.3 to 10.4% in cereal straw. In contrast, in the biomass of various plants grown in Portugal and Spain, like Cytisus multiflorus (L’Hér.) Sweet, Erica australis (L.), Ulex europaeus (L.) and Pterospartum tridentatum (L.), ash content was significantly lower and ranged between 1.32 and 1.58% [40]. In turn, Miscanthus sacchariflorus contained 4.3% of ash in the dry state [41]. In the light of the present results, it can be concluded that Spartina pectinata ash content was not high and was within the range of average values for other energy crops.

Heat of combustion and calorific value are the main characteristics determining the suitability of crops for energy production [42]. The highest heat of combustion of prairie cordgrass biomass in the dry state as well as in the natural state, as an average from treatment combinations, was recorded in the second year of research, while it was the lowest in the first (

Table 6. Heat of combustion (kJ kg⁻¹) of Spartina pectinata biomass in the natural and dry states.

Year of Research (B)	Experimental Plot (A)						Mean
	Control Plot	SS	SS75 + SMS25	SS50 + SMS50	SS25 + SMS75	SMS
	Biomass in the Natural State
Year 1	18,360	17,259	18,143	18,460	18,090	18,130	18,074
Year 2	19,003	18,734	18,879	18,877	18,581	18,874	18,825
Year 3	18,183	18,878	18,538	17,956	18,037	17,896	18,248
Mean	18,516	18,290	18,520	18,431	18,236	18,300	18,382
LSD0.05         A-NS                  B-309.3           A/B-932.6          B/A-757.5
Biomass in the dry state
Year 1	19,462	18,236	19,208	19,109	18,861	18,886	18,960
Year 2	19,817	19,543	19,762	19,735	19,369	19,729	19,660
Year 3	19,631	20,032	19,643	18,960	19,077	18,960	19,383
Mean	19,637	19,270	19,539	19,268	19,101	19,192	19,334
LSD0.05      A-502.2                B-288.4          A/B-869.8          B/A-706.5

SS—municipal sewage sludge dose introducing 170 kg N ha⁻¹; SMS—mushroom substrate dose introducing 170 kg Nha⁻¹; SS₇₅ + SMS₂₅; SS₅₀ +SMS₅₀; SS₂₅ + SMS₇₅—municipal sewage sludge used together with mushroom substrate in various proportions, each dose introducing 170 kg N ha⁻¹, NS—not significant difference.

). Natural-state biomass heat of combustion was lower than in the dry state. The heat of combustion of prairie cord grass in the natural state, averaged across years and treatment combinations, was 18,382 kJ kg⁻¹, with 19,334 kJ kg⁻¹ in the dry state. According to Kolowca and Knapik [43], Miscanthus gigantia heat of combustion was at a slightly higher level of 21.6 MJ kg⁻¹. In turn, Kościk [44] observed a lower value of prairie cordgrass heat of combustion at the level of 17.3 MJ kg⁻¹. For dry-state biomass from the control plot, heat of combustion was significantly higher than for plants treated with both sewage sludge and mushroom substrate at a dose of O₂₅ + PP₇₅. Lower heat of combustion values were recorded for other fertilizer plots than for the control plot, but these differences were not statistically significant.

The calorific value of prairie cordgrass in the dry state was 17,994 kJ kg⁻¹ and was much higher than in the natural state (17,116 kJ kg⁻¹) (

Table 7. Calorific value (kJ kg⁻¹) of Spartina pectinata biomass in the dry and natural states.

Years of Research (B)	Experimental Plot (A)						Mean
	Control Plot	SS	SS75 + SMS25	SS50 + SMS50	SS25 + SMS75	SS
	Biomass in the Natural State
Year 1	17,070	16,001	16,841	17,097	16,806	16,860	16,779
Year 2	17,734	17,461	17,610	17,605	17,316	17,608	17,556
Year 3	17,238	17,586	17,247	16,651	16,744	16,605	17,012
Mean	17,347	17,016	17,233	17,118	16,955	17,024	17,116
LSD0.05    A-NS                   B-306.9             A/B-925.5       B/A-751.8
Biomass in the dry state
Year 1	18,242	17,045	17,947	17,864	17,627	17,664	17,732
Year 2	18,598	18,321	18,548	18,517	18,154	18,517	18,109
Year 3	18,410	18,798	18,399	17,718	17,783	17,737	18,141
Mean	17,750	18,055	18,298	18,033	17,855	17,973	17,994
LSD0.05    A-NS                   B-360.6          A/B-883.2         B/A-1087

SS—municipal sewage sludge dose introducing 170 kg Nha⁻¹; SMS—mushroom substrate dose introducing 170 kg Nha⁻¹; SS₇₅ + SMS₂₅; SS₅₀ +SMS₅₀; SS₂₅ + SMS₇₅—municipal sewage sludge used together with mushroom substrate in various proportions, each dose introducing 170 kg N ha⁻¹, NS—not significant difference.

). According to the literature, the calorific value of cereal straw was at a similar level: from 16.1 MJ kg⁻¹ for barley to 17.3 MJ kg⁻¹ for wheat [45], and for sugar miscanthus it was 16,743 kJ kg⁻¹ [41]. Treatment did not cause significant differences in Spartina pectinata calorific value. For plants treated with organic waste, it was higher than for control ones, but the differences were not statistically significant. On the other hand, years of research significantly affected this feature, with the lowest calorific value in the first year.

The relationship between calorific value and other parameters of prairie cordgrass is presented in the form of linear correlation coefficients (

Table 8. Linear correlation coefficients between calorific value, moisture, ash content and heat of combustion of Spartina pectinata biomass.

Biomass Energy Parameter	Calorific Value	Moisture	Ash Content	Heat of Combustion
Calorific value	1.00
Moisture	−0.042	1.00
Ash content	−0.058	−0.516 *	1.00
Heat of combustion	0.988 *	−0.029	−0.077	1.00

* significant relationship p ≤ 0.05.

). A weak negative relationship between ash content and moisture on the one hand and the calorific value on the other was found. The correlation coefficients were, respectively, r = −0.058 and r = −0.042, which indicates that the higher the ash content, the lower the calorific value was. Similarly, as biomass moisture increased, there was a decrease in calorific value, which was confirmed by the research of Wilk [39]. A statistically significant positive relationship was noted between heat of combustion and calorific value (r = 0.988). This means that as one of them increased, the other also increased, i.e., the greater the heat of combustion, the greater the calorific value. A significant positive relationship was also found between ash content and biomass moisture. According to Bilandzija et al. [46], calorific value of all energy crops depends on their moisture content and increases with increasing dry matter content. A significant negative relationship (r = −0.516) was found between prairie cordgrass ash content and moisture content.

4. Conclusions

Municipal sewage sludge and mushroom substrate applied to Spartina pectinata in various combinations, each dose introducing 170 kg N ha⁻¹, significantly increased the biomass yield compared to the control plot. The combination of municipal sewage sludge with mushroom substrate at a dose of O₂₅ + PP₇₅ and mushroom substrate applied on its own (SMS) resulted in the highest yield (on average over years). More than 50% higher amounts of Spartina pectinata were recorded in those plots than in the control.

Ash content in prairie cordgrass biomass was not high. In the natural state, its average value was 5.03%, while in the dry state it was 5.30%. On average, the highest ash content was recorded in plants treated with mushroom substrate (5.73%) and the lowest (4.98%) in those treated with the highest dose of sludge together with the smallest dose of mushroom substrate (O₇₅ + PP₂₅). Ash content is very important for planning the use of plant biomass in the process of combustion on its own or with coal. Combined application of both sewage sludge and mushroom substrate is more effective than applying mushroom substrate only.

Heat of combustion and calorific value of prairie cordgrass significantly varied across growing seasons. Biomass energy parameters in the first year were slightly lower than in the second. The application of organic materials did not significantly affect calorific value. Due to varied yield and parameter content results of the studies on application of waste organic substances to energy crops, further research in this area is needed. The outcomes of such research are useful to the energy sector.