Jerusalem Artichoke: Nitrogen Fertilization Strategy and Energy Balance in the Production Technology of Aerial Biomass

Abstract

Jerusalem artichoke (Helianthus tuberosus L.) is a plant with considerable potential for energy generation due to its rapid growth, high biomass yield, and resistance to environmental stresses. The aim of this study was to determine the influence of the nitrogen fertilization strategy on the yield and energy balance in the production technology of Jerusalem artichoke (JA) in a perennial cropping system. The article presents the results of a three-year experiment which was conducted in Poland to determine the effect of different N rates (0, 50, 75, and 100 kg ha⁻¹) supplied with mineral fertilizers and liquid digestate on the energy balance in the production of JA aerial biomass. The experiment had a randomized block design with three replications. The demand for energy in JA cultivation reached 16.2–26.3 (year 1) and 2.9–14.6 GJ ha⁻¹ (years 2 and 3). Energy inputs in the cultivation technology were reduced by 17–19% (year 1) and 35–47% (years 2 and 3) when mineral fertilizers were replaced with digestate. Jerusalem artichoke yields were lowest in the technology without fertilization (12.5 Mg ha⁻¹ DM). Dry matter yield increased significantly (by 43–55%) after the application of 75 kg N ha⁻¹, regardless of fertilizer type. The energy output of biomass peaked (230.1 GJ ha⁻¹) in response to a mineral fertilizer rate of 75 kg N ha⁻¹. In turn, the highest energy gain (218.5 GJ ha⁻¹) was noted after the application of digestate at a rate equivalent to 75 kg N ha^–1. The energy efficiency ratio was highest in the technology without fertilization (20.1) and after the application of digestate at a rate equivalent to 75 kg N ha⁻¹ (19.7). Regardless of the factors that limit agricultural production, the energy balance of JA biomass production was most favorable when JA was fertilized with digestate at a rate equivalent to 75 kg N ha⁻¹. The results of this study may pave the way for future research on novel agronomic strategies for sustainable bioenergy production, including nutrient recycling.

1. Introduction

The use of fossil fuels in energy generation is regarded as the main cause of global climate change. Most efforts to minimize the adverse changes of climate change focus on the search for alternative sources of renewable energy [1,2,3,4]. Various sources of renewable energy are utilized in different regions of the world, depending on geographic location, climate, and the availability of natural resources. Countries with high solar energy potential may invest in solar farms, whereas wind farms may be preferred in countries with high wind energy capacity. Biomass is a highly versatile source of renewable energy [5,6]. Due to its widespread availability, biomass is an attractive energy source in many regions of the world with diverse climates. As a result, biomass can significantly contribute to the global energy transition and the implementation of sustainable and eco-friendly power generation systems [7]. Biomass has a number of additional advantages, in particular when it is produced as a dedicated energy crop: (i) it contributes to the management of marginal land and land protection against degradation through the use of best agronomic practices, and (ii) unlike wind and solar energy which fluctuate on a seasonal basis, biomass can be converted into electricity and heat throughout the year, thus stabilizing energy generation [8]. Biomass derived from lignocellulosic crops [8,9,10,11] that contain cellulose, hemicellulose (polysaccharides), and lignin (an aromatic polymer that acts as a binding agent for cellulose and hemicellulose) [12] is the most widely available source of renewable energy in the world. Lignocellulosic biomass is the third most important energy source after crude oil and coal [13]. It is one of the most sustainable sources of organic carbon (C_org) with net zero emissions of CO₂ [12]. In addition, most lignocellulosic crops, including dedicated energy crops, do not compete with food crops [14].

Maize (Zea mays L.) is the most popular lignocellulosic crop for bioenergy generation in Europe [15]. Maize is characterized by high biomass yields [16,17], high biochemical methane potential, and high biogas potential [18]. In Europe, the highest number of agricultural biogas plants (10,600) are operated in Germany [19]. Germany is the global leader in terms of installed electrical capacity of biogas plants (7500 MW_el) (59% and 41% of European and global electrical biogas capacity, respectively) [20]. Maize accounts for 73% of biomass feedstocks in German biogas plants [21]. In 2012, regulatory limits were imposed on the proportions of various feedstocks in biogas plants, and the so-called “maize cap” was introduced to promote the use of alternative substrates [22]. Maize is usually grown in the proximity of biogas plants to guarantee regular feedstock supplies. When grown as an energy crop, maize decreases the biodiversity of agricultural ecosystems and competes with food crops for arable land [23]. In Europe, maize is grown in high-input production technologies characterized by high fertilizer consumption, intensive tillage, and relatively high use of pesticides. High-input production technologies exert a negative impact on the environment, in particular when maize is grown in large-scale monocultures [8,24,25].

The biomass of Jerusalem artichoke (JA, Helianthus tuberosus L.) is an alternative substrate for renewable energy production [26,27,28,29,30,31]. Jerusalem artichoke has numerous advantages, including (i) low environmental and agronomic requirements [29,32,33,34]; (ii) tolerance to drought and soil salinity; and (iii) low fertilizer requirements [32,33,34]. Jerusalem artichoke can be grown as an annual plant (where tubers and aerial biomass are harvested each year) or as a perennial plant (where only aerial biomass is harvested annually) [28,29,35,36]. Both tubers and aerial biomass constitute valuable substrates for biogas and bioethanol production [26,29,37,38]. The yield potential of JA tubers and aerial biomass has been estimated at 70–100 Mg ha⁻¹ fresh matter (FM) [39,40] and 24–80 Mg ha⁻¹ dry matter (DM) [29,31,40,41,42,43,44]. The energy inputs (EIs) in the cultivation technology of JA range from 12–28 GJ ha⁻¹ (in a perennial cropping system, where only aerial biomass is harvested annually) [30,31] to 32–45 GJ ha⁻¹ (in an annual cropping system, where both tubers and aerial biomass are harvested each year) [45]. In perennial cropping systems, mineral fertilization is the most energy-intensive operation that accounts for 32–53% (in the year of plantation establishment) to 60–87% of total EI (successive years) [30,31]. Mineral fertilizers have a high share of total EI in crop production technologies mainly because fertilizer manufacture is an energy-intensive process [31]. In addition, intensive mineral fertilization, even at rates that are optimal for a given crop species under local conditions, has a negative impact on the ecosystem and the environment [46,47]. Organic fertilizers derived from agricultural (liquid and solid manure, slurry) and industrial production (municipal sewage sludge, digestate from agricultural biogas plants) offer an alternative to mineral fertilizers. Recycled fertilizers can decrease EI in crop production technologies [48,49] and eliminate/minimize the negative environmental impact of mineral fertilizers [47] without compromising agricultural productivity [50].

Digestate, a by-product from the anaerobic digestion of lignocellulosic biomass, is a popular organic fertilizer [16,51,52,53,54]. Biogas produced during anaerobic digestion is used for power generation, whereas digestate can be applied as organic fertilizer in agricultural production [55,56,57]. Digestate is a rich source of nitrogen (N), phosphorus, potassium, sulfur, micronutrients, and organic matter [58,59,60,61,62]. Digestate decreases the use of mineral fertilizers and mined resources (phosphate rock and natural gas) that are used in the production of mineral fertilizers [63]. Digestate contains mineral nutrients that are readily available to plants and significantly accelerate crop growth [64]. Digestate is also abundant in organic matter which increases the content of C_org in soil [65,66], initiates microbial processes [67,68], and stimulates the activity of soil enzymes [69]. As a result, digestate regulates the rate at which nutrients are released from organic compounds into the soil [64,65]. Digestate also improves soil structure [70] and soil’s ability to retain nutrients and water [46,52]. In the European Union (EU), around 180 Tg of digestate (a mixture of liquid manure and energy crops) is produced each year, mostly (67%) in agricultural farms [71]. Digestate is produced continuously in biogas plants, and it has to be adequately managed [72,73]. Prolonged storage of digestate is expensive and requires dedicated technical equipment [74]. Direct application in the field is the most popular and the cheapest method of digestate management [75,76]. However, effective groundwater protection measures are required when digestate is used as agricultural fertilizer. Soil nutrient levels have to be monitored, and legal standards concerning maximum nutrient loading rates have to be observed [77]. Directly after application, digestate should be incorporated into the topsoil layer to maximize productivity and protect the environment [78]. Adequate management of digestate in agricultural production is a direct and effective method of implementing the zero-waste concept as one of the pillars of a circular economy [71].

Research studies analyzing the effect of various methods of mineral fertilizer and digestate application on the yield of silage maize biomass, perennial grasslands, and winter triticale (×Triticosecale Wittm. ex A. Camus) intercropped with red clover (Trifolium pratense L.) demonstrated that digestate (solid and liquid fractions) induced a similar increase in biomass yields to mineral fertilizers [79]. Complete substitution of mineral fertilizers with digestate was possible in the intercropping system, and the greatest increase in biomass yields was noted when mineral fertilizers were combined with digestate, which reduced mineral fertilizer input by up to 66% [79]. In a study by Stolarski et al. [80], JA biomass yields increased when digestate was applied at a rate equivalent to 170 kg N ha⁻¹. At a lower fertilizer rate (85 kg N ha⁻¹), digestate induced a smaller increase in yields than mineral fertilizers [80]. In the work of Lee et al. [53], digestate applied at a rate equivalent to 150 kg N ha⁻¹ increased biomass yields in switchgrass (Panicum virgatum L.) by 14%. At lower N rates (50 or 100 kg ha⁻¹), digestate was less effective than mineral fertilizers. In turn, Nabel et al. [52] reported a correlation between fertilizer type, soil, and the biomass yield of Virginia fanpetals [Sida hermaphrodita (L.) Rusby]. In nutrient-poor soils (with a high content of the particle size fraction >2 mm and a low content of C_org), the biomass yield of Virginia fanpetals was 30% higher after the application of digestate than mineral fertilizers. However, mineral fertilizers induced a greater increase in biomass yields in nutrient-rich soils (with a high content of particle size fractions <2 mm). According to Dubis et al. [54], digestate and mineral fertilizers led to a comparable increase in the yields of Amur silvergrass [Miscanthus sacchariflorus (Maxim.) Hack.]. These observations indicate that digestate has a positive influence on the yields of energy crops. However, digestate’s impact on the energy balance in the cultivation technology of energy crops remains insufficiently investigated [49,54,81]. There is some evidence to indicate that recycled fertilizers (sewage sludge and solid digestate) improved the energy balance in the production of Amur silvergrass. Organic fertilizers decreased EI by 34–40% (sewage sludge) and 41–48% (digestate) [54]. In the work of Dubis et al. [54], the energy output (EO) was highest when M. sacchariflorus was supplied with 100 kg N ha⁻¹, regardless of fertilizer type (mineral fertilizer vs. digestate or sewage sludge). In turn, energy gain (EG) peaked in response to digestate applied at a rate equivalent to 160 kg N ha⁻¹. The energy efficiency ratio (EER) was also higher when M. sacchariflorus was fertilized with digestate or sewage sludge than mineral fertilizers [54]. In the cultivation of M. × giganteus, EI decreased by 32–34% (without affecting biomass yields) and the EER increased by 43−52% when mineral fertilizers were replaced with sewage sludge [82]. In the production technology of H. tuberosus, EI decreased by 27–32% and the EER increased by 44–48% when mineral fertilizers were replaced with sewage sludge [30].

The use of digestate as fertilizer in the production of non-food crops is a sustainable approach to waste management [81]. This biofertilizer can also increase yields in the production of JA biomass. At the same time, biomass produced with the use of digestate constitutes a valuable substrate or co-substrate for agricultural biogas plants; therefore, it is an important source of biogas and digestate in a circular economy. The production technology of JA biomass has to be intensified to ensure that EO exceeds EI. This is an important consideration because biomass conversion to energy will be environmentally and economically justified only if more energy is generated than input into the system. The influence of digestate fertilization on biomass yields and energy balance in the production of JA biomass has not been examined extensively to date. Therefore, the aim of this study was to determine the effect of N supplied with mineral fertilizers (ammonium nitrate, 34% N) and organic fertilizers (liquid digestate, 0.49–0.54% N) on biomass yields and energy balance (energy inputs, energy output, energy gain, and the energy efficiency ratio) in the cultivation technology of JA in a perennial cropping system (where only aerial biomass was harvested each year).

2. Materials and Methods

2.1. Field Experiment

The experiment was conducted in 2021–2023 at the Agricultural Experiment Station (AES) in Bałdy (Tomaszkowo village; 53°42′44′′ N, 20°25′54′′ E; elevation: 130 m above sea level; North-East Poland) which is part of the University of Warmia and Mazury in Olsztyn. The experimental variables were the N rate and two types of fertilizers: (i) control (without fertilization), (ii) 50 kg N ha⁻¹ supplied with mineral fertilizers, (iii) 50 kg N ha⁻¹ supplied with digestate, (iv) 75 kg N ha⁻¹ supplied with mineral fertilizers, (v) 75 kg N ha⁻¹ supplied with digestate, (vi) 100 kg N ha⁻¹ supplied with mineral fertilizers, and (vii) 100 kg N ha⁻¹ supplied with digestate. The experiment had a randomized block design with three replications. The experiment was established on Haplic Luvisol originating from boulder clay [83]. The plot area was 15 m² (6.0 × 2.5 m). Oat (Avena sativa L.) was the preceding crop. Before the experiment, soil samples (5 samples per plot) were collected at a depth of 0–20 cm. The experiment was established on slightly acidic soil (pH 5.8) with organic carbon (C_org) and total nitrogen (N_t) content of 8.1 and 1.06 g kg⁻¹, respectively. The content of available nutrients was determined at 206 mg P₂O₅ kg⁻¹, 165 mg K₂O kg⁻¹, 45 mg Mg kg⁻¹; 10 mg ${\mathrm{S}\mathrm{O}}_4^{2-}$ kg⁻¹, 0.3 mg B kg⁻¹; 93 mg Mn kg⁻¹; 1 mg Cu kg⁻¹, 7 mg Zn kg⁻¹, and 910 mg Fe kg⁻¹. The chemical composition of the soil was analyzed in the Chemical–Agricultural Station in Olsztyn according to the procedure described by Bardsley and Lancaster [84] and Houba et al. [85].

Nitrogen was applied as ammonium nitrate (34% N). In treatments with mineral fertilization, 65 kg P₂O₅ ha⁻¹ (enriched superphosphate, 40% P₂O₅) and 140 K₂O ha⁻¹ (potash salt, 50% K₂O) were applied in addition to N. Liquid digestate from a mesophilic digestion process was obtained from an agricultural biogas plant operated by EKODAMIR sp. z o.o., sp. k. in Jarnołtowo (situated 80 km from the experimental site). Digestate had alkaline pH (pH 7.8–8.9), DM content of 47 g kg⁻¹, and the following chemical composition: 4.9–5.4 g N_t kg⁻¹ DM, 1.2–1.4 g P kg⁻¹, 3.4–3.9 g K kg⁻¹, 1.2–1.3 g Ca kg⁻¹, 0.2 g Mg kg⁻¹ 0.02–0.03 mg Cd kg⁻¹, 1.1 mg Ni kg⁻¹, 0.34–0.76 mg Pb kg⁻¹, and 0.67 mg Cr kg⁻¹. Digestate rates (ii, iv, and vi) were calculated in each year of the experiment based on the content of N_t and DM (Table S1). The chemical composition of digestate was determined at the Aquanet Laboratorium sp. z o. o. in Poznań (Central-East Poland). In the first year of the experiment (plantation establishment), mineral fertilizers and digestate were applied before tuber planting. In years 2 and 3, fertilizers were applied before the beginning of the spring growing season. In all years of the study, only aerial biomass was harvested, and JA tubers were left in the soil for three years. The production process in the year of plantation establishment (year 1) and productive years (years 2 and 3) is presented in

Table 1. Production process of Jerusalem artichoke.

Farming Operation	Specification
Tillage a	Skimming (5–8 cm); harrowing, fall plowing (18–22 cm); two treatments with a cultivation unit (5–8 cm)
Planting a	20 April 2021; cv. Medius, row spacing: 75 × 30 cm; planting depth: 6–8 cm; density: 4.4 tubers m−2; disc hilling treatments between rows
Mineral fertilization b	50, 75, and 100 kg N ha−1 (ammonium nitrate, 34%); 65 kg P2O5 ha−1 (enriched superphosphate, 40%); 140 kg K2O ha−1 (potash salt, 60%) (20 April 2021; 13 April 2022; 20 April 2023)
Digestate application b	At the rates presented in Table S1 (20 April 2021; 13 April 2022; 20 April 2023)
Weed control b	Two disc hilling treatments between rows (after fertilization and after crop emergence)
Pathogen control b	200 g ha−1 azoxystrobin and 125 g ha−1 difenoconazole (25 June 2021; 30 June 2022; 3 July 2023)
Harvest and transport of aerial biomass b	21 September 2021, 21 September 2022, 22 September 2023

^a only in year 1 (2021); ^b treatments performed in each year of the experiment (2021, 2022, 2023).

.

2.2. Energy Input Analysis

The EIs associated with the cultivation technology of JA were determined based on diesel fuel consumption, the performance of tractors and agricultural machines, and human labor which were measured directly in the experimental fields in AES in Bałcyny (Table S2). The weight and operating time of tractors and agricultural machines were determined based on the work of Muzalewski [86,87]. The following factors were included in the calculation of EI in the production technology of JA: transport of agricultural equipment and materials from the farm to the experimental field, biomass transport from the experimental field to the farm (1 km), and digestate transport from the biogas plant to the experimental field (80 km). The energy equivalents for calculating EI are presented in

Table 2. Energy equivalents of inputs associated with the production of Jerusalem artichoke.

Source	Unit	Input	References
Labor	MJ h−1	80	Wójcicki [88]
Tractors	MJ kg−1	125	Wójcicki [88]
Machines	MJ kg−1	110	Wójcicki [88]
Diesel oil	MJ kg−1	48	Wójcicki [88]
Tubers	MJ kg−1	3.2	Fang et al. [45]
N	MJ kg−1	77	Wójcicki [88]
P2O5	MJ kg−1	15	Wójcicki [88]
K2O	MJ kg−1	10	Wójcicki [88]
Digestate a	MJ kg−1	0.2	Wójcicki [88]
Pesticides	MJ kg−1 active ingredient	300	Wójcicki [88]

^a based on the energy equivalents of organic fertilizers (liquid manure).

. Total EIs were calculated by summing up the EI associated with diesel oil consumption (energy carriers), labor, tractors and agricultural machines (fixed assets), JA tubers for planting, fertilizers, and pesticides (agricultural materials) according to the formula proposed by Bogucka and Jankowski [31].

2.3. Biomass Processing

The fresh matter yield (FMY) of JA was calculated directly after harvest, and the results were expressed per hectare. The moisture content of the harvested biomass was determined by drying 1 kg samples from each plot at a temperature of 105 °C in a Binder FD 53 ventilated oven (Binder GmbH, Tuttlingen, Germany) until constant weight. The results were used to calculate the DM content and dry matter yield (DMY) of JA biomass according to the formulas presented by Bogucka and Jankowski [31].

2.4. Energy Output Analysis

The higher heating value (HHV) of JA biomass was determined by adiabatic combustion in a C 6000 calorimeter (IKA-Werke GmbH & CO. KG, Staufen, Germany). The lower heating value (LHV) of JA biomass was calculated based on the formula presented by Kopetz et al. [89]. The EO of JA biomass was calculated as the product of FMY and LHV [31].

2.5. Energy Gain and the Energy Efficiency Ratio

The EG and EER were calculated to determine the energy balance in the cultivation technology of JA. Energy gain was calculated as the difference between EO and EI [31]. The EER was determined as the ratio of the volume of energy accumulated in JA biomass (EO) to the energy invested in the biomass production technology (EI) [31].

2.6. Statistical Analysis

The values of FMY, DMY, DM, LHV, EO, EG, and EER were processed by ANOVA with the use of a randomized block design model in Statistica v. 13.3 [90]. The F-values in ANOVA are presented in

Table 3. F-test statistics in ANOVA.

Parameter	Year	Fertilization Strategy	Year × Fertilization Strategy
Fresh matter yield (Mg ha−1)	921.301 **	12.993 **	1.688 ns
Dry matter content (g kg−1)	18.928 **	0.449 ns	0.854 ns
Dry matter yield (Mg ha−1)	211.137 **	3.153 **	0.364 ns
Lower heating value (MJ ha−1)	2.780 ns	2.490 **	1.153 ns
Energy output (GJ ha−1)	597.494 **	12.142 **	1.566 ns
Energy gain (GJ ha−1)	802.468 **	15.848 **	1.346 ns
Energy efficiency ratio	41.445 **	11.170 **	1.935 ns

** significant p ≤ 0.01; ns—not significant.

. Treatment means were compared based on studentized range distribution in Tukey’s Honest Significant Difference (HSD) test.

3. Results

3.1. Weather Conditions

Weather conditions varied across years during the three-year experiment (

Table 4. Weather conditions during the experiment (2021–2023) and the long-term average (1981–2010) in Tomaszkowo village (AES in Bałdy). Data were acquired with the use of the PM Ecology automatic weather station (PM Ecology sp. z o. o, Gdynia, Poland).

Month	Year			1981–2010
	2021	2022	2023
Total monthly rainfall (mm)
January	45.2	60.5	54.2	36.4
February	9.5	81.2	38.5	24.2
March	38.2	0.0	52.8	32.9
April	38.5	26.0	21.2	33.3
May	85.0	48.1	16.3	58.5
June	69.8	104.4	48.9	84.0
July	178.4	63.3	34.0	74.2
August	136.4	29.6	131.7	59.4
September	25.0	52.5	23.2	56.9
October	24.4	19.9	59.8	42.6
November	35.0	10.3	73.5	44.8
December	19.2	41.6	60.1	38.2
Total	704.6	537.4	614.2	581.8
Mean daily temperature (°C)
January	−2.0	0.4	2.1	−2.4
February	−3.9	1.8	0.8	−1.7
March	1.8	1.8	2.6	1.8
April	5.0	5.7	6.7	7.7
May	10.9	11.2	11.8	13.5
June	18.7	17.1	16.8	16.1
July	20.8	17.4	17.9	18.7
August	16.4	20.0	18.5	17.9
September	12.4	10.8	16.3	12.8
October	8.3	10.2	8.7	8.0
November	4.6	3.7	2.7	2.9
December	−1.9	−0.7	0.8	−0.9
Average	7.7	8.3	8.8	7.9

). In the first and second growing seasons (April-September), the mean daily temperature was 0.4 °C and 0.8 °C lower, respectively, than the long-term average (14.5 °C). In the third growing season, the mean daily temperature approximated the long-term average. The first growing season (2021) was characterized by very high precipitation. Total rainfall exceeded the long-term average by 46% (533 vs. 366 mm), and May, July, and August were particularly wet months. In the remaining growing seasons (2022 and 2023), precipitation was 12% and 25% below the long-term average, respectively. The third growing season was particularly dry with very low rainfall between April and July (36%, 72%, 42%, and 54% below the long-term average in April, May, June, and July, respectively) (Table 4).

3.2. Energy Inputs

The EIs associated with the cultivation technology of JA in a perennial cropping system (where only aerial biomass was harvested each year) were differentiated by the N rate, type of fertilizer, and year (

Table 5. Energy inputs in the production technology of Jerusalem artichoke biomass, per agricultural operation (2021, 2022, 2023).

Operation	2021							2022							2023
	C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1		C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1		C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1
		MF	D	MF	D	MF	D		MF	D	MF	D	MF	D		MF	D	MF	D	MF	D
Energy inputs (MJ ha−1)
Tillage	2257	2257	2257	2257	2257	2257	2257	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Planting	8072	8072	8072	8072	8072	8072	8072	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Fertilization	0	6307	2539	8232	3933	10,157	5078	0	6307	2351	8232	3651	10,157	4702	0	6307	2423	8232	3757	10,157	4844
Weed control	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258	1258
Pathogen control	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158	158
Harvest and transport	4421	4421	4421	4421	4421	4421	4421	2408	3414	3414	3020	3020	3020	3414	1474	2211	1474	1474	1474	1474	1474
Total	16,166	22,473	18,705	24,398	20,100	26,323	21,244	3824	11,138	7182	12,668	8088	14,593	9533	2890	9934	5313	11,122	6647	13,047	7734
Energy input structure (%)
Tillage	14.0	10.0	12.1	9.2	11.2	8.6	10.6	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Planting	49.9	35.9	43.2	33.1	40.2	30.7	38.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Fertilization	0.0	28.1	13.6	33.7	19.6	38.6	23.9	0.0	56.6	32.7	65.0	45.1	69.6	49.3	0.0	63.5	45.6	74.0	56.5	77.8	62.6
Weed control	7.8	5.6	6.7	5.2	6.3	4.8	5.9	32.9	11.3	17.5	9.9	15.6	8.6	13.2	43.5	12.7	23.7	11.3	18.9	9.6	16.3
Pathogen control	1.0	0.7	0.8	0.6	0.8	0.6	0.7	4.1	1.4	2.2	1.2	2.0	1.1	1.7	5.5	1.6	3.0	1.4	2.4	1.2	2.0
Harvest and transport	27.3	19.7	23.6	18.1	22.0	16.8	20.8	63.0	30.7	47.5	23.8	37.3	20.7	35.8	51.0	22.3	27.7	13.3	22.2	11.3	19.1
Total	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0

C–unfertilized control; MF–mineral fertilizers, D–digestate.

). In year 1 (plantation establishment), EIs in the JA biomass production technology without fertilization were determined at 16.2 GJ ha⁻¹. The N rates of 50, 75, and 100 kg N ha⁻¹ supplied with mineral fertilizers increased EI in biomass production by 39%, 51%, and 63%, respectively. In treatments fertilized with digestate, the EI was lower by 17–19% (i.e., by 3.8–5.1 GJ ha⁻¹). In years 2 and 3, no tillage or planting operations were performed, and EIs were lower by 45–56% (mineral fertilization), 55–72% (digestate), and 76–82% (no fertilization) relative to the year of plantation establishment. In years 2 and 3, EI in the JA biomass production technology without fertilization reached 3.8 and 2.9 GJ ha⁻¹, respectively. In these years, EIs were highest in cultivation technologies with mineral fertilizers (9.9–14.6 GJ ha⁻¹ y⁻¹). The application of digestate decreased EI in the JA cultivation technology by 35–47% (Table 5) due to the lower energy value of digestate relative to mineral fertilizers (6.2–10.1 vs. 3.1–4.1 GJ ha⁻¹) (

Table 6. Energy inputs in the production technology of Jerusalem artichoke biomass, per energy flux (2021, 2022, 2023).

Specification	2021							2022							2023
	C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1		C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1		C	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1
		MF	D	MF	D	MF	D		MF	D	MF	D	MF	D		MF	D	MF	D	MF	D
Energy inputs (MJ ha−1)
Labor	686	688	696	688	704	688	706	243	282	290	274	289	274	300	213	241	223	216	231	216	233
Tractors and machines	1956	2006	2265	2006	2497	2006	2574	835	1152	1412	1126	1617	1126	1721	640	872	949	689	1180	689	1258
Fuel	6386	6417	6566	6417	6701	6417	6746	2648	3381	3531	3021	3306	3021	3711	1939	2498	2119	1970	2254	1970	2299
Materials:	7138	13,363	9178	15,288	10,198	17,213	11,218	98	6323	1950	8248	2876	10,173	3802	98	6323	2022	8248	2982	10,173	3944
– tubers	7040	7040	7040	7040	7040	7040	7040	0	0	0	0	0	0	0	0	0	0	0	0	0	0
– mineral fertilizers	0	6225	0	8150	0	10,075	0	0	6225	0	8150	0	10,075	0	0	6225	0	8150	0	10,075	0
– digestate	0	0	2040	0	3060	0	4080	0	0	1852	0	2778	0	3704	0	0	1924	0	2884	0	3846
– pesticides	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98	98
Total	16,166	22,473	18,705	24,398	20,100	26,323	21,244	3824	11,138	7182	12,668	8088	14,593	9533	2890	9934	5313	11,122	6647	13,047	7734
Energy input structure (%)
Labor	4.2	3.1	3.7	2.8	3.5	2.6	3.3	6.4	2.5	4.0	2.2	3.6	1.9	3.1	7.4	2.4	4.2	1.9	3.5	1.7	3.0
Tractors and machines	12.1	8.9	12.1	8.2	12.4	7.6	12.1	21.8	10.3	19.7	8.9	20.0	7.7	18.1	22.1	8.8	17.9	6.2	17.8	5.3	16.3
Fuel	39.5	28.6	35.1	26.3	33.3	24.4	31.8	69.2	30.4	49.2	23.8	40.9	20.7	38.9	67.1	25.1	39.9	17.7	33.9	15.1	29.7
Materials:	44.2	59.5	49.1	62.7	50.7	65.4	52.8	2.5	56.8	27.1	65.1	35.6	69.7	39.9	3.4	63.6	38.0	74.2	44.9	78.0	51.0
– tubers	43.5	31.3	37.6	28.9	35.0	26.7	33.1	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
– mineral fertilizers	0.0	27.7	0.0	33.4	0.0	38.3	0.0	0.0	55.9	0.0	64.3	0.0	69.0	0.0	0.0	62.7	0.0	73.3	0.0	77.2	0.0
– digestate	0.0	0.0	10.9	0.0	15.2	0.0	19.2	0.0	0.0	25.8	0.0	34.3	0.0	38.9	0.0	0.0	36.2	0.0	43.4	0.0	49.7
– pesticides	0.6	0.4	0.5	0.4	0.5	0.4	0.5	2.5	0.9	1.4	0.8	1.2	0.7	1.0	3.4	1.0	1.8	0.9	1.5	0.7	1.3
Total	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0

C–unfertilized control; MF–mineral fertilizers, D–digestate.

). The EI related to digestate transport and application (labor, tractors and machines, and diesel fuel) were only 0.4 and 0.9 GJ ha⁻¹ higher than the EI associated with mineral fertilizer rates of 50 and 100 kg N ha⁻¹, respectively. The EI associated with digestate transport and application did not exert a negative impact on the EO of JA biomass because digestate has a lower energy value (by 3.1–6.0 GJ ha⁻¹) than mineral fertilizers (Table 6).

The EIs associated with each agronomic operation are presented in Table 5. In year 1, tuber planting was the most energy-intensive operation (50% of total EI) in the JA biomass production technology without fertilization. In years 2 and 3, the most energy-intensive operations were biomass harvest and transport (63% and 51% of total EI, respectively) and weed control (33% and 44% of total EI, respectively). Nitrogen fertilization not only increased the demand for energy in the JA cultivation technology, but also modified the structure of EI. In year 1, tuber planting (31–36% of total EI) and fertilization (28–39%) were the main EI in production technologies with mineral fertilization. In production technologies involving digestate, the most energy-intensive operations were tuber planting (38–43% of total EI), biomass harvest and transport (21–24%), and fertilization (14–24%). In years 2 and 3, N fertilization was the main EI regardless of fertilizer type. In these years, N fertilization accounted for 57–78% (mineral fertilizers) and 33–63% (digestate) of total EI (Table 5).

An analysis of the EI by energy fluxes revealed considerable differences between years (year 1 vs. years 2 and 3) and fertilizer types (mineral fertilizers vs. digestate) (Table 6). In year 1, the highest EIs were associated with materials (44–65%) and energy carriers (24–40%). The share of agricultural materials in total EI increased, whereas the share of energy carriers fuel decreased with a rise in the N rate (50, 75, and 100 kg ha⁻¹), regardless of fertilizer type. In years 2 and 3, fertilizer type (mineral vs. organic) induced the greatest differences in the structure of EI. In production technologies without fertilization and in technologies where digestate was applied as a source of nutrients, energy carriers were the main EI (67–69% and 39–49% of total EI, respectively). In production technologies with mineral fertilization, energy consumption was determined mainly by mineral fertilizers that accounted for 56–77% of total EI (Table 6).

3.3. Biomass Yield

The FMY of JA aerial biomass was highest (81.5 Mg ha⁻¹) in year 1. In years 2 and 3, the FMY was 52% and 79% lower, respectively (Figure 1a). The FMY of JA aerial biomass was lowest in the production technology without fertilization 34.3 Mg ha⁻¹) (

Table 7. Yield and moisture content of Jerusalem artichoke biomass (average for 2021–2023).

Parameter	Fertilization Strategy
	Unfertilized Control	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1
		Mineral Fertilizers	Digestate	Mineral Fertilizers	Digestate	Mineral Fertilizers	Digestate
Fresh matter yield (Mg ha−1)	34.3 c	48.1 ab	45.3 b	49.8 ab	53.3 a	46.0 b	46.1 b
Dry matter content (g kg−1)	367.3	369.6	366.6	364.1	366.9	369.4	365.0
Dry matter yield (Mg ha−1)	12.5 d	17.5 bc	16.3 c	17.9 b	19.4 a	16.8 bc	16.7 bc

Means with the same letter do not differ significantly at p ≤ 0.05 in Tukey’s HSD test.

). The FMY increased by 32–40% in response to the N rate of 50 kg N ha⁻¹, regardless of fertilizer type. An increase in the N rate to 75 kg N ha⁻¹ was justified only when JA was fertilized with digestate (FMY increased by 18%). A further increase in the N rate (100 kg N ha⁻¹) did not lead to a significant increase in FMY, irrespective of fertilizer type (Table 7). The DM content of JA biomass differed across each year. This parameter was lowest in year 1 (357.0 g kg⁻¹), and it increased by 4% in years 2 and 3 (Figure 1b). The DM content of JA biomass was not significantly affected by the N rate or fertilizer type (Table 3 and Table 7).

The DMY of JA biomass was highest (29.1 Mg ha⁻¹) in the first year of the experiment. In years 2 and 3, the DMY of aerial biomass was lower by 14.4 and 22.6 Mg ha⁻¹ (by 50% and 78%), respectively (Figure 1c). This parameter was lowest (12.5 Mg ha⁻¹ DM) in the JA biomass production technology without fertilization. The N rate of 50 kg ha⁻¹ increased DMY by 30–40%, regardless of fertilizer type. The N rate of 75 kg ha⁻¹ increased DMY (by 11–19%) only after the application of digestate. The highest N rate (100 kg ha⁻¹) did not induce a significant increase in DMY, irrespective of fertilizer type (Table 7).

3.4. Energy Output

The LHV of JA aerial biomass did not differ significantly across the experimental years (years 1–3). This parameter was lowest in the JA biomass production technology without fertilization (3.98 MJ kg⁻¹). A significant increase in the LHV of biomass (by 11%) was observed after the application of 50 kg N ha^–1 in mineral form (Figure 2).

The EO of JA biomass was highest in the year of plantation establishment (342.6 GJ ha⁻¹), and it decreased by 50% in year 2 and by 79% in year 3 (Figure 3a). The EO was lowest in the JA biomass production technology without fertilization (134.8 GJ ha⁻¹) (

Table 8. Energy balance in the production technology of Jerusalem artichoke biomass (average for 2021–2023).

Parameter	Fertilization Strategy
	Unfertilized Control	50 kg N ha−1		75 kg N ha−1		100 kg N ha−1
		Mineral Fertilizers	Digestate	Mineral Fertilizers	Digestate	Mineral Fertilizers	Digestate
Energy output (GJ ha−1)	134.8 c	208.7 ab	191.2 b	230.1 a	210.7 ab	198.1 ab	197.0 ab
Energy gain (MJ ha−1)	127.2 c	194.2 ab	180.8 b	194.6 ab	218.5 a	180.2 b	184.2 b
Energy efficiency ratio	20.1 a	13.4 bc	17.7 ab	12.6 c	19.7 a	10.2 c	14.6 bc

Means with the same letter do not differ significantly at p ≤ 0.05 in Tukey’s HSD test.

). Nitrogen fertilization increased the EO by 42–55% (50 kg ha⁻¹), 56–71% (75 kg ha⁻¹), and 46–47% (100 kg ha⁻¹). The EO of aerial biomass was highest (230.1 GJ ha^–1) in the production technology where mineral N was supplied at 75 kg ha⁻¹ (Table 8).

3.5. Energy Gain and the Energy Efficiency Ratio

The EG in JA cultivation was highest in the year of plantation establishment (321.3 GJ ha⁻¹). This parameter was 49% and 80% lower in years 2 and 3, respectively (Figure 3b). During the entire three-year experiment, the average EG was lowest (127.2 GJ ha⁻¹) in the JA biomass production technology without fertilization and highest (218.5 GJ ha⁻¹) in the production technology where digestate was applied at a rate equivalent to 75 kg N ha⁻¹. An increase in the N rate to 100 kg ha⁻¹ decreased EG by 16–18%, regardless of fertilizer type (Table 8).

The EER of the examined JA cultivation technologies was very high (6.5 to 29.1) in all years and fertilization regimes (Figure 3b). The EER peaked (29.1) in year 1, and it was two times and 4.5 times lower in years 2 and 3, respectively. The EER was highest in the JA biomass production technology without fertilization (20.1) and in the technology where digestate was applied at a rate equivalent to 75 kg N ha⁻¹ (19.7). The remaining N rates led to a significant decrease in the EER. It should be noted that mineral fertilizers induced a greater decrease in the EER than digestate (33–49% vs. 12–27%). During the entire three-year experiment, the average EER was 21% higher in the biomass production technologies where JA was fertilized with digestate than in technologies involving mineral fertilizers (14.6 vs. 12.1) (Table 8).

4. Discussion

4.1. Energy Inputs

Energy crops should be characterized by high biomass yields, high EO, and relatively low EI in the production process [82]. These parameters can be improved by modifying the energy intensity of the cultivation technology [91,92,93,94]. In the work of Fang et al. [45], the EI associated with JA cultivation in North-Eastern and Eastern China reached 34.7 and 44.7 GJ ha⁻¹, respectively. According to Stolarski et al. [81], the EI in the cultivation technology of JA in North-Eastern Poland ranged from 10.8–19.8 to 32.3–59.1 GJ ha⁻¹. In the present study, EI ranged from 16.2 (no fertilization) to 18.7–22.5 and 21.2–26.3 GJ ha⁻¹ (50 and 100 kg N ha⁻¹, respectively) in the year of plantation establishment. In years 2 and 3, EI decreased by 45–56% (mineral fertilizers), 55–72% (digestate), and 76–82% (no fertilization). In the work of Jankowski et al. [30] and Bogucka and Jankowski [31], the demand for energy also decreased in the second and third year of biomass production (by 35–47% relative to the year of plantation establishment). In the perennial cropping system, EIs were lower due to the absence of planting and tillage operations in successive years of the experiment. Perennial cropping promotes soil organic matter accumulation, carbon sequestration, and the diversity of soil-dwelling microorganisms. It also reduces the demand for fossil fuels and decreases greenhouse gas emissions.

In the biomass production of perennial crops, the demand for energy is influenced mainly by agricultural materials (root cuttings, rhizomes, and tubers) and mineral fertilizers in the first year of cultivation. In perennial cropping systems, mineral fertilization is the most energy-intensive agricultural operation in successive years of cultivation [30,31,49,82]; present study, Table 6. In modern cultivation technologies, the demand for energy can be reduced through the use of alternative fertilizers [95], including digestate [16,51,54], digested biogas crops and slurry [96], municipal sewage sludge [30,49,54,82], and meat and bone meal [48]. Energy inputs were reduced by 27–34% when mineral fertilizers were replaced with sewage sludge in the cultivation of JA [30], cup plant (Silphium perfoliatum L.) [49], and giant miscanthus [Miscanthus × giganteus (Greef et Deuter)] [82]. In the current experiment, the replacement of mineral fertilizers with digestate decreased the demand for energy in JA production by 17–19% (year 1) to 55–72% (years 2 and 3). In the cultivation technology of Amur silvergrass, EIs were also reduced (by 41–48%) when digestate was used instead of mineral fertilizers [54].

The present study demonstrated that digestate is an alternative fertilizer that significantly decreases the demand for mineral fertilizers in the production of JA biomass. Mineral fertilizers are among the most energy-intensive inputs in farming, mainly because their production requires considerable amounts of energy (non-renewable resources, including fossil fuels). The results of this experiment indicate that EI associated with JA production in a perennial cropping system can be reduced by up to 55–72% when mineral fertilizers are replaced with digestate. This decrease can significantly reduce energy consumption in large-scale farms. The replacement of mineral fertilizers with recycled fertilizers not only lowers EI in the production process, but also decreases agricultural producers’ dependence on external energy suppliers. As a result, the production process will be less influenced by fluctuations in the prices of mineral fertilizers and unstable supplies, thus providing farmers with greater autonomy in managing the production process. Biomass producers who use recycled fertilizers are less vulnerable to changes in the energy market.

4.2. Biomass Yield

The biomass yield of JA is influenced mainly by the availability of water [97,98,99]. Drought, in particular during flowering and tuber formation, decreases the accumulation of DM [100] and biomass yields in JA by 20% [97,101] to 25% [102]. In the present study, the FMY and DMY of JA aerial biomass were highest in the wet and warm year of 2021 (the first growing season). In turn, biomass yields were lowest in the third growing season which was characterized by prolonged drought (April-July). The biomass yields of JA were also affected by weather conditions in the work of Jankowski et al. [31], Piskier [35], Sawicka [43], and Skiba [103].

Jerusalem artichoke yields are strongly determined by climate [28,29,31,34,40,43,102,104,105,106,107]. The yield potential of this crop species is lowest (<15 Mg DM ha⁻¹) in the subarctic climate (Dfc—Köppen climate classification) of North America, Northern Europe, Eastern Europe, and North Asia [28,108,109,110]. Aerial biomass yields can reach 30 Mg ha⁻¹ DM in the humid continental climate of Central Europe (Dfb) and the cold semi-arid climate (BSk) of the Middle East ([31,34,40,43,107,111,112]; present study, Figure 1c). In Poland, the aerial biomass yield of JA ranges from 6–28 Mg ha⁻¹ DM on nutrient-poor soils [43,113] to 35–50 Mg ha⁻¹ DM on soils that are more abundant in nutrients and water [43,101]. The present experiment was established on slightly acidic Haplic Luvisol originating from boulder clay, and the DMY of JA aerial biomass was determined at 6.5 to 29.1 Mg ha⁻¹. In studies conducted by Stolarski et al. [81] and Jankowski et al. [30] in North-Eastern Poland, the DMY of JA aerial biomass reached 5.9–13.5 and 5.1–16.4 Mg ha⁻¹, respectively. The highest JA yields (up to 40 Mg ha⁻¹ DM) are noted in the Mediterranean climate (Cs) of Western and Southern Europe and in the cold desert climate (BWk) of East Asia [98,102,104,105,106,114,115,116,117].

Nitrogen fertilization is essential for the growth and development of JA plants. The optimal N rate in JA cultivation ranges from 52 to 190 kg ha⁻¹ [29,34,102,104,117]. In Northern China, aerial biomass yields increased in response to 50–75 kg N ha⁻¹, whereas maximum tuber yields were achieved already after the application of 25 kg N ha⁻¹ [118]. In another study conducted in China, the optimal N rate in JA production was 20–50 kg ha⁻¹ [117]. In the United Kingdom, the N rate of 56 kg ha⁻¹ induced the greatest increase in JA biomass yields [119]. In Spain, JA biomass yields peaked in response to 100 kg N ha⁻¹ [99]. In Central Germany, N fertilization increased biomass yields up to the rate of 120 kg ha⁻¹. In this study, the DMY of JA aerial biomass was highest (19.4 Mg ha⁻¹) after the application of 75 kg N ha⁻¹. An increase in the N rate to 100 kg ha⁻¹ decreased DMY by 14%. However, in some studies, N fertilization exerted no yield-forming effects in JA cultivation [28,104]. Research studies conducted in various parts of the world (China, United Kingdom, Spain, Germany) have shown that the optimal N rates are determined by local agroecological conditions [28,29,34,99,102,104,117,118,119]; present study, Table 7. The above implies that N fertilization strategies should be adapted to soil conditions and climate in a given region. Further research is needed to develop optimal N rates for JA biomass production in different climate zones with the aim of promoting sustainable management of agricultural resources, reducing eutrophication of surface waters, and decreasing greenhouse gas emissions.

The biomass yields of JA can be improved not only through the application of mineral fertilizers containing N, but also through the use of organic fertilizers, including digestate [80,120]. Fertilization is regarded as the cheapest method of digestate management [120,121], which promotes the recovery and recycling of organic and mineral compounds [58]. Research has shown that digestate has a positive impact on crop yields [59,122,123]. In a pot experiment conducted by Szymańska et al. [124], digestates obtained from typical agricultural substrates and wastes from the agri-food industry significantly increased maize biomass yields. In comparison with the unfertilized control treatment, maize biomass yields were more than three times higher in treatments fertilized with agricultural digestates and more than two times higher in treatments supplied with agri-food waste digestates. According to Siebielec et al. [125], digestate enhanced maize and winter wheat (Triticum aestivum L.) yields, but decreased winter rapeseed (Brassica napus L.) yields. Digestate exerted similar or greater yield-forming effects than mineral fertilizers in the work of Dubis et al. [54], Sapp et al. [126], Barłóg et al. [127,128], and Jamison et al. [129]. In the current study, digestate was more productive than mineral fertilizers only when applied at a rate equivalent to 75 kg N ha⁻¹. Lower (50 kg N ha⁻¹) and higher (100 kg N ha⁻¹) equivalent rates of digestate exerted a similar effect on biomass yields to mineral fertilizers.

In some studies, digestate had no effect on crop yields [130], whereas other researchers reported on the negative effects of digestate fertilizers [131,132,133]. According to Alburquerque et al. [134], some digestates contain heavy metals (mainly zinc and copper) at concentrations that are toxic to plants. Digestates with a high content of potassium, sodium, and chlorine may also exert phytotoxic effects. Seed germination and plant growth may be inhibited in soils that are abundant in these elements [135,136]. According to Tigini et al. [136], McLachlan et al. [137], and Teglia et al. [138], the phytotoxic effects of digestate can also be attributed to a high concentration of ${\mathrm{N}\mathrm{H}}_4^{+}$-N which inhibits germination. In turn, Sogn et al. [139] observed that the fertilizing value of digestate increases with a rise in ${\mathrm{N}\mathrm{H}}_4^{+}$-N levels. Digestate did not exert phytotoxic effects in the present study. The highest N rate (100 kg ha⁻¹) decreased aerial biomass yields in JA plants that were fertilized with digestate as well as in plants that were supplied with mineral fertilizers.

The study demonstrated that the use of digestate as fertilizer promotes nutrient recycling, reduces waste, and contributes to the sustainable management of non-renewable resources. This fertilization strategy minimizes waste storage and its adverse impact on the environment. Digestate decreases the demand for mineral fertilizers, thus reducing greenhouse gas emissions and minimizing the chemical pollution of soils and groundwater. However, the presence of heavy metals and high concentrations of potassium, sodium, and chlorine in digestates can contribute to soil salinity and phytotoxicity. Therefore, the chemical composition of digestate should be monitored to reduce these risks. The results of this experiment indicate that digestate can be as effective or, in some cases, more effective than mineral fertilizers. Digestate offers a cheaper alternative to mineral fertilizers, thus decreasing costs in agricultural production. The use of digestate in the farming sector also contributes to effective waste management.

4.3. Energy Output

In energy crops, the EO of agricultural phytomass is determined mainly by the energy intensity of the production process [26,91,92,93,94]. An increase in EI associated with cultivation technology generally increases the EO of biomass [82,91,92,93,94,140,141]. In a study by Epie et al. [28], the EO of JA aerial biomass ranged from 182 to 269 GJ ha⁻¹. Ivanova et al. [142] estimated the EO of JA aerial biomass at 495 GJ ha⁻¹. In Poland, the EO of JA aerial biomass ranges from 60 to 321 GJ ha⁻¹ ([30,31,35]; present study, Figure 3a). In this study, the EO of JA aerial biomass peaked (230.1 GJ ha⁻¹) in response to 75 kg N ha⁻¹ in mineral form. An equivalent N rate supplied with digestate decreased EO by 8%. Seleiman et al. [143] reported that sewage sludge increased the EO of maize and hemp (Cannabis sativa L.) by 5–6% and 20%, respectively. Dubis et al. [82] demonstrated that the EO of giant miscanthus biomass peaked after the application of 160 kg N ha⁻¹, regardless of fertilizer type (mineral fertilizers vs. sewage sludge). In a study by Dubis et al. [82], the greatest increase in the EO of Amur silvergrass was noted in response to 100 kg N ha⁻¹ in mineral form or 160 kg N ha⁻¹ supplied with sewage sludge and digestate. Jankowski et al. [30] found that the EO of JA aerial biomass was influenced by the type of N fertilizer and plantation age. In year 1, EO was higher in treatments fertilized with sewage sludge. In year 2, higher EO was achieved in treatments supplied with mineral fertilizers. In year 3, the EO of biomass was not differentiated by the type of fertilizer.

Considerable variations in the EO of JA biomass supplied with different types and rates of N fertilizers (mineral vs. organic) indicate that this macronutrient should be rationally managed. Large-scale use of recycled fertilizers not only contributes to a circular economy, but also promotes the sustainable production of energy crops. The results of research studies examining the influence of various fertilization strategies on the yield of energy crops will enable farmers to adopt agricultural practices to plant requirements, thus increasing the energy efficiency of the entire biomass production system. The prices of mineral fertilizers have risen significantly in recent years, which is why recycled fertilizers can generate substantial economic benefits for agricultural producers. Digestates offer an attractive option for farmers due to lower fertilization costs and comparable or even higher energy value of the obtained biomass. The current study revealed that the EO of JA biomass is also strongly influenced by local agroecological conditions. The above implies that agricultural producers should adapt their fertilization strategies to local requirements by selecting the most appropriate type (mineral vs. organic) and rate of N fertilizer.

4.4. Energy Gain and the Energy Efficiency Ratio

An increase in the EI associated with the production technology of energy crops generally increases the EG (by increasing DMY and EO) and decreases the EER (the increase in EI exceeds the increase in EO) [30,82]. The EG in the production technology of JA aerial biomass ranges from 41–88 to 168–346 GJ ha⁻¹ ([30,31,45,81]; present study, Figure 3b). In the current experiment, EG was lowest (127 GJ ha⁻¹) in the JA biomass production technology without fertilization and highest (218 GJ ha⁻¹) when digestate was applied at a rate equivalent to 75 kg N ha⁻¹. An increase in the N rate to 100 kg ha⁻¹ decreased EG by 16–18%, regardless of fertilizer type. In the work of Jankowski et al. [30], EG peaked in the cultivation technology where JA was fertilized with sewage sludge at a rate equivalent to 100 kg N ha⁻¹. Organic fertilizers also improved EG in the cultivation of giant miscanthus (160 kg N ha⁻¹ supplied with sewage sludge] and Amur silvergrass (160 kg N ha⁻¹ supplied with digestate] [54,92].

In Europe, the EER in the production technology of JA biomass ranges from 7–18 ([30,35,81]; present study, Figure 3c) to 30 [31]. An increase in EI can decrease the EER of the JA biomass production technology by 32% to 80% [30,31,81]. According to Jankowski et al. [30] and Stolarski et al. [81], production technologies without fertilization are characterized by the highest EER values. In JA production, fertilization decreases the EER by 51–71% (85–100 kg N ha⁻¹) and 47–79% (160–170 kg N ha⁻¹). The EER is affected not only by the N rate, but also by the type of fertilizer [30,35,81]. In the work of Piskier [35], the EER was 31–41% higher in JA cultivation technologies involving mineral fertilizers than mineral-organic fertilizers (50:50) or organic fertilizers. Stolarski et al. [81] reported that the use of dry digestate and torrefied digestate decreased the EER in the JA biomass production technology by 46–55% and 49–62%, respectively, relative to production technologies involving mineral fertilizers. In the present study, the EER was highest in the JA biomass production technology without fertilization (20.1) and in the technology where liquid digestate was applied at a rate equivalent to 75 kg N ha⁻¹ (19.7). Mineral fertilizers decreased the EER by 33–37% (50–70 kg N ha⁻¹) and 49% (100 kg N ha⁻¹). Jankowski et al. [30] also found that the EER was nearly two times higher when JA was supplied with organic fertilizers (sewage sludge) than mineral fertilizers. The replacement of mineral fertilizers with sewage sludge or digestate also exerted a positive impact on the EER in the production of giant miscanthus and Amur silvergrass [54,82].

The study demonstrated that digestate induced a greater increase in EG and the EER in the production technology of JA biomass than mineral fertilizers. This suggests that mineral fertilizers can be replaced with digestate when JA biomass is produced in a perennial cropping system, thus reducing the carbon footprint associated with biomass production and transport. Digestate can also decrease fertilization costs, which is a very important consideration in the face of the recent increase in the prices of mineral fertilizers. The transition to organic fertilizers (in particular recycled fertilizers) can generate economic benefits, especially for small-area farms, where capital is the main factor that limits agricultural production. Regardless of farm size (scale of production), the application of digestate as fertilizer constitutes an attractive management strategy by improving the energy balance in the production of energy crops. When JA biomass is produced in a perennial cropping system, the replacement of mineral fertilizers with digestate has positive implications for the environment and agricultural practice by promoting sustainable development and reducing fertilizers’ contribution to climate change.

5. Conclusions

In the year of plantation establishment, the demand for energy in the JA biomass production technology without fertilization reached 16.2 GJ ha⁻¹. The application of mineral fertilizers at 50, 75, and 100 kg N ha⁻¹ increased EI by 39%, 51%, and 63%, respectively. Energy inputs were 45–82% lower in productive years (years 2 and 3). The replacement of mineral fertilizers with digestate decreased EI by 17–19% (year 1) and 35–47% (years 2 and 3). Higher EIs associated with digestate transport and application (by 0.7 GJ ha⁻¹) were compensated by the lower energy value of digestate relative to mineral fertilizers (difference of 4.2–6.4 GJ ha⁻¹). Biomass yields peaked (19.4 Mg ha⁻¹ DM) in the production technology where digestate was applied at a rate equivalent to 75 kg N ha⁻¹. The EO of JA biomass was highest in year 1 (342.6 GJ ha⁻¹), and it was 50% lower in year 2 and 79% lower in year 3. The EO of JA aerial biomass was lowest (134.8 GJ ha⁻¹) in the production technology without fertilization. Nitrogen increased the EO of JA aerial biomass by 42–71%. Jerusalem artichoke biomass was characterized by the highest EO (230.1 GJ ha⁻¹) in the production technology where N was applied at 75 kg ha^–1 in mineral form. In turn, EG peaked (218.5 GJ ha⁻¹) in response to digestate applied at a rate equivalent to 75 kg N ha⁻¹. The EER was highest in the JA biomass production technology without fertilization (20.1) and in the technology where digestate was applied at a rate equivalent to 75 kg N ha⁻¹ (19.7). The remaining N rates significantly decreased the EER of the production technology. Mineral fertilizers induced a greater decrease in the EER than organic fertilizers (33–49% vs. 12–27%). Production technologies involving digestate were characterized by higher EER (by 21% on average) than technologies involving mineral fertilizers (14.6 vs. 12.1). The energy balance (EG and EER) in the production of JA aerial biomass was most favorable when JA was fertilized with digestate at a rate equivalent to 75 kg N ha⁻¹, regardless of farm size and factors that limit agricultural production (land resources in small-area farms and capital in large-area farms). The results of this study may pave the way for future research on novel agronomic strategies for sustainable bioenergy production, including nutrient recycling. In the future, a long-term study should also be undertaken to assess the impact of digestate fertilizers on JA biomass yields, the chemical and biological properties of soil, and greenhouse gas emissions under different agroecological conditions.