Thermochemical Valorisation of Apple Pomace-Derived Biochar: Temperature-Driven Structural Evolution, Soil Chemical Modulation, and Agronomic Performance in Wheat Germination

Abstract

Apple pomace represents an important agro-industrial residue with high moisture content and significant environmental burden if improperly managed. This study investigated its thermochemical valorisation into biochar via two processes, followed by comprehensive physicochemical characterization and agronomic evaluation. Elemental analysis revealed carbon enrichment from 47.89% in raw material to 77–78% after the thermal process, evidencing a progressive aromatization. Scanning electron microscopy, Fourier transform infrared spectroscopy, and Raman analysis confirmed a temperature-dependent transition from partially amorphous carbon (400 °C) to more ordered aromatic structures (450 °C), while excessive thermal treatment (550 °C) increased structural defects. ICP-OES revealed an enrichment in thermally stable metals (Fe, Al, Mn) and limited Cd accumulation. Germination assays using Triticum aestivum L. demonstrated that biochar produced at 400 °C significantly improved the germination uniformity and seedling height (14.1 mm), as well as biomass accumulation compared to the control soil sample. The fertilizer addition increased the soluble Na and electrical conductivity (up to 643 µS/cm), potentially inducing transient salinity stress. Soil chemical analysis indicated increased K availability in soils amended with biochar produced at 400 °C, whereas the combination of biochar obtained at 450 °C with fertilizer conducted to elevated concentrations of certain trace metals, mainly Ni and Cr, highlighting the demand for careful monitoring. Overall, the biochar produced at 400 °C yielded to an optimal balance between structural stability, nutrient enrichment, and agronomic performance, evidencing that apple pomace may be a viable feedstock for sustainable biochar production within circular bioeconomy frameworks.

1. Introduction

Efficient waste management constitutes one of the most important environmental challenges faced by modern society. Accelerated technological development, economic growth, urbanization, population expansion, and evolving consumption patterns have collectively driven a substantial increase in global waste generation, with no indication of a significant slowdown soon. Food and green waste constitute the largest fraction of global waste streams, accounting for approximately 44% of the worldwide waste [1]. A considerable part of this category comes from the agro-food processing industry, mainly from fruits and vegetables processing.

According to the United Nations Environment Programme, around 1.05 billion tonnes of food waste were generated globally in 2022, corresponding to nearly 19% of total food production [2]. Of this amount, 60% originated from households, 28% from the food service sector, and 12% from retail activities [3]. These figures underscore the urgency of developing effective waste management and valorisation protocols within the framework of a circular economy.

Apple pomace is among the most abundant and problematic agro-industrial residues [4]. The apples rank as the third most cultivated fruit globally, with a production of about 97 million metric tonnes in 2023, the juice and cider industries alone generating an annual estimated 12 to 20 million metric tonnes of pomace [4]. Despite containing valuable organic compounds, its recovery rate remains limited, while direct disposal or landfilling remains the most prevalent management practice [5]. This management approach raises environmental concerns because the reside contains high moisture content (exceeding 70%), fermentable sugars, and organic acids accentuate methane emissions and generate high-BOD (Biochemical Oxygen Demand) leachate that threatens soil and groundwater stability [5]. Furthermore, the presence of seeds may introduce amygdalin, a cyanogenic glycoside with potential toxicity, although acute poisoning is unlikely under typical conditions [6].

In this respect, apple pomace valorisation represents an important strategy to mitigate these risks, while exploiting its economic and energetic potential [6]. Several studies demonstrated its use as a source of bioactive compounds for industrial biotechnology [6,7]. Recently, the apple pomace has attracted increasing attention as a promising feedstock for the bioenergy and carbon sequestration, including bioethanol, biogas, and biochar, offering a pathway to reduce the greenhouse gas emissions [6,7,8]. Although the high initial moisture content is often considered a limitation for thermochemical conversion, this challenge can be mitigated through drying and through the proximity of fruit-processing facilities to potential conversion sites. Such conditions support the development of decentralized “short-loop” circular bioeconomy models, in which agro-industrial residues are locally converted into value-added products, such as biochar, that can be reutilized as soil amendments in the same agricultural systems that generate the biomass. Within this framework, the thermochemical valorisation of apple pomace represents a promising pathway for sustainable waste management and carbon stabilization.

Among the available valorisation pathways, thermochemical conversion technologies (pyrolysis, gasification, or hydrothermal carbonization) have attracted increasing attention [9]. Pyrolysis, defined as the thermal decomposition of biomass in the absence of oxygen, is the most widely applied procedure, resulting in the simultaneous production of bio-oil, biochar, and gaseous products [10,11]. Biochar derived from apple pomace may be used in environmental and agricultural applications, including soil amendment, fertility enhancement, or wastewater treatment [12,13]. Its efficiency is correlated with the feedstock composition and pyrolysis parameters (temperature, heating rate, and residence time) [14]. Biochar application can improve soil physical and chemical properties—such as bulk density, porosity, water retention capacity, and pH—making it particularly beneficial for acidic soils and contaminated environments [12,15].

Although various studies have reported the production of biochar from fruit-processing residues and described some general improvements in soil physicochemical characteristics after biochar amendment [8,10,13], a systematic investigation mainly focusing on the apple pomace-derived biochar and its direct influence on germination kinetics and early seedling development of cereal crops remain limited. The existing literature focused either on the optimization of the pyrolysis conditions for yield and structural characterization or on the long-term soil amendment effects [8,10,13], without establishing mechanistic correlations between the temperature-driven structural evolution, trace element redistribution, and early-stage plant physiological responses. Moreover, a comparative assessment of different pyrolysis temperatures in relation to the germination synchronization, mean germination time, and biomass allocation patterns in wheat (Triticum aestivum L.) are scarce. Consequently, a clear structure–function–response relationship between the thermochemical processing parameters and the measurable agronomic performance indicators is still insufficiently addressed.

This study investigated the thermochemical valorisation of apple pomace through controlled thermochemical processing (400, 450, and 550 °C) to produce structurally tailored biochar and to evaluate its physicochemical characteristics and agronomic performance in wheat germination. By integrating structural characterization, elemental and soil chemical analysis, and germination indices, the work evidenced direct structure–function relationships between biochar obtaining conditions, carbon structural evolution, nutrient availability, and early plant development.

The novelty of the present study lies in the integrated evaluation of the apple pomace-derived biochar across multiple levels, from thermochemical structural evolution (elemental composition, Raman ordering, FTIR functionality, and mineral redistribution) to the soil chemical modulation and quantitative germination kinetics in wheat. By directly correlating processing temperature with the germination synchronization, mean germination time, biomass allocation, and trace metal dynamics, this work provided a mechanistic structure–function–agronomic response framework that has not been systematically reported for apple pomace biochar.

2. Materials and Methods

2.1. Raw Material Preparation and Moisture Content Assessment

Apple pomace was collected from a local fruit-processing facility engaged in juice production. The material was dried in a laboratory oven at 105 °C for 6 h/day over two consecutive days until constant mass was achieved. After drying, the pomace was homogenized manually. A portion of the material was preserved for characterization, while the remainder was stored for subsequent pyrolysis experiments.

The moisture content of the biomass (apple pomace) represents a critical parameter in evaluating its suitability for pyrolysis processes. A high moisture level requires additional energy consumption for water evaporation and may result in reducing thermochemical conversion efficiency, as well as in diluting the pyrolytic products. To quantify the moisture content of fresh apple pomace, a gravimetric simulation was performed based on weighing multiple samples before and after oven drying at 105 °C. Four fresh apple pomace samples were subjected to drying, and the mass difference was used to calculate water content.

2.2. Pyrolysis Procedure

Pyrolysis experiments were performed using a previously developed laboratory-scale fixed-bed reactor system [16]. The reactor was electrically heated and equipped with automatic PID temperature control, operating under an inert nitrogen atmosphere. The dried and non-milled apple pomace was subjected to pyrolysis at 400 °C and 450 °C, using a constant heating rate of 5 °C/min. For each experimental run, 130 g of apple pomace were loaded into the reactor. Prior to heating, the system was intertied by purging with high-purity nitrogen (99.999 vol%) for 30 min. During the pyrolysis process, nitrogen was continuously supplied at a constant flow rate of 50 mL/min (±5%) to maintain oxygen-free conditions.

Each pyrolysis experiment had a total duration of about 120 min, consisting of 60 min of heating to the target temperature, followed by an isothermal residence time of 60 min. Upon completion, the reactor and the entire system were maintained under nitrogen flow until complete cooling. Water-assisted cooling was applied to reduce the system temperature below 10 °C, thereby preventing oxidation of the pyrolysis products.

2.3. Calcination of Apple Pomace

Due to the operational temperature limitation of the pyrolysis reactor (≤450 °C), the apple pomace thermal treatment at higher temperatures was performed by calcination using a Nabertherm muffle furnace (Nabertherm GmbH, Lilienthal, Germany). The dried apple pomace was subjected to calcination at 550 °C under reduced oxygen conditions, following a stepwise heating program designed to ensure controlled thermal decomposition and the formation of a stable carbonaceous material. The temperature program consisted in three successive steps: heating to 150 °C, followed by 350 °C, and finally 550 °C, where the temperature was maintained for 6 h. When the process had finished, the furnace was allowed to naturally cool to the room temperature before sample removal, and the resulting material was stored in airtight containers for further characterization and application studies. This staged calcination protocol allowed gradual removal of the organic constituents and minimized the formation of unstable structures or undesired by-products.

2.4. Characterization of Raw Material and Resulting Solid Products

The raw material and the obtained biochar were characterized in terms of the physicochemical composition and morpho-structural properties using complementary analytical techniques, including elemental analysis (EA), inductively coupled plasma optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. All reagents were purchased from Merck Group (Darmstadt, Germany) and used without further processing.

Elemental composition (C, H, N, S) was determined using a Flash EA 2000 elemental analyser (Thermo Fisher Scientific Inc., Waltham, MA, USA) based on combustion and pyrolysis methods coupled with gas chromatography [17].

The multi-elemental metal content was quantified by ICP-OES using a PlasmaQuant 9100 spectrometer (Analytik Jena, Jena, Germany). Sample preparation was performed by a microwave-assisted acid digestion using a CEM MARS 6 system (CEM Corporation, Matthews, NC, USA). About 0.1 g of biochar sample was digested in two stages: first with 10 mL H₂SO₄, followed by the addition of 10 mL HNO₃ after cooling. After digestion, the solutions were filtered, transferred into 50 mL volumetric flasks, and diluted to volume with ultrapure water. Additional dilutions were applied prior to quantitative analysis, depending on matrix concentration.

The morphology and surface microstructure of the biochar were investigated by field-emission scanning electron microscopy (FESEM VP) using a Carl Zeiss microscope (Carl Zeiss, Oberkochen, Germany) operated at an accelerating voltage of 30 kV, with resolutions of 0.8 nm and 2.5 nm in variable-pressure mode. Elemental surface composition was assessed by EDS analysis.

Functional groups were evidenced by ATR-FTIR spectroscopy using a Cary 630 spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA). Prior to analysis, solid samples were ground in an agate mortar and dried under vacuum at 80 °C to remove physically adsorbed moisture. FTIR spectra were recorded in the range 4000–400 cm⁻¹, with 32 scans, a resolution of 8 cm⁻¹, and a threshold of 0.002. Spectral interpretation was performed using Micro-Lab Expert v1.0.0 software (Agilent Technologies Inc., Santa Clara, CA, USA).

The carbon structure and degree of aromaticity of the biochar were further investigated by confocal Raman spectroscopy using a Renishaw Raman microscope (Renishaw plc, Gloucestershire, UK). Spectra were collected in the range 3200–400 cm⁻¹ under identical acquisition conditions: 473 nm excitation laser, 4 mW laser power, 3 accumulations, 10 s exposure time, and 2 cm⁻¹ spectral resolution.

The characteristic D (~1350 cm⁻¹) and G (~1580 cm⁻¹) bands were fitted using Lorentzian functions, which are commonly applied for carbonaceous materials due to their suitability for describing disordered graphitic structures. Peak fitting was performed within defined spectral windows (1250–1450 cm⁻¹ for the D band and 1500–1650 cm⁻¹ for the G band). The full width at half maximum (FWHM) was extracted directly from the Lorentzian fitting parameter (Γ), corresponding to the peak width at half of the maximum intensity. The ratio between the intensities of D and G bands (I_D/I_G) provided insight into the degree of disorder and crystallite size. According to the Tuinstra–Koenig relationship [18], the I_D/I_G ratio is inversely proportional to the in-plane crystallite size (L_a) for graphitic carbon materials. This relationship is excitation-wavelength dependent [19,20] and can be expressed as:

L_a = 2.4 × 10⁻¹⁰ × [Laser Wavelength laser (nm)]⁴/I_D:I_G Ratio

(1)

where L_a represents the crystallite size (nm) and λ is the laser wavelength (532 nm in this study).

2.5. Germination Experiment

2.5.1. Experimental Design

A germination assay was conducted to evaluate the effect of apple pomace (AP) and apple pomace-derived biochars (BAP400 and BAP450) on wheat (Triticum aestivum L.) seed germination and early seedling development. Each treatment consisted of three independent containers (biological replicates), each containing 10 seeds. For each treatment, approximately 217–220 g of soil was weighed into individual containers. Based on literature recommendations [21], the following biochar application rates were established: 2.17 g biochar (≈1% w/w), 4.34 g biochar (≈2% w/w), 2.17 g biochar + organic fertilizer. An organic liquid fertilizer (Natura, Agro CS Romania S.R.L., Brasov, Romania; N–P–K: 6.4–1.7–9) enriched with potassium and micronutrients was used. The fertilizer contained molasses derivatives and ground phosphates suitable for both open-field and container cultivation. The fertilizer solution was prepared by diluting 15 mL of fertilizer in 1000 mL ultrapure water. Prior to mixing with the soil, the biochar samples were pre-moistened (activated) with 50 mL ultrapure water or 50 mL fertilizer solution and left to equilibrate for several hours. The treated biochar was then homogenized with the soil and allowed to stabilize for 24 h before sowing.

Ten wheat seeds were placed in each container. Seeds were pre-hydrated in water for several hours before planting. After sowing, the containers were transferred to a controlled climate chamber (SANYO Versatile Environmental Test Chamber, model MLR-351H—Sanyo Electric Co., Ltd., Osaka, Japan) and maintained for 10 days under controlled photoperiod conditions (16 h light/8 h dark). Temperature and illumination were regulated via the chamber software v0.3.0-beta.

During the 10-day experiment, seedlings were irrigated as needed (every 2 days) and daily monitored. Germination rate, seedling emergence, and growth parameters were recorded throughout the experiment.

The experimental treatments are summarized in

Table 1. Germination experiment—soil amendments and treatment conditions.

Treatment Code	Description	Seeds per Container	Soil (g)	Biochar/AP (g)	Fertilizer Solution
S	Soil (control)	10	217.2	–	–
SAP	Soil + raw apple pomace	10	217.2	2.17	–
SBAP400	Soil + BAP400	10	217.2	2.17	–
SBAP400+	Soil + BAP400 (2%)	10	217.2	4.34	–
SBAP400F	Soil + BAP400 + fertilizer	10	217.2	2.17	50 mL
SBAP450	Soil + BAP450	10	217.2	2.17	–
SBAP450+	Soil + BAP450 (2%)	10	217.2	4.34	–
SBAP450F	Soil + BAP450 + fertilizer	10	217.2	2.17	50 mL

.

2.5.2. Germination Indices

Germination parameters were calculated using the following equations [22]:

Mean Germination Time (MGT):

$$\mathrm{M}\mathrm{G}\mathrm{T}={\displaystyle \frac{\sum T_{i }\times N_{i }}{\sum N_{i }}}$$

(2)

where T_i is the day of germination and N_i is the number of seeds germinated on day i.

Mean Germination Rate (MGR):

$$\mathrm{M}\mathrm{G}\mathrm{R}={\displaystyle \frac{\sum {\mathrm{N}}_{\mathrm{i}}}{\mathrm{T}}}$$

(3)

where T represents the total number of days until germination completion.

Germination Speed (GS):

$$\mathrm{G}\mathrm{S}=\sum {\displaystyle \frac{N_i}{T_{i }}}$$

(4)

Coefficient of Variation of Germination Time (CV_TG):

$${\mathrm{C}\mathrm{V}}_{\mathrm{T}\mathrm{G}}=\left({\displaystyle \frac{\mathsf{\sigma }}{\mathrm{T}\mathrm{M}\mathrm{G}}}\right)\times 100$$

(5)

Standard Deviation (σ):

$$\mathsf{\sigma }=\sqrt{{\displaystyle \frac{\sum {\mathrm{N}}_{\mathrm{i}}{({\mathrm{T}}_{\mathrm{i}}-\mathrm{M}\mathrm{G}\mathrm{T})}^2}{\sum {\mathrm{N}}_{\mathrm{i}}}}}$$

(6)

2.5.3. Determination of Inorganic Macro- and Microelements and Electrochemical Parameters

Prior to the elemental analysis, the samples (soil, plant tissues, and apple pomace-derived biochar treatments) were subjected to an acid digestion using a closed-vessel microwave mineralization system (CEM MARS 6 system, Matthews, NC, USA). About 0.5 ± 0.05 g of the dried and homogenized sample was accurately weighed into the digestion vessel. A mixture of 3.0 mL concentrated nitric acid (HNO₃, 65%) and 9.0 mL hydrochloric acid (HCl, 37%) was added. Digestion was carried out using a three-step program: (i) ramp to 170 °C within 5 min at 40 bar, followed by a 10 min hold; (ii) ramp to 200 °C within 1 min at 40 bar, followed by a 15 min hold; and (iii) a cooling phase. After completion, vessels were allowed to cool to room temperature. The digested solutions were quantitatively transferred into 50 mL volumetric flasks and diluted to volume with ultrapure water.

The concentrations of Ca, Mg, K, Na, and Fe were determined by flame atomic absorption spectroscopy (FAAS) using a ZEEnit Analytik Jena atomic absorption spectrophotometer (Analytik Jeba, Jena, Germany). Calibration was performed using certified standard solutions, and the results were expressed on a dry weight basis.

The pH and electrical conductivity (EC) of the samples were measured in aqueous suspensions prepared at a 1:5 (w/w) sample-to-distilled water ratio. The suspensions were stirred for 60 ± 10 min and then allowed to equilibrate for at least 1 h (but no longer than 3 h) prior to measurement. pH was measured using a calibrated pH meter (SevenExcellence^TM, Mettler Toledo, Greifensee, Switzerland), while electrical conductivity was determined with a conductivity meter (InoLab COND 720, WTW, Weilheim in Oberbayern, Germany) under standardized laboratory conditions.

Trace element concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Sample preparation and analytical conditions followed the same digestion protocol described above and the instrumental parameters previously detailed in Section 2.4. Results were expressed as mg/kg dry weight.

2.6. Statistical Analysis

Physicochemical characterization, including elemental analysis (CHNS/O) and SEM–EDS, were performed on representative homogenized samples and were reported as single measurements, as they were intended for material identification and compositional assessment rather than statistical comparison of biological responses. No statistical analysis was applied to these datasets.

Quantitative elemental data obtained by ICP-OES were expressed as mean ± standard deviation (SD) based on three independent analytical replicates (n = 3) for each sample.

Statistical analysis was performed using Microsoft Excel (Version 2508, Microsoft Office Home and Business 2019, Microsoft Corporation, Redmond, WA, USA). Prior to statistical testing, data normality and homogeneity of variances were verified. Differences in the elemental concentration among apple pomace and the derived biochars were evaluated using one-way analysis of variance (ANOVA). To ensure methodological consistency, the biochar sample obtained at 550 °C (BAP550), produced via calcination under oxidative conditions, was excluded from the statistical analysis. Statistical evaluations (ANOVA and Tukey’s HSD test) were therefore performed exclusively on the pyrolysis-derived samples (AP, BAP400, and BAP450), which were generated under comparable thermochemical conditions. When significant differences were detected (p < 0.05), Tukey’s honestly significant difference (HSD) post hoc test was applied to identify pairwise differences between treatments.

Germination and seedling growth data (final germination, maximum shoot height—h_max, and minimum shoot height—h_min) were obtained from three independent biological replicates (n = 3) for each treatment. Results were expressed as mean ± standard deviation (SD). Statistical differences among treatments were assessed using one-way analysis of variance (ANOVA). When significant effects were detected (p < 0.05), means were compared using Tukey’s honestly significant difference (HSD) post hoc test.

3. Results and Discussion

3.1. Biochar Preparation and Elemental Analysis

The apple pomace moisture content was determined from four independent measurements and expressed as mean ± standard deviation. The results indicated a consistently high moisture level of 87.8 ± 0.1%, reflecting the intrinsically wet nature of the raw material. The low standard deviation confirmed an enhanced reproducibility of the measurements and the homogeneity of the analysed samples. This high initial moisture content justified the necessity of thermochemical treatment prior to further valorisation, as it may significantly influence mass yield, energy efficiency, as well as the physicochemical characteristics of the resulting biochar. These results were in good agreement with values reported in the literature, which typically ranged between 80% and 90% for this type of agro-industrial residue [8,23].

During the pyrolysis of apple pomace, a series of complex chemical reactions occurred, including cracking, hydrogenation, dehydrogenation, cyclization, and aromatization. The resulting products comprised gaseous, liquid, and solid fractions (biochar). Gas evolution began at temperatures of approximately 225–230 °C, occurring around 60 min after the onset of reactor heating. Volatile formation was also observed at lower temperatures (170–175 °C), in significantly smaller quantities. The evolved gases exhibited a whitish, dense appearance and a sharp odour characteristic to hydrocarbon-rich mixtures containing traces of H₂S, suggesting potential applicability as an alternative fuel. Another reaction product obtained during apple pomace pyrolysis was pyrolytic oil, which began to condense in most experiments within the temperature range of 200–230 °C. The pyrolytic oil displayed a dark brown coloration and a characteristic hydrocarbon scent. The solid residue remaining after pyrolysis, denoted as apple pomace-derived biochar, accounted for approximately 22–23% of the initial dry biomass mass. About 32.99 g of biochar were obtained from pyrolysis at 400 °C, while 32.24 g were recovered from pyrolysis at 450 °C. The biochar samples were labelled according to pyrolysis temperature as BAP400 and BAP450, respectively.

The first stage (up to 150 °C) was intended to remove physically adsorbed water and residual moisture, preventing excessive vapor pressure and structural disruption during subsequent heating. The second stage (up to 350 °C) promoted the decomposition of volatile components and hemicellulose, together with the release of light organic fractions and the initial development of aromatic carbon structures. The final stage (up to 550 °C), with a residence time of 6 h, represented the main carbonization step, during which cellulose and lignin decomposition was achieved, volatile matter was fully eliminated, and partial graphitization occurred. This phase also contributed to the stabilization of the carbon matrix and the porosity development, properties essential for applications such as adsorption, catalysis, and or amendment.

The stepwise calcination of the apple pomace significantly reduced the risk of pore collapse, particle aggregation, and formation of the unstable amorphous structures, thereby improving the final quality and performance of the resulting biochar. Maintaining the material at 550 °C for 6 h resulted in an advanced degree of thermal transformation and carbonization, as typically associated with prolonged high-temperature treatment of biomass residues. This thermal treatment was consistent with the literature reports describing temperature staging as an efficient approach to modify the biomass composition and increase the structural ordering during the thermos-conversion [24,25]. After cooling, a homogeneous black solid was obtained and designated as BAP550, where the notation reflects the final calcination temperature.

The elemental analysis of the biochar obtained from apple pomace is presented in

Table 2. Elemental analysis of the raw material and derived biochar.

Element (%)	AP	BAP400	BAP450	BAP550
C	47.89	77.44	78.10	75.35
H	6.69	2.74	2.34	3.40
S	nd	nd	nd	nd
N	1.25	1.98	1.90	2.39

nd = not detected; AP = apple pomace (raw material); BAP400, BAP450 = biochar obtained from apple pomace by pyrolysis at 400 and 450 °C, respectively; BAP550 = biochar obtained from apple pomace by calcination at 550 °C.

.

The carbon (C) content of the biochar showed a slight increase with the pyrolysis temperature, a behaviour characteristic of progressive aromatization and elimination of volatile fractions. The sample obtained at 450 °C (BAP450) had the highest carbon content (78.10%), indicating a higher degree of carbonization. At elevated temperatures, apple pomace biochar presented a relatively constant carbon fraction and reduced content of thermal labile components, evidencing the formation of a condensed aromatic carbon structure. This structural stabilization, demonstrated by the elemental composition and spectroscopic analysis, supported the idea of suitability for applications demanding enhanced structural integrity and long-term carbon persistence [2].

During pyrolysis, the hydrogen (H) content decreased with the temperature, reflecting thermal dehydration and the loss of hydrogen-rich volatile compounds. Particularly, hydrogen content decreased from 2.74% (BAP400) to 2.34% (BAP450). A similar trend was observed for nitrogen (N), its concentration slightly decreasing from 1.98% to 1.90% with the increase of the pyrolysis temperature. This reduction can be attributed to the thermal decomposition and volatilization of nitrogen-containing organic compounds. Sulphur (S) was not detected in any of the apple pomace-derived samples, indicating concentrations below the detection limit of the applied analytical method. Overall, a pyrolysis temperature of 450 °C appeared optimal for maximizing carbon content while minimizing excessive losses of other heteroatoms.

At 550 °C, under calcination conditions, the process favoured the concentration of residual inorganic elements rather than the preservation of the biochar structure observed during pyrolysis. The biochar obtained under the calcination conditions (BAP550) presented enhanced chemical stability under severe oxidative and thermal environments, suggesting the formation of a partially hybrid organic–inorganic structure that can contribute to improved carbon stabilization.

3.2. ICP-OES Analysis

The ICP-OES results are presented in

Table 3. ICP-OES analysis of apple pomace and derived biochar.

Sample	Zn (mg/kg)	Al (mg/kg)	Fe (mg/kg)	Cu (mg/kg)	Cr (mg/kg)	Pb (mg/kg)	Co (mg/kg)	Ni (mg/kg)	Cd (mg/kg)	Mn (mg/kg)
AP	309.7 ± 0.95 a	366.4 ± 0.54 c	472.6 ± 1.30 c	30.92 ± 0.10 c	43.40 ± 0.12 c	0.392 ± 0.003 b	3.034 ± 0.014 b	11.30 ± 0.03 b	0.0567 ± 0.0003 a	18.86 ± 0.04 c
BAP400	47.54 ± 0.57 c	411.5 ± 0.25 b	543.7 ± 1.48 b	146.2 ± 0.72 a	45.22 ± 0.20 c	3.649 ± 0.008 a	0.0656 ± 0.0002 c	11.70 ± 0.03 b	0.109 ± 0.0004 a	38.04 ± 0.17 b
BAP450	74.74 ± 0.41 b	670.6 ± 0.24 a	946.6 ± 3.37 a	56.53 ± 0.30 b	28.50 ± 0.17 d	2.589 ± 0.011 b	0.369 ± 0.003 a	7.113 ± 0.024 c	<LOD	57.97 ± 0.24 a
BAP550	75.38 ± 0.71	315.5 ± 0.36	373.7 ± 0.77	54.86 ± 0.13	<LOD	2.776 ± 0.013	0.317 ± 0.0001	13.94 ± 0.06	<LOD	44.15 ± 0.19

AP = apple pomace (raw material); BAP400 and BAP450 = biochar obtained from apple pomace by pyrolysis at 400 and 450 °C, respectively; BAP550 = biochar obtained from apple pomace by calcination at 550 °C, excluded from statistical analysis; LOD = limit of detection; values are expressed as mean ± SD (n = 3); different superscript letters within the same column indicate statistically significant differences according to one-way ANOVA followed by Tukey’s HSD test (p < 0.05).

.

The ICP-OES analysis evidenced important differences of the trace metal concentrations between the raw apple pomace and the derived biochar, demonstrating the combined influence of the feedstock composition and pyrolysis temperature. It is well known that the elemental profile of the biochar is strongly controlled by the intrinsic mineral content of the precursor material, while the thermal conditions control the metal concentration, volatilization, redistribution, or immobilization within the solid matrix [26,27]. The BAP550 sample, obtained via calcination under oxidative conditions, was excluded from statistical analysis and was only qualitatively discussed, as its production pathway differed fundamentally from pyrolysis.

The zinc (Zn) content decreased in the biochars compared to the raw biomass, indicating a partial volatilization during the pyrolysis. The measured concentrations (47.54–74.74 mg/kg) were within the broad range reported for the biochar derived from agricultural and agro-industrial residues (9.61–138 mg/kg) [26]. Similar Zn depletion trends with increasing pyrolysis temperature have been reported and assigned to the moderate volatility of Zn and its low retention in ash-rich biochar produced at elevated temperatures [27,28]. This behaviour suggested that the resulting biochar may be suitable for applications such as Zn-contaminated soil remediation, ensuring that Zn bioavailability is carefully controlled [26].

Aluminium (Al) and iron (Fe) concentrations increased in all the samples compared to the apple pomace, probably due to the high thermal stability of aluminosilicate and iron-bearing mineral phases and their progressive concentration following organic matter decomposition [17]. This enrichment was consistent with other studies that investigated the accumulation of refractory elements in the biochar matrix at higher pyrolysis temperatures, mainly for the plant-based feedstocks containing soil-derived mineral fractions [17,26]. The limited variation observed for Fe further suggests that its behaviour is strongly influenced by the stable mineral associations rather than the volatilization process.

Copper (Cu) presented relatively high concentrations in the biochars (56.53–146.20 mg/kg), surpassing many values reported for woody or straw-derived biochar (3.45–40.27 mg/kg) [26]. This increase highlights the pronounced feedstock-specific influence, as fruit-processing residues are known to retain Cu resulting from the agricultural inputs and plant metabolism [29]. The relative thermal stability of Cu supports its retention and concentration in the solid residue, potentially enhancing the biochar adsorption capacity or catalytic performance [29].

Chromium (Cr) concentrations revealed a slight decrease with the pyrolysis temperature (28.50–45.22 mg/kg), remaining within the range reported for other biochars (5.88–69.42 mg/kg) [26]. On the other hand, nickel (Ni) showed moderate enrichment at higher temperatures, with concentrations (7.11–11.70 mg/kg) within the reported values (4.60–33.38 mg/kg) [26]. This behaviour suggests the increased mobilization and concentration of Cr and Ni under the thermal treatment, probably associated with the loss of oxygen-containing functional groups and reduced surface complexation capacity at higher temperatures [28,30].

Cobalt (Co) content increased in all the samples compared to the apple pomace, highlighting its concentration in the solid residue during the thermo-conversion, a pattern frequently observed for the transition metals with low volatility [27]. Cadmium (Cd) was detected at very low concentrations in the biochar produced at 400 °C and became undetectable at higher temperatures, confirming its high volatility under severe thermal conditions. The measured values (0.004–0.11 mg/kg) were comparable to those reported in the literature (0.12–0.17 mg/kg) [26], indicating a minimal cadmium presence, which is favourable for environmental applications [31].

Lead (Pb) was detected at relatively low concentrations (0.88–3.65 mg/kg), remaining within the range reported within the literature (1.70–35.99 mg/kg) [26]. This limited variability suggested an efficient Pb immobilization of Pb within the stable mineral phases, under neutral to slightly alkaline conditions [30].

A substantial increase in manganese (Mn) content was observed in all samples compared to the raw apple pomace, with a slight additional enrichment at higher pyrolysis temperature. This tendency may be important given the role of Mn-containing biochar in increasing the adsorption efficiency for heavy metals such as Pb²⁺, Cd²⁺, Cu²⁺, and Zn²⁺ through surface complexation and redox-mediated mechanisms [32]. Nevertheless, an excessive Mn accumulation may also result in its release under acidic conditions, causing potential ecological risks [33]. In this respect, a comprehensive characterization of Mn content and stability under varying environmental conditions is crucial prior to biochar practical application.

The statistical evaluation of the ICP-OES data (Table 3), restricted to the pyrolysis-derived samples (AP, BAP400, and BAP450), highlighted significant differences (p < 0.05) in the concentration of several elements as a function of the temperature. The analysis revealed that the thermal treatment under inert conditions had a strong influence on both macro- and trace-element distributions. Zn concentrations significantly decreased, with the raw material exhibiting the highest content, while both biochar samples (BAP400 and BAP450) had substantially lower values, indicating a partial volatilization or immobilization during the thermal process. In contrast, Al and Fe concentrations increased in the pyrolysis-derived biochars, reflecting ash enrichment and concentration effects associated with the progressive degradation of organic matter. Cu and Cr revealed statistically significant variations between the pyrolysis temperatures, suggesting a temperature-dependent mineral redistribution and transformation mechanisms within the biochar matrix. Cd concentrations remained at very low levels, approaching or below the detection limit, with no statistically significant differences observed between the pyrolysis samples, indicating negligible accumulation and minimal environmental risk. The relatively low relative standard deviation values (generally below 1%) confirmed the high analytical precision and reproducibility of the measurements, supporting the robustness of the observed trends.

Overall, the statistical analysis demonstrated that pyrolysis temperature is a key parameter governing elemental redistribution in apple pomace-derived biochar, with direct implications for its agronomic performance and environmental safety. Overall, the ICP-OES analysis confirmed that the trace metal composition of the apple pomace-derived biochar may be governed by both feedstock-specific mineral inputs and temperature-dependent thermochemical transformation, highlighting the demand for tailored pyrolysis conditions, resulting in biochar with controlled metal content and optimized environmental performance [17,27,28].

From the environmental perspective, the observed trace metal distribution and soil chemical responses highlighted the potential of apple pomace-derived biochar as a sustainable soil amendment. The consistently low concentrations of Pb and Cd, remaining below commonly reported phytotoxic thresholds, indicated a limited environmental risk associated with biochar application. This behaviour can be attributed to the strong affinity of these metals for stable mineral phases and carbonaceous surfaces, promoting immobilization through adsorption, precipitation, or co-precipitation under neutral to slightly alkaline soil conditions.

3.3. Thermogravimetric Analysis

Figure 1 presents the thermogravimetric analysis of the raw apple pomace and the corresponding biochar samples obtained at 400 °C, 450 °C, and 550 °C.

Thermogravimetric analysis (TGA) is essential for biochar characterization, providing insights into degree of carbonization, thermal stability, and mineral residue content. The TGA and DTG profiles allowed the identification of successive mass-loss stages corresponding to the complex lignocellulosic structure of apple pomace.

The TGA curve of dried apple pomace revealed several distinct thermal degradation stages characteristic of lignocellulosic biomass. A mass loss of approximately 4% was observed in the 20–150 °C temperature range, corresponding to the evaporation of free and weakly bound water. The DTG signal remained low (<–0.5%/min), indicating a physical dehydration process rather than a chemical degradation. This behaviour is typical for apple pomace, known for its initially high moisture content [34]. An additional weight loss of approximately 12% occurred within 150–260 °C, attributed to the thermal decomposition of hemicelluloses. The DTG peak reached a maximum around 260 °C, with a degradation rate of about –2.77%/min. This stage reflected the thermolabile nature of hemicellulosic polysaccharides, being consistent with values reported in the literature for similar lignocellulosic residues [34,35]. The 260–370 °C interval registered the most pronounced degradation stage, with a cumulative mass loss of approximately 25%. The main DTG peak appeared at 310–320 °C, reaching a maximum degradation rate of about 4.99%/min. This temperature range corresponded to the breakdown of the ordered cellulose structure, which degrades at higher temperatures compared to hemicellulose [34,35]. Beyond 370 °C, the degradation of lignin becomes dominant. The associated mass loss is approximately 12%, with broader and less defined DTG peaks, reflecting the complex and gradual decomposition of this highly aromatic and thermally stable polymer. The degradation rate in this region is lower (–1.0 to –1.5%/min), consistent with the known thermal resistance of lignin. Above 450 °C, the TGA curve gradually stabilized, indicating the formation of a thermally resistant carbonaceous residue. The final residual mass represented approximately 47% of the initial mass, highlighting the high potential of apple pomace for biochar production. This relatively high char yield is comparable to other lignocellulosic biomasses rich in structural polymers [35].

Overall, the TGA/DTG analysis confirmed that the apple pomace exhibited a thermal degradation profile typical to plant-based biomaterials, with well-defined mass-loss steps corresponding to moisture, hemicellulose, cellulose, and lignin. The significant stable residue supports its suitability as a feedstock for pyrolysis, yielding a carbon-rich material with potential application in agriculture or environmental remediation.

In contrast to the raw apple pomace, the thermogravimetric profiles of derived biochars revealed a simplified degradation behaviour. The TGA curves of the biochars produced at 400, 450, and 550 °C evidenced only minor mass losses over the entire temperature range up to 800 °C, confirming the advanced carbonization degree achieved during the thermochemical conversion [34]. The initial mass loss below 150 °C was minimal (<2%) and was attributed to the desorption of physically adsorbed moisture and residual volatile species. The absence of the distinct degradation phases in the biochar indicated that the thermal labile biomass components, such as hemicellulose and cellulose, had already been decomposed during the pyrolysis or calcination. In consequence, the remaining solid matrix mainly contained condensed aromatic carbon structures and mineral phases, which were characterized by a high thermal resistance [34,35]. This explained the similar TGA and DTG profiles observed for all biochar samples, regardless of the processing temperature. Small differences in the residual mass at higher temperatures (>600 °C), with slightly lower values for the biochar produced at 550 °C, were correlated with the removal of the residual functional groups and the partial decomposition of the ash-associated carbon species under more severe thermal conditions [34,35]. Overall, the comparable thermogravimetric behaviour of the biochars emphasised the dominance of the stable carbon frameworks formed at temperatures ≥ 400 °C, confirming their suitability for applications requiring thermal and structural stability.

3.4. Morphological and Elemental Characterization of Apple Pomace-Derived Biochar (SEM–EDS)

SEM micrographs of the biochar revealed a clear temperature-dependent evolution of surface morphology (Figure 2).

BAP400 exhibited a relatively compact structure with limited pore development, indicating incomplete carbonization and partial preservation of the original lignocellulosic matrix. The restricted porosity at this temperature was consistent with reports showing that lower pyrolysis temperatures lead to denser and less porous carbon structures [36]. BAP450 displayed a significantly more developed porous network, characterized by increased pore number and size. The enhanced porosity reflected intensified thermal decomposition and volatilization of organic fractions, promoting mesopore formation, in agreement with the literature data for intermediate pyrolysis temperatures [37,38]. BAP550 exhibited an expanded and more interconnected pore structure. Nevertheless, partial collapse of pore walls and localized structural rearrangements were also observed, probably due to the sintering effects and carbon matrix contraction at elevated temperatures. Similar morphological trends have been reported for biochar produced above 500 °C, where excessive thermal treatment may reduce microporosity despite increased overall pore development [39].

Overall, the SEM analysis demonstrated a transition from a compact morphology (400 °C) to a highly porous structure (450 °C), followed by partial structural rearrangement at 550 °C [40]. The sponge-like texture observed at higher temperature may enhance water retention and heavy metal adsorption capacity, relevant for environmental and agricultural applications [41,42].

The elemental composition of the raw apple pomace and derived biochars was evaluated by EDS (

Table 4. Elemental composition by EDS (wt%).

Element	AP	BAP400	BAP450	BAP550
C	66.49	88.43	87.59	60.31
O	21.02	8.62	9.15	29.43
K	10.21	1.65	2.10	8.81
Mg	0.45	0.30	0.30	0.44
P	1.08	0.31	0.48	0.67
Al	–	–	–	0.13
Si	–	0.12	–	0.22
Ca	0.19	0.57	0.38	–
Cl	0.33	–	–	–
S	0.23	–	–	–

), revealing significant chemical transformations induced by thermal treatment.

Carbon content increased from 66.49 wt% in AP to 88.43 wt% and 87.59 wt% in BAP400 and BAP450, respectively, confirming efficient volatilization and carbon enrichment during pyrolysis [25]. At 550 °C, carbon content decreased to 60.31 wt%, probably due to the structural degradation and enhanced exposure of mineral phases. Simultaneously, oxygen content decreased from 21.02 wt% (AP) to 8.62 wt% (BAP400) and 9.15 wt% (BAP450), indicating progressive elimination of oxygen-containing functional groups and formation of a more aromatic and hydrophobic carbon matrix [42]. The increase in oxygen content observed in BAP550 (29.43 wt%) suggested the surface reoxidation or the presence of mineral oxides. Potassium, initially abundant in AP (10.21 wt%), decreased significantly after pyrolysis at 400–450 °C, due to the volatilization and redistribution. An increased potassium content in BAP550 (8.81 wt%) reflected the reconcentration in stable mineral forms. Magnesium remained relatively constant (0.30–0.45 wt%), indicating a high thermal stability. Phosphorus decreased after pyrolysis but showed a slight increase at 550 °C, probably due to the formation of stable phosphate phases. Chlorine and sulphur were eliminated after thermal treatment, representing an advantage for soil application by reducing phytotoxicity risks [15]. The highest C/O ratio was observed for BAP400 and BAP450, indicating an optimal carbonization and structural stabilization at intermediate temperatures. These results suggested that 400–450 °C provided a favourable balance between carbon enrichment, structural integrity, and mineral retention in apple pomace-derived biochar.

3.5. Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectra of raw apple pomace (AP) and the corresponding biochars are presented in Figure 3.

The FTIR spectrum of AP exhibited characteristic absorption bands corresponding to the major organic constituents of lignocellulosic biomass, including polysaccharides, lignin, pectin, proteins, and phenolic compounds [43,44]. A broad band located at around 3280–3320 cm⁻¹ was attributed to O–H stretching vibrations from hydroxyl groups present in cellulose, hemicellulose, and phenolic compounds, as well as to absorbed moisture [39]. This band indicated the hydrophilic nature of the apple pomace and the presence of extensive hydrogen bonding networks [42,45]. The bands observed within the 2950–2850 cm⁻¹ region corresponded to the C–H stretching vibrations of –CH₃ and –CH₂– groups, typical for the aliphatic chains found in lignocellulosic components and fatty acids [46]. The band near 1735 cm⁻¹ was assigned to C=O stretching vibrations of ester and carboxylic groups, mainly associated with the pectin and acetylated hemicellulose structures characteristic for the fruit residues [47]. The relatively pronounced intensity of this band reflected the high pectin content of apple pomace. The absorption bands within the 1600–1510 cm⁻¹ region were attributed to the aromatic C=C stretching vibrations from lignin and phenolic compounds, as well as to potential contribution from bound water (H–O–H bending) [35,48]. The region between 1460–1360 cm⁻¹ corresponded to the bending vibrations of methyl and methylene groups, associated with lignin and minor lipid components. The carbohydrate fingerprint region (1200–900 cm⁻¹) exhibited strong C–O–C and C–O stretching vibrations typical of polysaccharides. The prominent peaks around 1030–1050 cm⁻¹ and near 1140 cm⁻¹ confirmed the presence of cellulose, hemicellulose, and pectin [45,47].

Overall, the FTIR profile of the apple pomace confirmed the lignocellulosic and pectic nature of apple pomace, highlighting its richness in polysaccharides and phenolic structures. This supports its suitability as a precursor for the thermochemical valorisation processes such as pyrolysis for the biochar production.

The FTIR spectra of the biochars revealed a progressive structural transformation with increasing pyrolysis temperature, consistent with the literature reports for similar carbonaceous materials [49]. The intensity of the functional group bands was the highest in BAP400 and decreased progressively toward BAP550, reflecting gradual elimination of oxygen-containing groups and increased aromatization. The bands within the 2960–2850 cm⁻¹ range, corresponding to aliphatic C–H stretching vibrations, indicated the presence of residual cellulose and hemicellulose fragments in lower-temperature samples [50,51]. These bands significantly diminished at higher temperatures due to the enhanced devolatilization. The weak bands detected in the 1840–1860 cm⁻¹ region corresponded to the C=O stretching in ketene-type structures formed during the thermal degradation [51,52]. The 1550–1554 cm⁻¹ region was mainly correlated with the C=C stretching vibrations of the aromatic structures derived from hemicellulose and lignin transformation. The intensity of these bands increased with the pyrolysis temperature, indicating progressive aromatic condensation. The bands from 1400–1480 cm⁻¹ were attributed to the C–H deformation in cellulose and hemicellulose residues, with slight shifts toward higher wavenumbers at elevated temperatures [53,54]. The aromatic ring C=C stretching and H–O–H bending vibrations may also contribute in the 1600 cm⁻¹ region, mainly for the lower-temperature samples [55]. The strong bands observed between 1010–1030 cm⁻¹ corresponded to the C–O stretching vibrations from the residual cellulose and hemicellulose structures [49,51,56]. These peaks decreased with the increased temperature, indicating progressive degradation of carbohydrate structures. The absorptions observed within the 800–890 cm⁻¹ region were attributed to the C=C stretching of vinylidene-type alkenes and aromatic ring structures, with increasing intensity at higher pyrolysis temperatures [49,51,57]. The carbonyl groups originally present in the hemicellulose and lignin components were gradually reduced during pyrolysis, and their concentration depended on the holocellulose-to-lignin ratio within the biomass [58]. Once the pyrolysis temperature increased, soft carbon components were progressively removed, while more condensed and stable aromatic (hard carbon) structures remained. This structural evolution enhanced the adsorption potential of the obtained biochar, essential for the wastewater treatment or environmental remediation applications [59].

The FTIR analysis indicated that an intermediate temperature (around 450 °C) may provide a balance between functional group retention and structural stability, which may constitute an advantage, depending on the intended application (adsorption, soil amendment, or environmental remediation).

3.6. Raman Spectroscopic Analysis

Raman spectroscopy represents a powerful tool for evaluating the structural organization of carbonaceous materials, enabling assessment of graphitic ordering and defect density in biochars produced by pyrolysis. The Raman spectra of the are presented in Figure 4. The BAP550 sample, obtained via calcination under oxidative conditions, is not directly comparable with the pyrolysis-derived samples (BAP400 and BAP450). Therefore, it is discussed only as a qualitative structural reference and not as part of the pyrolysis temperature-dependent trend.

The spectra were dominated by two characteristic bands:

-

the G band (~1580 cm⁻¹) corresponded to the E2g vibrational mode of sp²-hybridized carbon atoms in graphitic hexagonal lattices and reflects structural ordering of the carbon matrix [60];

-

the D band (~1350 cm⁻¹) was associated with A_1g vibrational modes activated by structural defects, edge sites, sp³ hybridization, or lattice distortions [60].

The I_D/I_G ratio and the crystallite sizes were calculated and are presented in

Table 5. Raman spectral parameters of apple pomace-derived biochars (λ = 532 nm).

Sample	D Position (cm−1)	D Intensity	G Position (cm−1)	G Intensity	ID/IG	La (nm)	FWHM(D) (cm−1)	FWHM(G) (cm−1)
BAP400	1377.63	714.44	1594.84	968.58	0.738	16.28	301.33	81.65
BAP450	1376.09	695.49	1597.56	985.24	0.706	17.02	255.93	67.94
BAP550	1374.54	612.96	1599.06	801.06	0.765	15.70	312.64	69.76

[20].

BAP400 exhibited a broad and relatively intense D band and a less defined G band, indicating a predominantly amorphous carbon structure with a high density of defects. The I_D/I_G (0.738) and calculated crystallite size (L_a ≈ 16.3 nm) suggested an initial aromatization but significant structural disorder. This behaviour is typical for biochar produced at lower temperatures, where lignin decomposition is incomplete and sp³-hybridized carbon structures remain prevalent [60]. Compared with the BAP400 sample, the BAP450 sample presented an increased G-band intensity (985.24 vs. 968.58) and a slightly lower D-band intensity (695.49 vs. 714.44), reflecting a relative decrease in the structural defects and a progressive development of the ordered sp² carbon domains [54]. The I_D/I_G (0.706) and the corresponding crystallite size (L_a ≈ 17.0 nm) confirmed an enhanced aromatic reorganization and improved graphitic ordering at 450 °C. This temperature appeared to promote optimal lignin conversion into extended aromatic structures, consistent with active structural rearrangement reported in the literature [61]. The balance between structural ordering and defect density suggested an improved carbon network stabilization. While increasing the pyrolysis temperature from 400 to 450 °C enhanced the overall aromatization, excessively severe thermal conditions may induce partial fragmentation, structural collapse, or internal reorganization of graphitic domains [62]. This indicated that, while the carbon matrix became thermally stable, it may also contain a higher density of defects compared to the 450 °C sample.

The crystallite sizes obtained (around 15–17 nm) indicated partially ordered carbon structures with moderately developed graphitic domains. As the pyrolysis temperature increased, soft carbon fractions were progressively eliminated, and more condensed, aromatic (hard carbon) structures remained. This evolution enhanced the thermal stability and may improve the performance in adsorption, catalysis, or energy storage applications [63].

In addition to the I_D/I_G ratio, the full width at half maximum (FWHM) of the D and G bands was evaluated to further characterize structural ordering in the biochar (Table 5). The FWHM values provided insight details regarding defect density and crystallite size distribution, with broader peaks indicating increased structural disorder. BAP400 exhibited a broad D band (FWHM ≈ 301 cm⁻¹) and a broad G band (≈82 cm⁻¹), consistent with a predominantly amorphous carbon structure and an incomplete aromatization at low pyrolysis temperature. These results indicated a heterogeneous distribution of small sp² domains embedded within a disordered carbon matrix. At 450 °C, both D and G bands became significantly narrower (FWHM(D) ≈ 256 cm⁻¹; FWHM(G) ≈ 68 cm⁻¹), reflecting an enhanced structural reorganization and development of more uniform aromatic clusters. The reduced FWHM values, together with the lowest I_D/I_G ratio and the highest crystallite size (L_a), confirmed that 450 °C represented the optimal temperature for carbon network ordering in apple pomace-derived biochar.

For the BAP550 sample, the D band broadened (≈313 cm⁻¹), while the G band showed a slight increase in intensity. In this case, the I_D/I_G slightly increased (0.765), and the crystallite size decreased (L_a ≈ 15.7 nm). This suggested that under calcination conditions, the enhanced thermal severity may promote further carbon condensation but may also induce structural rearrangement, increased defect density, or partial disruption of graphitic domains. Such behaviour was consistent with the thermally induced fragmentation reported for biochars treated at elevated temperatures [62,63].

Overall, Raman analysis confirmed a temperature-dependent structural transition within the pyrolysis domain, from predominantly amorphous carbon (400 °C) to semi-ordered aromatic networks (450 °C). The results suggested that intermediate pyrolysis temperatures may provide the most favourable trade-off between structural ordering and defect minimization in apple pomace-derived biochar. The BAP550 sample, produced via calcination, exhibited a distinct structural behaviour characterized by increased defect density, and was therefore considered only as a qualitative reference rather than part of the pyrolysis evolution pathway.

3.7. Germination and Early Growth Performance

The biochar obtained at 550 °C (BAP550) was solely produced as a high-temperature structural reference material to evaluate the upper limit of carbon structural ordering and mineral concentration under severe thermal treatment. This sample was generated via calcination rather than a controlled inert pyrolysis and therefore it was considered outside the optimal agronomic processing window investigated in this study (400–450 °C). Preliminary physicochemical assessment indicated substantial structural reorganization and increased mineral exposure at 550 °C, which could potentially alter salinity, trace metal availability, and surface functionality. For this reason, and to maintain a focused comparison within the pyrolysis temperature range compatible with soil amendment applications, BAP550 was not included in the germination bioassay. The agronomic evaluation concentrated on biochars obtained under controlled pyrolysis conditions (400 and 450 °C), which represent the most relevant and scalable processing range for agricultural use.

The effects of the raw apple pomace (AP) and biochars obtained at 400 and 450 °C on wheat seed germination and early seedling development are summarized in

Table 6. Final germination and seedling height parameters (Day 10) for apple pomace-derived treatments.

Treatment	Code	Final Germination (Seeds/10)	hmax (mm)	hmin (mm)
Soil (Control)	S	9 *	12.9 ± 0.10 c	6.2 ± 0.17 d
Soil + AP	SAP	7 *	13.4 ± 0.10 b	4.5 ± 0.20 e
Soil + BAP400	SBAP400	10 *	14.1 ± 0.10 a	7.0 ± 0.10 c
Soil + BAP400 (2%)	SBAP400+	10 *	14.1 ± 0.20 a	5.3 ± 0.10 e
Soil + BAP400 + F	SBAP400F	9 *	13.5 ± 0.10 ab	7.3 ± 0.10 b
Soil + BAP450	SBAP450	10 *	12.7 ± 0.10 c	7.1 ± 0.20 c
Soil + BAP450 (2%)	SBAP450+	9 *	13.5 ± 0.20 ab	7.6 ± 0.17 b
Soil + BAP450 + F	SBAP450F	9 *	13.5 ± 0.26 ab	10.1 ± 0.30 a

h_max = average maximum seedling height; h_min = average minimum seedling height; values are expressed as mean ± SD (n = 3); different superscript letters within the same column indicate statistically significant differences according to Tukey’s HSD test (p < 0.05). * Final germination showed no variance across replicates and was therefore not subjected to statistical testing.

(final germination and shoot height) and

Table 7. Root length and biomass parameters for apple pomace-derived treatments.

Treatment	Code	Root Max (cm)	Root Min (cm)	Fresh Leaf (g)	Dry Leaf (g)	Fresh Root (g)	Dry Root (g)
Soil (Control)	S	23.5 ± 0.30 a	11.0 ± 0.10 c	0.531 ± 0.00 c	0.371 ± 0.00 c	0.608 ± 0.01 a	0.589 ± 0.01 a
Soil + AP	SAP	23.5 ± 0.36 a	14.0 ± 0.17 a	0.353 ± 0.00 d	0.259 ± 0.00 d	0.347 ± 0.00 c	0.337 ± 0.00 c
Soil + BAP400	SBAP400	22.5 ± 0.10 b	11.0 ± 0.17 c	0.408 ± 0.00 c	0.291 ± 0.00 c	0.506 ± 0.00 b	0.485 ± 0.00 b
Soil + BAP400 (2%)	SBAP400+	23.0 ± 0.10 a	13.0 ± 0.10 b	0.483 ± 0.00 b	0.368 ± 0.00 b	0.560 ± 0.00 a	0.535 ± 0.01 a
Soil + BAP400 + F	SBAP400F	14.0 ± 0.10 c	8.0 ± 0.10 d	0.711 ± 0.00 a	0.420 ± 0.00 a	0.598 ± 0.01 a	0.571 ± 0.01 a
Soil + BAP450	SBAP450	22.0 ± 0.10 b	12.0 ± 0.10 c	0.495 ± 0.00 b	0.369 ± 0.00 b	0.402 ± 0.00 c	0.389 ± 0.00 c
Soil + BAP450 (2%)	SBAP450+	22.6 ± 0.10 b	10.0 ± 0.10 c	0.442 ± 0.00 c	0.340 ± 0.00 c	0.333 ± 0.00 d	0.323 ± 0.00 d
Soil + BAP450 + F	SBAP450F	19.5 ± 0.20 d	13.5 ± 0.10 a	0.568 ± 0.01 b	0.431 ± 0.00 a	0.486 ± 0.00 b	0.472 ± 0.00 b

Values are expressed as mean ± SD (n = 3); different superscript letters within the same column indicate statistically significant differences (one-way ANOVA, Tukey’s HSD test, p < 0.05).

(root development and biomass parameters).

3.7.1. Final Germination and Seedling Height

The control soil (S) exhibited a final germination of nine seeds out of 10 (no variance across replicates) and a maximum seedling height (h_max) of 12.9 ± 0.10 mm after 10 days (Table 6).

The application of raw apple pomace (SAP) resulted in reduced germination (7/10 seeds), although the maximum seedling height reached 13.4 mm. The lower germination rate suggested that the untreated apple pomace may contain inhibitory compounds or create suboptimal microenvironmental conditions. Among the biochar treatments, the sample denoted as SBAP400 and SBAP400+ (1% and 2% application rates, respectively) exhibited the highest final germination (10/10 seeds) and the highest maximum seedling height (14.1 mm). These treatments outperformed the control, indicating a stimulatory effect of the biochar produced at 400 °C. The treatment with biochar 400 °C and fertilizer (sample SBAP400F) resulted in slightly reduced germination (9/10 seeds) and a lower maximum height (13.5 mm) compared to the sample SBAP400+, suggesting that the fertilizer addition did not provide a clear synergistic benefit. For the biochar produced at 450 °C, the sample SBAP450 achieved full germination (10/10 seeds), but a lower maximum height (12.7 mm) compared to SBAP400. The double-dose treatment (SBAP450+) slightly improved the height (13.5 mm), while SBAP450F showed no additional growth benefit, despite fertilizer addition, fact that may indicate a more uniform but not necessarily more vigorous growth pattern.

The statistical evaluation of the germination and early growth parameters (Table 6) highlighted significant treatment-dependent responses on the seedling growth. One-way ANOVA followed by Tukey’s HSD post hoc test (p < 0.05) revealed that, while the final germination presented no variance among replicates (SD = 0.00) and it was not subjected to inferential statistics, both maximum and minimum heights significantly varied among the treatments. These results confirmed the experimental conclusion that the biochar produced at 400 °C had a more pronounced stimulatory influence on the shoot elongation, probably due to an enhanced nutrient availability and preservation of the surface functional groups. The minimum seedling height presented even stronger statistical differentiation. The SBAP450F sample exhibited significantly higher h_min values than all other treatments (p < 0.05), indicating an improved uniformity of the seedling development. On the other hand, the raw apple pomace (SAP) and the double-dose treatment (SBAP400+) revealed the lowest h_min values, statistically grouped at the lowest significance level, suggesting less uniform early growth. The control soil displayed intermediate behaviour.

The statistical analysis confirmed the experimental observation that the apple pomace-derived biochar significantly influenced the early wheat seedling growth, the biochar obtaining temperature having a crucial role.

Overall, biochar produced at 400 °C demonstrated superior performance in promoting both germination rate and shoot elongation, reflecting a favourable balance between nutrient availability and preservation of surface functional groups, as previously evidenced by the soil chemical analysis.

3.7.2. Root Development and Biomass Accumulation

The control treatment (S) exhibited a maximum root length of 23.5 cm and the highest total dry biomass (leaf + root), confirming adequate baseline soil fertility (Table 7). Raw apple pomace (SAP) showed similar maximum root length (23.5 cm) but significantly lower dry biomass, suggesting elongation without proportional biomass accumulation—possibly indicating mild stress conditions.

The SBAP400 and SBAP400+ samples maintained substantial root development (22.5–23.0 cm) and evidenced increased biomass accumulation compared to the SAP sample. The SBAP400+ sample produced one of the highest dry biomass values (0.9027 g total dry mass), indicating enhanced plant growth at the 2% application rate.

Contrary, SBAP400F highlighted reduced root length (14.0 cm), despite relatively high fresh biomass, suggesting altered biomass allocation when fertilizer was added.

Biochar produced at 450 °C (SBAP450 and SBAP450+) exhibited moderate root development (22.0–22.6 cm), but did not surpass the performance of SBAP400. The SBAP450F sample presented slightly reduced root growth and intermediate biomass values.

The root-to-shoot dry biomass ratio (calculated from Table 7) was the highest for SBAP400, indicating preferential allocation toward root development, beneficial for water and nutrient uptake. Fertilized samples showed more balanced or slightly reduced root allocation, suggesting potential shifts in physiological resource distribution.

The combined data presented in Table 6 and Table 7 demonstrated that apple pomace-derived biochar positively influenced wheat germination and early growth, mainly when produced at 400 °C and applied at moderate to double doses.

The results clearly demonstrated that biochar derived from apple pomace at 400 °C significantly improved uniform germination and ensured complete seed emergence, indicating favourable physicochemical interactions within the soil–plant system. Furthermore, this biochar type promoted superior shoot elongation and enhanced biomass accumulation, suggesting improved nutrient availability, water retention, and root–soil interface dynamics. Contrary, the fertilizer incorporation did not consistently enhance plant performance and, in certain treatments, failed to produce additive or synergistic effects. These findings highlighted the crucial role of the pyrolysis temperature and amendment strategy in optimizing the agronomic efficiency of the apple pomace-derived biochar.

One-way ANOVA analysis highlighted statistically significant differences (p < 0.05) among the treatments for all root length and biomass-related parameters (Table 7). The Tukey’s HSD test showed that the biochar amendment significantly modulated both the root architecture and the biomass accumulation compared to the control soil. The highest root elongation was observed for the samples amended with the biochar produced at 400 °C, mainly for the application rate of 2%, evidencing a significant increased root length than both the control and apple pomace samples.

These results support the hypothesis to use moderate pyrolyzed apple pomace biochar as an effective soil amendment for improving early-stage crop development, consistent with the literature, indicating that biochar produced at moderate temperatures keeps functional groups beneficial for nutrient retention and plant growth [64].

3.7.3. Germination Parameters and Seedling Performance Under Apple Pomace Treatments

Based on the experimental data, several germination parameters were calculated to evaluate the influence of raw apple pomace and apple pomace-derived biochar on the wheat seed performance.

Figure 5 presents the evolution of the mean germination time (MGT) and the coefficient of variation of germination time (CV_MGT), which reflects the uniformity of the seed emergence.

The sample treated with raw apple pomace recorded the lowest MGT (1.86 days), evidencing a significant stimulatory effect on the germination initiation (Figure 5a). This response may be associated with the presence of the soluble organic acids, available carbohydrates, or low concentrations of phenolic compounds able to stimulate early seed metabolism.

Contrary, the samples involving biochar produced at 400 °C (SBAP400, SBAP400+, SBAP400F) presented slightly higher MGT values, suggesting that a moderate pyrolysis temperature may not completely remove the volatile compounds or residual phytotoxic intermediates. Although the 400 °C biochar generally preserved oxygen-containing functional groups (such as –OH or –COOH), able to enhance the cation exchange capacity and nutrient retention [63], the incomplete thermal degradation of various compounds could temporarily delay germination.

The SBAP450F sample (2.43 days) revealed a relative improvement compared to SBAP400F, highlighting that a higher pyrolysis temperature combined with fertilization may generate a more stable carbon matrix with reduced phytotoxicity and improved water–nutrient retention. These findings are consistent with studies that reported biochars produced at moderate-to-high temperatures exhibiting enhanced structural stability and reduced volatile fractions, favouring seed establishment [64].

The coefficient of variation of germination time (CV_MGT) (Figure 5b) was calculated as a descriptive indicator of germination uniformity among treatments. Lower coefficient values (Figure 5b) indicated synchronized germination, an essential parameter for crop establishment. The control soil sample (S) exhibited moderate variability (26.23%), whereas the sample treated with apple pomace (SAP) produced a very high coefficient (109.33%), indicating strong heterogeneity in seed emergence. This significant dispersion suggested potential localized phytotoxic effects or uneven moisture distribution associated with undecomposed organic matter.

For the samples treated with biochar produced at 400 °C, the coefficient values were dependent on the dose and fertilization. The double-dose SBAP400+ treated sample significantly reduced variability (11.76%), indicating a uniform germination. This result supported the hypothesis that an increased biochar content can improve the soil structure, aeration, or moisture buffering capacity. On the other hand, the fertilized sample presented an intermediate variability (29.44%), suggesting that fertilizer addition may slightly disturb the germination synchrony.

The presence of the biochar produced at 450 °C resulted in coefficient values ranging from 15.31% (SBAP450), indicating a uniform germination, to 51.28% (SBAP450F), evidencing an increased heterogeneity when fertilizer was added. These findings demonstrated that the fertilizer–biochar interactions may alter osmotic balance or nutrient dynamics, mainly for the highly aromatized biochars.

The statistical analysis of the germination kinetics highlighted a strong treatment-dependent response (Figure 5). One-way ANOVA demonstrated a significant difference between different treatments, for mean germination time (MGT). Subsequent Tukey’s HSD post hoc analysis revealed that all treatments were statistically distinct from each other, reflecting the very low within-group variability and the pronounced influence of the applied amendments. The biochar produced at 400 °C, mainly SBAP400+, presented the highest MGT values, whereas the raw apple pomace treatment (SAP) exhibited the lowest MGT, evidencing accelerated but highly heterogeneous germination. The small standard deviations obtained for all replicates confirmed the high reproducibility of the experimental design and strengthened the robustness of the statistical discrimination among different treatments. On the other hand, the lowest CV_MGT values were observed for SBAP400+ and SBAP450, suggesting an improved synchronization of the germination.

The results emphasized that careful optimization of the pyrolysis temperature, biochar application rate, and fertilizer incorporation may be essential to achieve an optimal nutrient balance, while mitigating the potential inhibitory effects during the early stages of germination. Overall, low coefficient values, such as those observed for SBAP400+, indicated synchronized germination and agronomically favourable establishment, whereas elevated values (>50%) highlighted the demand for careful optimization of the treatment combinations.

Figure 6 presents the mean germination rate (MGR), a parameter inversely proportional to MGT.

The high MGR values indicated a faster and more efficient germination. The MGR values ranged from 0.70 (slow germination) to 1.00 (optimal germination). The control soil sample revealed a value of 0.90, while the highest values (1.00) were recorded for SBAP400, SBAP400+, and SBAP450. These treatments confirmed that the apple pomace biochar produced at 400–450 °C, can significantly enhance the germination yield by improving the soil microenvironmental conditions, including the water retention and nutrient availability [64]. The lowest MGR (0.70) was recorded for the SAP and SBAP450F samples, indicating either inhibitory effects of the undecomposed organic matter or unfavourable interaction between the fertilizer and the highly aromatized biochar. Intermediate values (0.90) were observed for SBAP400F and SBAP450, indicating acceptable but slightly delayed germination, probably due to the residual volatile compounds or nutrient imbalances [64].

One-way ANOVA evidenced significant treatment effects on the mean germination rate (MGR) (p < 0.05). Tukey’s HSD post hoc analysis differentiated treatments into three statistically distinct groups corresponding to MGR values of 1.00 (group a), 0.90 (group b), and 0.70 (group c). Biochar treatments produced at 400 and 450 °C generally had the highest MGR values, highlighting an increased germination performance compared with the raw apple pomace and fertilized variants.

These results highlighted that the application of biochar produced at moderate temperatures, mainly without fertilizer addition, supported a rapid crop establishment.

Figure 7 shows the germination speed (GS), which reflects the promptness of seed response once favourable conditions are established.

Overall, the values ranged from 1.83 to 3.33. The control sample exhibited a moderate GS (2.75). The sample treated with apple pomace (SAP) resulted in a lower value (1.83), reinforcing the hypothesis of potential inhibitory phenolic compounds or transient acidity effects in untreated biomass. Among the samples treated with biochar, SBAP400+ registered the highest GS (3.33), revealing a synergistic effect between the moderate pyrolysis temperature and the increased application rate. The samples SBAP400 and SBAP450 also had high GS values (2.70–2.82), confirming that the biochar produced at 400–450 °C enhanced the early metabolic activation. On the other hand, SBAP450F presented a reduced GS (2.00), indicating a potential chemical interference between the fertilizer components and the more condensed aromatic structure of high-temperature biochar.

The statistical evaluation of the germination speed (GS) evidenced a significant treatment-dependent effect (one-way ANOVA, p < 0.001). Tukey’s HSD post hoc test (p < 0.05) revealed a clear differentiation among different treatments (Figure 7). The highest GS was observed for SBAP400+, evidencing as a distinct statistical group and indicating a pronounced stimulatory effect of the double-dose biochar produced at 400 °C. The SBAP450 sample formed a separate statistical group, highlighting an intermediate enhancement compared to the control soil. The samples SBAP400, SBAP400F, and SBAP450+ had similar values. The lowest GS values were observed for SBAP450F and SAP, both statistically inferior to all the biochar-only treatments. The very low within-treatment variability (SD ≤ 0.006) confirmed the high reproducibility of the experiment and supported the robustness of the statistical separation.

These findings were consistent with the literature, demonstrating that biochar produced at moderate temperatures may keep some functional groups that improve water-holding capacity and nutrient exchange, thereby promoting seedling vigour [63], while excessive fertilization may disrupt osmotic balance or microbial interactions.

3.7.4. Soil Chemical Properties After Application of Apple Pomace and Derived Biochars

Table 8. Soluble macroelements, pH, and electrical conductivity of treated soils (final stage of the experiment).

Treatment	Ca (mg/kg)	Mg (mg/kg)	Na (mg/kg)	K (mg/kg)	pH	EC (µS/cm)
Initial soil	30.46 ± 1.51 c	5.70 ± 0.10 c	8.04 ± 1.92 e	185.34 ± 3.12 d	7.4 c	165.5 c
S	39.38 ± 0.59 b	4.34 ± 0.11 d	23.07 ± 1.87 d	141.29 ± 3.13 e	7.5 c	110.2 d
SAP	32.66 ± 0.82 c	3.87 ± 0.25 d	31.26 ± 2.22 c	118.68 ± 3.28 f	7.7 b	125.0 d
SBAP400	23.94 ± 1.28 d	7.72 ± 0.17 c	27.29 ± 2.06 cd	133.35 ± 3.15 e	7.6 b	110.0 d
SBAP400+	28.88 ± 0.05 c	3.80 ± 0.19 d	28.82 ± 2.19 c	173.18 ± 3.20 d	7.8 a	214.0 c
SBAP400F	53.95 ± 0.01 b	12.89 ± 0.23 b	133.50 ± 2.17 b	346.90 ± 3.12 b	7.9 a	440.0 b
SBAP450	18.19 ± 0.18 e	14.92 ± 0.15 b	44.92 ± 2.13 c	101.04 ± 3.16 f	7.7 b	178.0 c
SBAP450+	25.50 ± 0.88 cd	6.33 ± 0.07 c	48.45 ± 2.19 c	143.04 ± 3.14 e	7.9 a	127.8 d
SBAP450F	80.65 ± 0.53 a	18.16 ± 0.11 a	193.55 ± 2.03 a	447.89 ± 3.15 a	7.7 b	643.0 a

Values are expressed as mean ± SD (n = 3); Different superscript letters within the same column indicate statistically significant differences (one-way ANOVA followed by Tukey’s HSD test, p < 0.05).

presents the soluble macroelement concentrations (Ca, Mg, Na, K), soil pH, and electrical conductivity (EC) at the end of the experiment.

The soil pH remained within the neutral to slightly alkaline range (7.4–7.9). The highest values were observed for SBAP400+ and SBAP450+, consistent with the known liming effect of biochar due to the ash enrichment and alkaline mineral content [58]. These pH levels are generally favourable for macroelement availability. Nevertheless, the values approaching 8 may begin to reduce Fe, P, or Zn bioavailability through precipitation reactions [64].

The electrical conductivity (EC) revealed substantial variation across treatments. The highest value was observed for SBAP450F (643 µS/cm), followed by SBAP400F (440 µS/cm). Both samples involved fertilizer addition, indicating that combined biochar–fertilizer applications markedly increase soluble salt content. The elevated EC values suggested an increased ionic strength in the soil solution and potential osmotic stress, which may partially justify the reduced germination performance observed in some fertilized variants. Previous studies reported similar relationships between biochar–fertilizer interactions and salinity increase [63].

One-way ANOVA evidenced some statistically significant differences (p < 0.05) within the concentration of the soluble macroelements, pH, and electrical conductivity (Table 8). The Tukey’s post hoc analysis showed that the soils amended with the biochars, mainly those combined with a fertilizer, presented higher soluble Ca, Mg, Na, and K concentrations than the control and untreated soil. The most pronounced increases were registered for the SBAP450F and SBAP400F samples, forming distinct statistical groups for the soluble K and the electrical conductivity, revealing a substantial increase of the ionic strength and nutrient availability. Soil pH remained within a neutral to slightly alkaline range, with only moderate but statistically significant shifts among the samples. These values were generally favourable for the nutrient uptake, although the combination of elevated EC and high Na levels in fertilizer-amended treatments may impose osmotic stress, explaining the reduced germination performance [65]. The statistical analysis confirmed once again the strong influence of the biochar obtaining temperature, but also of the fertilizer addition, on the soil characteristics and on the nutrient dynamics.

Table 9. Total macroelement concentrations (mg/kg) in soils treated with apple pomace and derived biochars.

Treatment	Ca (mg/kg)	Mg (mg/kg)	Na (mg/kg)	K (mg/kg)	Fe (mg/kg)
Initial soil	9626.9 ± 0.5 f	4809.0 ± 0.1 f	78.5 ± 0.2 f	3890.8 ± 0.3 f	17,363.5 ± 0.8 c
S	9704.5 ± 0.1 f	4888.2 ± 0.1 f	79.1 ± 0.5 f	3913.7 ± 0.2 f	17,372.2 ± 0.2 c
SAP	9135.0 ± 0.3 g	4601.5 ± 0.1 g	102.9 ± 0.1 e	3756.5 ± 0.1 g	15,022.0 ± 0.3 d
SBAP400	11,047.8 ± 0.2 d	5829.8 ± 0.1 c	214.6 ± 0.2 c	5997.8 ± 0.7 a	19,046.2 ± 0.1 a
SBAP400+	10,745.7 ± 0.6 e	5298.9 ± 0.01 d	313.2 ± 0.1 b	4759.1 ± 0.1 c	16,993.2 ± 1.6 c
SBAP400F	11,884.8 ± 0.1 c	5751.3 ± 0.02 c	423.8 ± 0.2 a	5419.2 ± 0.02 b	19,007.6 ± 0.4 a
SBAP450	12,945.2 ± 1.2 b	5676.3 ± 0.1 c	362.7 ± 1.0 b	4039.6 ± 0.3 e	17,697.1 ± 0.7 c
SBAP450+	13,024.0 ± 0.3 a	5690.6 ± 0.03 c	401.4 ± 0.2 a	4514.0 ± 0.8 d	17,734.5 ± 1.3 c
SBAP450F	10,738.6 ± 1.8 e	5540.4 ± 0.02 d	533.8 ± 1.4 a	5024.0 ± 0.1 c	17,534.0 ± 1.5 c

Values are expressed as mean ± SD (n = 3); different superscript letters within the same column indicate statistically significant differences (one-way ANOVA followed by Tukey’s HSD test, p < 0.05).

presents the total macroelement concentrations determined after acid digestion.

Ca concentrations increased in all samples compared to the initial soil. Nevertheless, the soluble fraction accounted less than 1% of the total Ca, consistent with precipitation in carbonate or phosphate forms under neutral–alkaline conditions. Mg followed a similar pattern, though SBAP450 and SBAP450F exhibited slightly higher soluble fractions (0.25–0.33%), reflecting an enhanced mobilization at higher pyrolysis temperature. Na exhibited the most substantial proportional increase in the soluble fraction (10–40% of the total), mainly in the fertilized variants. Increased soluble Na levels corresponded with high EC values and may contribute to transient salinity stress. K displayed the most agronomically relevant modification. The total K was the highest in SBAP400 (5997 mg/kg), while soluble K peaked in SBAP450F (447.89 mg/kg). The increase in soluble K within the fertilized treatments supported the improved nutrient availability, but may induce ionic antagonism with Ca and Mg if excessive. Overall average soluble/total ratios were 0.2–0.8% for Ca, 0.1–0.3% for Mg, 10–40% for Na and 3–9% for K. These data indicated a higher mobility of Na and K compared to Ca and Mg, a behaviour consistent with ash-derived alkali metal dynamics in biochar-amended soils [63].

One-way ANOVA indicated statistically significant differences (p < 0.05) in the total macroelement concentrations (Table 9). Tukey’s post hoc test evidenced that the biochar-amended soils presented higher total Ca, Mg, Na, and K concentrations compared to the samples treated with the initial soil and with the raw apple pomace.

The highest total Ca concentrations were observed in soils treated with biochar produced at 450 °C, particularly at the higher application rate, indicating effective incorporation of Ca-rich mineral phases during high-temperature thermochemical conversion [66,67]. Total Mg followed a similar trend, with biochar treatments forming statistically distinct groups relative to non-amended soils.

Total Na content significantly increased in the fertilizer-amended variants, mainly in the SBAP400F and SBAP450F samples, reflecting the contribution of both biochar ash and fertilizer inputs. K presented the most pronounced response to biochar addition, with the SBAP400 sample exhibiting the highest concentration, consistent with the retention of K-bearing phases at moderate pyrolysis temperatures [68].

In contrast, total Fe concentrations exhibited smaller relative variations among treatments, suggesting that Fe is intensively controlled by the native soil matrix and is less affected by biochar addition [69].

Overall, these results demonstrate that thermochemical valorisation of apple pomace significantly alters the total nutrient reservoir of amended soils, while the agronomic relevance of these changes depends on the fraction converted into soluble and plant-available forms.

Table 10. Heavy metal concentrations in soils treated with apple pomace and derived biochars.

Treatment	Zn (mg/kg)	Al (mg/kg)	Fe (mg/kg)	Cu (mg/kg)	Cr (mg/kg)	Pb (mg/kg)	Co (mg/kg)	Ni (mg/kg)	Cd (mg/kg)	Mn (mg/kg)
Initial soil	112.3 ± 0.25 b	7544 ± 0.37 c	22,230 ± 4.95 b	24.83 ± 0.06 c	43.02 ± 0.10 b	15.81 ± 0.04 c	11.82 ± 0.03 b	21.98 ± 0.05 c	0.613 ± 0.001 b	425.6 ± 0.10 c
SAP	108.7 ± 0.42 b	10,460 ± 4.04 b	24,150 ± 9.33 a	26.71 ± 0.10 b	54.98 ± 0.21 b	18.92 ± 0.07 b	18.42 ± 0.07 a	26.29 ± 0.10 b	0.555 ± 0.002 c	574.3 ± 0.22 b
SBAP400	119.6 ± 0.35 a	16,170 ± 4.67 a	24,580 ± 7.09 a	28.40 ± 0.08 b	73.79 ± 0.21 b	18.79 ± 0.05 b	13.19 ± 0.04 b	38.22 ± 0.11 b	0.736 ± 0.002 a	437.9 ± 0.13 c
SBAP400F	117.2 ± 0.20 a	13,140 ± 2.25 a	24,840 ± 3.98 a	27.25 ± 0.05 b	47.82 ± 0.08 b	17.72 ± 0.03 b	12.87 ± 0.02 b	35.21 ± 0.06 b	0.624 ± 0.001 b	432.7 ± 0.07 c
SBAP400+	114.5 ± 0.28 a	17,100 ± 4.11 a	24,280 ± 5.84 a	33.59 ± 0.08 a	71.41 ± 0.17 b	19.33 ± 0.05 b	12.98 ± 0.03 b	28.49 ± 0.07 b	0.701 ± 0.002 a	430.8 ± 0.10 c
SBAP450	117.9 ± 0.20 a	13,520 ± 2.24 a	24,360 ± 4.20 a	32.73 ± 0.06 a	367.5 ± 0.61 a	18.73 ± 0.03 b	15.78 ± 0.03 a	199.1 ± 0.33 a	0.709 ± 0.001 a	614.6 ± 0.10 b
SBAP450+	73.54 ± 0.08 c	6322 ± 0.65 c	7348 ± 0.76 c	26.76 ± 0.03 b	18.60 ± 0.02 c	4.94 ± 0.01 c	9.24 ± 0.01 c	18.46 ± 0.02 c	0.433 ± 0.0004 c	354.2 ± 0.04 c
SBAP450F	112.4 ± 0.37 b	10,910 ± 3.54 b	23,270 ± 8.18 b	27.68 ± 0.09 b	45.36 ± 0.15 b	19.69 ± 0.06 b	22.06 ± 0.07 a	26.89 ± 0.09 b	0.646 ± 0.002 b	662.3 ± 0.22 a

Values are expressed as mean ± SD (n = 3); different superscript letters within the same column indicate statistically significant differences according to one-way ANOVA followed by Tukey’s HSD post hoc test (p < 0.05).

illustrates the concentrations of trace metals determined by ICP-OES for apple pomace treatments.

It must be stated that the trace metal concentrations determined for the initial soil were within typical background ranges for the agricultural soils [69]. For example, the measured Pb concentration (15,810 µg/kg) corresponded to 15.81 mg/kg, and Cd (612.6 µg/kg) corresponded to 0.61 mg/kg. These values were within the commonly reported natural background ranges for uncontaminated agricultural soils (Pb: 10–30 mg/kg; Cd: 0.1–1 mg/kg) [70,71]. Therefore, the initial soil cannot be considered contaminated but rather representative of a typical agricultural substrate with naturally occurring trace elements.

Zn generally increased compared to initial soil, mainly for SBAP400. Nevertheless, decreases were observed in SBAP450+, indicating a potential immobilization. The statistical analysis evidenced a significant treatment-dependent difference within the samples (Table 10). Zn concentrations were significantly higher in the soils treated with biochar produced at 400–450 °C, compared with the control and raw apple pomace, while a pronounced decrease was observed for the double-dose biochar produced at 450 °C. This was an indication of the partial immobilization at higher thermal severity [28,30].

Al concentrations increased in most cases, mainly in SBAP400+. Under neutral to slightly alkaline pH conditions, Al mobility remains limited. Fe revealed variable behaviour, with reductions in SBAP450+, suggesting potential immobilization or precipitation. Al and Fe revealed a statistically significant enrichment in most biochar-amended soils compared with the control, mainly for the samples involving moderate pyrolysis temperature, The lowest concentrations were registered for the double-dose 450 °C biochar, suggesting an increased metal stabilization or some dilution effects.

Cu moderately increased across all variants, consistent with the adsorption–desorption equilibrium influenced by biochar surface chemistry [30].

Cr and Ni presented pronounced increases in SBAP450, where Ni reached 199,100 µg/kg. They exceeded the typical background levels reported for agricultural soils in Europe (Cr: 10–100 mg/kg; Ni: 5–50 mg/kg) and may approach or surpass commonly applied precautionary thresholds (Cr: 150–200 mg/kg; Ni: 75–100 mg/kg) [69]. Therefore, although the observed values indicated significant accumulation relative to the control soil, they remained below the upper limit thresholds generally adopted under EU soil protection frameworks [72]. As discussed by other researchers, the commonly applied LV ranges under national transpositions of the European regulations are about 500–2500 mg/kg for Cr and 300–500 mg/kg for Ni, depending on soil characteristics such as pH and texture [73]. In this context, the concentrations measured in the present study remained below the upper LV thresholds generally applied within the EU regulatory framework. Moreover, under the EU Fertilising Products Regulation (EU) 2019/1009, specific maximum limits were established for Cd in fertilising products, but not for total Cr and Ni in soils, evidencing that the regulatory evaluation primarily concerns product compliance rather than the post-application soil total concentrations [74]. Nevertheless, these results underscored the importance of dose control and monitoring when applying high-temperature biochar to agricultural soils. From a statistic point of view, Cr and Ni presented the most pronounced treatment-induced variability. This fact indicated an enhanced mobilization of these elements under the high-temperature chemical conversion. From a mechanistic perspective, the elevated concentrations observed in SBAP450 may be attributed to the thermochemical transformations occurring during higher-temperature pyrolysis. At temperatures around 450 °C, progressive devolatilization and carbon matrix reorganization can lead to the relative enrichment of inorganic mineral fractions originally present in the apple pomace feedstock. In consequence, trace metals such as Ni and Cr may become more concentrated in the residual biochar matrix. Moreover, higher pyrolysis temperature may reduce the abundance of oxygen-containing surface functional groups (e.g., –COOH and –OH), known to contribute to metal complexation and surface adsorption. The reduced density of these functional groups may limit the capacity of the biochar surface to immobilize certain trace elements, increasing their apparent mobility in the amended soil system [75,76]. Although these values indicated a treatment-induced enrichment, it should be emphasized that the measurements corresponded to the end of a short-term germination experiment, where the observed concentrations reflected a combined contribution of the initial soil composition and the mineral fraction introduced through the biochar amendment. However, such increased values may further require careful monitoring, as high concentrations can present phytotoxic risks depending on speciation.

Cd remained below typical phytotoxic thresholds in all treatments [77]. Both Pb and Cd remained stable across all treatments, with no statistically significant enhancement beyond the beyond the initial soil levels, but also the background levels [30]. This fact can be explained by their strong affinity for the stable mineral phases and their efficient immobilization through adsorption, precipitation, or co-precipitation in neutral to slightly alkaline soil conditions [27,78,79,80].

Mn significantly increased in SBAP450F and AP treatments. While Mn represents an essential micronutrient, excessive levels may become toxic under certain conditions [77,78]. Its concentrations were significantly higher in the fertilized biochar treatments, highlighting the combined influence of the biochar mineral composition and the fertilizer-derived input [79,81].

The Tukey grouping demonstrated that both pyrolysis temperature and amendment formulation may significantly influence the heavy metals behaviour within the soil, underscoring the demand of a controlled biochar application to balance the nutrient availability and metal mobility.

It must be once again emphasized that the heavy metal concentrations (Table 10) were determined at the end of the 10-day germination experiment, therefore representing the integrated soil system after amendment incorporation, incubation, and early plant development. During this period, dynamic processes (such as dissolution–reprecipitation of ash-derived mineral phases, ion exchange reactions, competitive sorption, surface complexation, and initial plant uptake) may affect the metals distribution within the soil matrix. Consequently, the reported values reflected a stabilized post-interaction condition rather than the direct elemental contribution of the amendments alone. In a broader European context, the European Environment Agency (EEA) indicates that trace metals are widely present in agricultural soils due to both natural background and cumulative anthropogenic inputs, with regional variability influenced by soil properties and land-use history [71]. Thus, the interpretation of total heavy metal concentrations must consider an accumulation of several factors (such as soil chemistry, short-term equilibration processes, and regulatory benchmarks) rather than some absolute concentration values in isolation. This integrated perspective may support a balanced evaluation of the biochar application effects within realistic agronomic scenarios.

Overall, the application of the biochar obtained from apple pomace modified the soil chemical characteristics in a temperature-dependent manner. The biochar obtained at 400 °C promoted a more balanced nutrient profile, mainly corelated with the K enrichment and moderate ionic strength. Contrary, the 450 °C treatment, particularly when combined with a fertilizer, yielded in higher soluble Na levels and increased electrical conductivity, evidencing a higher contribution of the ash-derived salts to the soil solution chemistry. While most of the trace metal concentrations remained within the agronomic acceptable levels, the elevated Cr and Ni values highlight the demand for careful optimization of the pyrolysis temperature and the application rate. These findings accentuate the conclusion that the biochar production conditions and amendment formulation critically influence the balance between the nutrient enhancement and potential risks related to salinity and trace element accumulation. These remarks are consistent with the conclusions from previous studies addressing temperature-dependent biochar behaviour [63,64].

The stimulatory effect observed for the biochar produced at 400 °C was explained by the combined influence of its physicochemical characteristics on the soil–plant interface. Biochar produced at moderate pyrolysis temperatures may retain more oxygen-containing functional groups (such as –OH or –COOH), contributing to the increase of the cation exchange capacity and improve the nutrient retention in soil. Moreover, the porous microstructure observed in the SEM analysis may contribute to improved water retention and aeration within the rhizosphere, which could favour seed imbibition and early root development [64]. The redistribution of mineral elements detected by ICP-OES also indicated that the moderate pyrolysis temperature may promote the stabilization of the nutrient-containing mineral phases, while avoiding excessive ash accumulation that could increase salinity or osmotic stress. Together, these features may improve the nutrient accessibility, regulate the soil moisture, and stimulate the early metabolic activity during germination. Previous studies have also shown that such improvements in soil microenvironment may enhance enzymatic activities associated with seed germination, including amylase and protease activity, facilitating the mobilization of seed reserves, and promoting uniform seedling establishment [63,64,67].

4. Conclusions

The present study demonstrated that apple pomace may constitute an appropriate feedstock for thermochemical valorisation into biochar with tuneable structural and chemical characteristics. The extremely high initial moisture content (about 88%) confirmed the demand of pre-drying prior to conversion. Pyrolysis at 400–450 °C significantly influenced the carbon content and promoted aromatic condensation, as confirmed by FTIR and Raman spectroscopy, while maintaining a balanced defect density. The intermediate temperature (450 °C) conducted to the highest structural ordering (lowest I_D/I_G and narrowest FWHM), whereas calcination at 550 °C induced additional structural rearrangements and defect formation.

From the agronomic perspective, the biochar produced at 400 °C evidenced the most favourable performance in wheat germination assays, enabling the complete seed emergence, improved uniformity, enhanced shoot elongation, and increased biomass accumulation. These effects were associated with improved soil physicochemical characteristics, including moderated pH, enhanced K availability, and improved microstructural water retention potential.

On the other hand, the fertilizer addition in combination with high-temperature biochar increased the electrical conductivity and soluble Na concentrations, probably inducing a transient salinity stress and reducing germination uniformity. The trace metal concentrations generally remained within the agronomic acceptable limits; nevertheless, the Ni and Cr increase in certain high-temperature treatments highlights the importance of continuous monitoring and dose optimization.

Overall, the pyrolysis at 400 °C provided the most balanced compromise between the structural stability, nutrient enrichment, and plant performance. These findings support the integration of apple pomace-derived biochar into sustainable soil management protocols and circular bioeconomy models, simultaneously contributing to agro-waste reduction, carbon stabilization, and soil fertility enhancement.