Poultry Manure-Derived Biochar Synthesis, Characterization, and Valorization in Agriculture: Effect of Pyrolysis Temperature and Metal-Salt Modification

Abstract

In the present work, six biochars were produced from the pyrolysis of poultry manure at 400 °C and 600 °C (PM-B-400 and PM-B-600), and their post-modification with, respectively, iron chloride (PM-B-400-Fe and PM-B-600-Fe) and potassium permanganate (PM-B-400-Mn and PM-B-600-Mn). First, these biochars were deeply characterized through the assessment of their particle size distribution, pH, electrical conductivity, pH at point-zero charge, mineral composition, morphological structure, and surface functionality and crystallinity, and then valorized as biofertilizer to grow spring barley at pot-scale for 40 days. Characterization results showed that Fe- and Mn-based nanoparticles were successfully loaded onto the surface of the post-modified biochars, which significantly enhanced their structural and surface chemical properties. Moreover, compared to the control treatment, both raw and post-modified biochars significantly improved the growth parameters of spring barley plants (shoot and root length, biomass weight, and nutrient content). The highest biomass production was obtained for the treatment with PM-B-400-Fe, owing to its enhanced physico-chemical properties and its higher ability in releasing nutrients and immobilizing heavy metals. These results highlight the potential use of Fe-modified poultry manure-derived biochar produced at low temperatures as a sustainable biofertilizer for soil enhancement and crop yield improvement, while addressing manure management issues.

1. Introduction

The poultry sector accounts for around 40% of the total meat consumption in the world, and the largest producer countries are the USA (21.0 million tons (MT)), Brazil (15.1 MT), and China (15.0 MT) [1]. As a consequence, large amounts of poultry manure (PM) are yearly produced around the world. According to the United States Department of Agriculture (USDA), almost 105 million tons were generated in 2023 [2]. If mismanaged, the direct application of raw PM as a soil conditioner could lead to negative effects on soil, water resources, and air due to its high content in organic matter, nitrogen, and phosphorus [3]. Additionally, raw PM may act as a potential reservoir for several zoonotic bacteria, such as Salmonella spp. and Campylobacter jejuni, which represents an important risk to human health [4]. Hence, recent studies have focused on the management of these organic wastes by using several thermo-chemical conversion techniques including combustion, gasification, hydrothermal carbonization, and pyrolysis [5]. The pyrolysis process, which consists of biomass carbonization in absence of oxygen, is usually considered an attractive and cost-effective method for sustainable biomass management [6,7]. Moreover, this technology is ecofriendly, since it fully converts biomass into biofuel (syngas and bio-oil) that can be used for energetic purposes, and a solid carbonaceous residue, called biochar. Biochars can be valorized as effective adsorbents for pollutant removal from liquid and gaseous effluents [8] as well as a biofertilizer for agricultural soil amendment [9]. The latter reuse option improves the physical, chemical, and microbiological properties of soils and therefore enhances crop productivity [10]. Moreover, application of biochar to soil sequesters carbon and can reduce greenhouse gas emissions [11].

Biochars derived from PM can exhibit increased specific surface areas and porosity relative to the raw PM feedstock [12], although these values are typically lower than those of biochars produced from lignocellulosic materials. These properties allow for a marked improvement in the water retention capacity and hydraulic conductivity of the amended soils [13], and also a better stability of their aggregates [14]. In this context, Adekiya et al. [15] showed that the addition of PM-derived biochar (PM-B) at 50 t ha⁻¹ increased soil porosity and moisture content by around 14% and 6%, respectively. Moreover, the use of PM-B can significantly improve the chemical properties of agricultural soil such as the pH, electrical conductivity (EC), and cation exchange capacity (CEC) [15]. Furthermore, these biochars may offer more bioavailable nutrients for plants, including potassium, calcium, and magnesium [16,17,18]. In addition, agricultural soils usually contain a complex community of organisms such as bacteria, fungi, protozoa, and other invertebrates [19]. It has been shown that adding biochars to agricultural soils can stimulate their microbial activity potential, including enzymatic activities [20]. Soil enzymes such as dehydrogenase (intracellular) and glucosidase (extracellular) are considered good indicators of soil quality and are directly linked with the quantity and quality of soil organic matter content. For instance, Akça and Namlı [21] showed that the enzymatic activities following the addition of PM-Bs increased by about 63% compared to the control soil (without biochar). In addition, owing to their relatively high nutrient contents, PM-B have significantly increased the yield of cherry tomatoes [22], black cumin [23], and sweet potatoes [24]. This PM-B amendment improved mainly the growth of the shoot and root parts, and the number and dimensions of leaves in all crop cycles, which is dependent on several parameters related to feedstock and crop types [11]. For instance, Subedi et al. [25] proved that an amendment of 2% of poultry litter- and pig manure-derived biochars produced at 400 °C increased the dry matter yield of ryegrass by 50% and 127%, respectively.

It is worth mentioning that biochar modification with specific metal salts such as FeCl₃, CaCl₂, or MgCl₂ can significantly improve plant growth [26]. For instance, Wen et al. [27] showed that Fe-modified biochar from Platanus orientalis branches increased the grain yield by 60% due to the improvement in soil physico-chemical properties and nutrient availability. In addition, this operation reduced the bioavailability of As, Pb, and Cd in the amended soil. A similar trend was observed for lettuce growth and Cd immobilization in an amended soil with 1% of a KMnO₄-modified water hyacinth-derived biochar produced at 300 °C [28]. However, most of these studies have dealt with the valorization of modified biochars of lignocellulosic origin. Moreover, researchers have usually investigated either the effect of the type of metal salt used for modification or the pyrolysis temperature on soil properties and plant growth. Hence, to address these research gaps, this work aimed to evaluate the impact of using PM-Bs produced at 400 and 600 °C and their post-modified forms with Fe and Mn on spring barley growth for 40 days at pot-scale. The specific objectives were (i) to synthesize raw and Fe-, Mn-post-modified PM-Bs, (ii) to deeply characterize these PM-Bs by using different analytical techniques, (iii) to investigate, at pot-scale, the effect of PM-Bs on the growth of spring barley, as well as the nutrient content in their roots and shoots, and (iv) to address possible correlations between the amendment modalities and the nutrient content in roots and shoots.

2. Materials and Methods

2.1. Soil Collection and Characterization

The soil used in this study was collected from the top layer (0–25 cm) of a sandy loam agricultural soil in the Limerick region (Ireland). The soil was initially air-dried to a constant weight and then ground manually. In this work, we use the soil fraction with a particle size less than 2 mm. Particle size distribution was determined by a laser diffraction analyzer (Malvern Mastersizer STD06, Malvern, Worcestershire, UK). Moreover, soil hydrogen potential (pH) and electrical conductivity (EC) were measured in 1:10 soil–water slurry using a Fisher Scientific Accumet AB150 benchtop meter (Thermo Fisher Scientific, Waltham, MA, USA). Furthermore, the mineral composition was analyzed using an X-ray Fluorescence (XRF) apparatus (S8 TIGER Series 2).

2.2. Biochar Preparation and Characterization

Poultry manure was collected from a local farm in the province of Ben Arous, Northeast of Tunisia. Prior to pyrolysis, PM preparation was carried out according to the experimental protocol given in our previous investigation [12]. Briefly, the PM was firstly air-dried for around 10 days, then manually grinded, and the fraction with a particle size lower than 1 mm was retained. Pyrolysis was then performed in a laboratory furnace (Lenton). During this biochar production step, the sample was heated from room temperature using a heating gradient of 5 °C min⁻¹, then kept at the final chosen temperature for a residence time of 3 h. After that, the biochar was cooled down overnight. Two final pyrolysis temperatures of 400 °C and 600 °C were selected for this study; the resulting biochars were labeled PM-B-400 and PM-B-600, respectively. The choice of these temperatures was based on previous differential thermal analysis (DTA) and differential thermogravimetry (TGA) [12]. Afterwards, 50 g of each biochar sample was post-modified through agitation for 24 h in 1000 mL aqueous solution containing either 0.3 M of FeCl₃ or 0.3 M of KMnO₄. The agitation was performed using a magnetic stirrer (IKA, RT15 Power IKAMAG, IKA, Staufen, Germany). The resulting materials were rinsed thoroughly with distilled water before drying for 16 h at 60 °C. Depending on the used pyrolysis temperature (400 °C or 600 °C) and the modification treatment (with Fe or Mn), four post-modified biochars were synthesized, and, respectively, named PM-B-400-Fe, PM-B-400-Mn, PM-B-600-Fe, and PM-B-600-Mn.

The produced biochars were deeply characterized through the determination of their (i) particle size distribution, by way of Malvern Mastersizer STD06 laser granulometry; (ii) pH and EC, using a Fisher Scientific Accumet AB150 benchtop meter (Thermo Fisher Scientific, Waltham, MA, USA); (iii) pH of point-zero charge (pHpzc) through the pH drift method [29]; (iv) mineral composition through XRF analysis (S8 TIGER Series 2); (v) morphological properties using scanning electron microscopy (SEM) (SU-70 Hitachi, Tokyo, Japan) coupled with Energy Dispersive X-ray (EDS) (Oxford INCA x-act, Oxford instruments, Abingdon, UK); (vi) surface functionality by Fourier-transform infrared spectroscopy (FTIR) (Perkin Elmer-2000, Perkin Elmer, Waltham, MA, USA); and (vii) crystallinity properties through X-ray diffraction (XRD) (Bruker D8, Bruker, Billerica, MA, USA) equipped with a copper anode. The corresponding diffractograms and crystalline phases were identified with the Panalytical X’Pert High Score program and the International Center for Diffraction Data (ICDD) database.

2.3. Agricultural Valorization

The agronomic assays were carried out at pot-scale in a controlled plant growth chamber (A1000 Adaptis, Conviron, Manitoba, Germany). The temperature was regulated at day/night temperatures of 25/20 °C for a 12 h cycle. The relative humidity and the photo-active radiation were kept constants to 70% and 320 µmoles m⁻² s⁻², respectively. Each pot has four holes (diameter 0.5 cm) at its bottom to facilitate water drainage. Each pot was filled with the agricultural soil (1 kg) amended with 1% of each of the six biochars (PM-B-400, PM-B-400-Fe, PM-B-400-Mn, PM-B-600, PM-B-600-Fe, and PM-B-600-Mn). The agricultural soil sampling coordinates are located in Limerick, Ireland (52.6680° N, 8.6305° W). Soil samples were collected from the top layer (0–20 cm) using a stainless-steel auger following a composite sampling protocol. The fraction with dimension lower than 2 mm was used in the current study. This soil was classified as a sandy loam medium according to the USDA soil texture classification system. To allow for a relevant statistical analysis of the experimental data, all treatments including controls (agricultural soil without amendment) were carried out in quadruplicate. A total of 28 pots were prepared and then sown with eight spring barley seeds each. Subsequently, 100 mL of demineralized water was sprayed onto each pot to maintain the moisture of the soil throughout the cultivation cycle. At the end of the experiment, the harvested plant biomass from each replicate treatment was immediately weighed to determine the fresh weight. Moreover, for the determination of the mineral content, the harvested materials were firstly carbonized at 550 °C, then 0.2 g of these samples were dissolved in a solution containing 4 mL of hydrofluoric acid, 4 mL of hydrogen peroxide, and 6 mL of nitric acid at 190 °C in a microwave digestion oven (MARS 6 system). After complexation with boric acid, the content of micro- and macro-nutrient (P, K, Ca, Mg, Si, Fe, Cu, Mn, and Zn) were measured using an inductively coupled plasma optical emission spectrometry apparatus (Agilent 5100 ICP-OES, Agilent, Santa Clara, CA, USA) fitted with an SPS4 auto-sampler.

2.4. Statistical Analysis

Data were analyzed by ANOVA with post hoc Duncan’s multiple range test at p ≤ 0.05 for mean separation using XLSTAT software, version 2014 (Addinsoft, Paris, France). Multivariate statistical methods including Pearson Correlation (PC) and Principal Component Analysis (PCA) were also applied. These analyses aimed to assess the effect of the used biochars on plant growth parameters.

3. Results and Discussion

3.1. Agricultural Soil Characterization

The agricultural soil has a typical fine and uniform sandy loam texture (Table S1). Indeed, it is mainly composed of sand and loam with respective percentages of 55.1% and 40.2%, with a relatively low average diameter (0.4 mm). This agricultural soil can be considered a heterogeneous medium since its uniformity coefficient (UC) is more than 2 [30] (Table S1). Soil pH is neutral, and its electrical conductivity is low (185 µS cm⁻¹), suggesting that the contents of leachable minerals would be relatively low (Table S1). Furthermore, the soil mineral analysis indicates that it is moderately nutrient-rich with moderate potassium, calcium, and magnesium contents (Table S2). However, phosphorus content is low, which indicates the need for a supplementary amendment with a nutrient-rich material like biochars. Despite the soil’s high aluminum content (4.13%, Table S2), its near-neutral pH would likely prevent aluminum from limiting nutrient bioavailability [31]. Furthermore, the presence of low sodium levels confirms the low electrical conductivity (Table S1). The experimental soil also has low contents of chromium and zinc and concentrations below the detection limits for cadmium, arsenic, lead, and copper.

3.2. Biochar Characterization

According to the particle size distribution analysis, the synthetized biochars had a fine texture with an average particle size varying between 0.11 and 0.42 mm (

Table 1. Main physico-chemical composition of the used biochar (dx (mm): x% of particles have lower diameter than dx).

Properties	PM-B-400	PM-B-400-Fe	PM-B-400-Mn	PM-B-600	PM-B-600-Fe	PM-B-600-Mn
d10 (mm)	0.10	0.11	0.08	0.07	0.14	0.08
d50 (mm)	0.35	0.30	0.16	0.30	0.42	0.11
d60 (mm)	0.42	0.34	0.19	0.37	0.50	0.13
Uniformity coefficient (UC = d60/d10)	4.03	3.08	2.25	5.19	3.59	1.70
Electrical conductivity (µs cm−1)	333.0	750.3	376.7	1056	1335	1038
pHpzc	8.35	6.14	8.07	9.85	7.32	8.43

). Additionally, the pHpzc of the two raw biochars were evaluated to 8.35 and 9.85 for PM-B-400 and PM-B-600, respectively. The increase in the pHpzc value with the increase in pyrolysis temperature is due to the existence of higher alkali salts and metal oxides in the ash fraction [32]. Biochar post-modification with Fe and Mn resulted in a significant decrease in the pHpzc (Table 1). For instance, at a temperature of 600 °C, the pHpzc decreased by 2.5 and 1.4 pH units after post-modification with Fe and Mn, respectively, compared to non-modified biochar. This behavior is due to the formation of acidic functional groups of Fe and Mn on the surface of these biochars. A comparable trend was reported by Fu et al. [33] when investigating the effect of KMnO₄ post-modification on swine manure-derived biochar and by Park et al. [34] for a FeCl₃-pretreatd cattle manure-derived biochar at 500 °C. The analysis of CHNOS content showed that the PM-B-400 had high contents of C and O, at respective percentages of 22.04% and 75.45%. The H, N, and S were detected at lower contents of 0.87%, 0.95%, and 0.68%, respectively. Increasing the temperature to 600 °C (PM-B-600) resulted in a slight compositional change, with C, H, N, O, and S contents reaching 21.22%, 0.54%, 0.64%, 76.90%, and 0.66%, respectively. These changes are attributed to dehydration and decarboxylation reactions occurring during the pyrolysis process. Additionally, the H/C ratio decreased with temperature, indicating an enhanced aromaticity and carbon stability of the biochar at higher pyrolysis temperatures [12]. It is worth mentioning that the carbon content in hydrochars impregnated with Fe was found to be higher than Fe-modified biochars, a difference attributed to the distinct thermal decomposition behaviors of the two processes [35]. Moreover, Fe modification led to a significant reduction in hydrogen, nitrogen, and oxygen contents, particularly in the pyrolyzed samples, with a pronounced effect on biochars compared to hydrochars. According to the H/C, O/C, and (N + O)/C atomic ratios, the hydrothermal process preserved more aliphatic structures and nitrogen- or oxygen-containing functional groups compared to the pyrolysis process [36].

On the other hand, the content of mineral elements (i.e., Na, Mg, P, and K) significantly increased with temperature increase (

Table 2. Main physico-chemical composition of the used biochars (mean value ± standard deviation; DL: detection limit-: not measured). For each parameter, means with the same lowercase letters are not statistically different at p < 0.05.

Nutrient (%)	PM-B-400	PM-B-400-Fe	PM-B-400-Mn	PM-B-600	PM-B-600-Fe	PM-B-600-Mn
Mn	0.08 ± 0.015 a	0.01 ± 0.0008 b	11.26 ± 0.06 b	0.09 ± 0.0005 a	0.02 ± 0.0008 b	8.01 ± 0.01 b
Si	1.82 ± 0.024 a	1.1 ± 0.035 c	1.03 ± 0.075 c	1.61 ± 0.0590 a,b	0.82 ± 0.092 c	1.39 ± 0.027 b
Ca	7.15 ± 0.538 a	0.98 ± 0.071 b	8.69 ± 0.034 a	11.41 ± 0.078 c	1.77 ± 0.078 b	10.85 ± 0.069 c
K	3.93 ± 0.710 a	0.28 ± 0.07 b	4.24 ± 0.080 a	4.82 ± 0.0810 a	0.45 ± 0.035 b	4.76 ± 0.067 a
Mg	1.45 ± 0.244 a	-	1.31 ± 0.071 b	1.81 ± 0.148 a	-	1.63 ± 0.071 b
P	4.63 ± 0.272 a	4.56 ± 0.417 b	3.62 ± 0.132 b	5.85 ± 0.758 a	4.39 ± 0.516 b	4.45 ± 0.419 b
Al	0.36 ± 0.312 a,b	0.40 ± 0.080 a,b	0.23 ± 0.081 a	0.29 ± 0.085 b	0.43 ± 0.068 a,b	0.29 ± 0.027 a,b
Fe	0.36 ± 0.123 a	6.11 ± 0.085 b	0.26 ± 0.068 a	0.33 ± 0.101 a	5.27 ± 0.106 c	0.76 ± 0.063 d
Na	1.10 ± 0.011 a	<DL	0.36 ± 0.004 b	1.50 ± 0.068 c	-<DL	0.66 ± 0.009 d
S	0.74 ± 0.116 a	0.20 ± 0.039 b	0.20 ± 0.003 b	0.72 ± 0.035 a	0.24 ± 0.07 b	0.32 ± 0.07 b
Cr	0.0061 ± 0.0007 a	0.0034 ± 0.0016 b	0.0027 ± 0.0007 a	0.0041 ± 0.0002 a	0.0036 ± 0.0005 a	0.0027 ± 0.0004 a
Zn	0.08 ± 0.0260 a,b	0.01 ± 0.0030 a	0.04 ± 0.0160 a,b	0.08 ± 0.0300 a,b	0.02 ± 0.008 a,b	0.08 ± 0.0008 b
Ba	0.0048 ± 0.0006 a,b	0.0039 ± 0.0001 a	-	0.0052 ± 0.0001 b	0.0033 ± 0.00001 a	-
Ni	0.0209 ± 0.007 a	0.0210 ± 0.005 a	-	0.0216 ± 0.001 a	0.0238 ± 0.0003 a	-
Cu	0.0122 ± 0.0002 a	0.0137 ± 0.0004 a	-	0.0142 ± 0.0052 a	0.0139 ± 0.0065 a	-
Pb	-	0.0007 ± 0.00005 a	-	0.0003 ± 0.00006 b	0.0009 ± 0.00001 c	-

) due to the concentrating effect of the pyrolysis. Based on the statistical analysis, the contents of mineral elements show significant differences for studied biochars. In this context, Ibn Ferjani et al. [37] demonstrated an increase in K, Mg, Na, and P by 83%, 155%, 112%, and 143%, respectively, when the temperature increased from 300 °C to 600 °C. Moreover, the nutrient content (i.e., Ca, P, K, and Mg) of the raw and modified biochars were relatively high (Table 2). For instance, the potassium concentration dropped from around 3.9% in PM-B-400 to 0.3% in PM-B-400-Fe and from 4.8% to 0.5% at 600 °C, suggesting that K⁺ ions were likely exchanged with Fe species during modification. A similar observation applies to Mg, which was entirely undetected in both Fe-treated samples, supporting the hypothesis that Mg²⁺ was also displaced by Fe through cation exchange mechanisms. This exchange was likely facilitated by the high affinity of Fe for the active surface sites on biochars. Overall, Fe modification enhances cation exchange via surface complexation, replacing native elements like K⁺ and Mg²⁺. In contrast, Mn modification primarily promotes redox reactions. Mn oxide deposition on biochar surfaces imparts redox activity, enabling the reduction of Mn⁴⁺ to Mn²⁺, which facilitates the removal of redox-sensitive contaminants (Table 2). Moreover, the Mn-modified biochars also exhibit distinct surface charge characteristics, influencing cationic metal adsorption differently than Fe-modified biochars [38]. For these reasons, the Mn exhibits limited direct cation exchange with K⁺ and Mg²⁺ [39].

Moreover, Table 2 shows that the increase in the pyrolysis temperature slightly increased heavy metal content, which was expected due to a concentrating effect. No significant effect on the concentration of these elements was observed after the post-modification process, suggesting that they were chemically retained onto the surface of these biochars.

Moreover, according to SEM analysis (Figure 1), the raw PM-Bs (PM-B-400 and PM-B-600) had heterogeneous and irregular porosity and displayed a crystal-like structure that is most probably composed of mineral oxides. After modification with Fe and Mn, the pore structure of the synthetized biochars became obstructed and acquired a honeycomb-like shape. In addition, the modified biochars appeared smoother and gradually covered with nanoparticles of Fe- or Mn-based nanoparticles. A comparable result was reported by Chen et al. [40] when studying the post modification of cow litter with FeSO₄·7H₂O and Fe(NO₃)₃.

It is worth mentioning that the textural properties of biochars (BET surface area, total pore volume, and average pore width) may vary greatly with the pyrolysis temperature and the used modification process. For example, in a previous work of the current research team, it was shown that compared to a raw PM-B-400, modification with KMnO₄ increased the BET surface area by 1.7 times and decreased its average pore size from 15 to 8 nm [41]. A similar trend was also observed by Wang et al. [42] for both chicken manure biogas residues and pig manure biochars at 550 °C, respectively.

The FTIR spectra of the raw biochar samples (PM-B-400 and PM-B-600) (Figure 2) indicated the presence of numerous alkaline and acidic functional groups, confirming surface functionality heterogeneity [43]. The primary identified functional groups include hydroxyls, alcohols, and phenols (–OH at 3400–3200 cm⁻¹) [44]; carbonyls (C=O at 1650–1600 cm⁻¹), which involve the C=O stretching of carboxyl groups (CO₂H) and primary amides [45]; –CH₂ deformations (at 1440–1400 cm⁻¹) corresponding to the stretching of saturated fatty acids and cellulose [46]; acetyl groups (C–O: 1100–1000 cm⁻¹) related to carbohydrate stretching [47]; and aromatic C–H bonds (1002–876 cm⁻¹), which correspond to the stretching of C–H and represent the out-of-plane bending vibrations of aromatic structures [48]. The post modification of PM-derived biochar with Fe and Mn induced important changes in the intensity of some existing peaks (Figure 2). For instance, the O–H stretching vibration transmittance of PM-B-400-Fe was around 51% higher than PM-B-400, which is attributed to the hydrolysis of Fe(OH)₃ to produce a more stable crystalline form of iron hydroxide (FeO–OH) [49]. Furthermore, the peaks detected at 1419 cm⁻¹ and 871 cm⁻¹, related to the aromatic –C–H bands and aliphatic C–H stretches, respectively, have disappeared after the treatment with Fe (for both PM-B-400-Fe and PM-B-600-Fe) (Figure 2). Since these peaks are associated with potential C–H deformations inaromatic bends in carbohydrates, their disappearance may suggest the successful incorporation of Fe during the doping process. Similar results were found by Godlewska et al. [50] for Fe(NO₃)₃ post-modified wheat straw biochar. Additionally, Fe impregnation on PM-B-600-Fe resulted in an increased intensity of the carbonyl (C=O) peak at 1628 cm⁻¹. The C–O bond (at 1233 cm⁻¹) also showed greater intensity, associated with the stretching of –COR in aliphatic ethers and –COH in alcohols [51]. In addition, after modification with Fe, new peaks appeared at wavelengths lower than 500 cm⁻¹, which correspond to the characteristic peaks of Fe–O and confirmed that FeCl₃ was successfully loaded onto PM biochar [52]. However, it is worth mentioning that no important changes were observed after Mn post-modification. This can be ascribed to the fact that Fe is more readily ionized than Mn in the presence of free oxygen, leading to a higher production of metal oxides [41]. However, Mn–O bands were observed in both PM-B-400-Mn and PM-B-600-Mn spectra at a wavelength of 712 cm⁻¹. Mn oxide formation at the surface of the biochar contributes to the enhancement of its textural properties [41].

As shown in Figure 3, XRD analysis indicated the presence of crystalline structure for raw and doped biochars, attributed mainly to calcium carbonate (CaCO₃; ICDD: 00-005-0586) and quartz (SiO₂; ICDD: 00-033-1161 and ICDD: 00-046-1045). Under both carbonization temperatures, biochars presented large bands between 25° and 35° (2θ), more distinguishable for the ones synthesized at 400 °C, which can be attributed to the amorphous carbon structure of the biochars [35]. Notably, the aromatic matrix of cellulose seems to be more pronounced for Fe- and Mn-loaded biochars (Figure 3). In fact, peaks corresponding to amorphous carbon are detected at 31.4° (2θ) for PM-B-400 and PM-B-400-Mn, while this peak shifts for the case of PM-B-400 to a diffraction angle of 26.0°(2θ). This suggests that iron and manganese deposited on the biochar may exist in a nonstoichiometric, amorphous phase [53]. Likewise, Liang et al. [54] reported a notable reduction in both the number and intensity of several peaks in the XRD spectra following the impregnation of manganese onto raw biochar generated from swine waste. This was imputed to a partial disruption of the ordered arrangement within the biochar, possibly due to the formation of new manganese–biochar complexes or amorphous phases. On the other hand, Jia et al. [55] reported that the impregnation under air of Fe²⁺/Fe³⁺ onto biochar could lead to the formation of oxidized forms of iron which are easily removed in the aqueous solution. Consequently, the increase in peak the intensity of amorphous carbon after modification with Mn and Fe could be partially attributed to an overlapping with those related to Fe- and Mn-oxides (e.g., MnO₂, Fe₂O₃, or Fe₃O₄) [56,57].

3.3. Impact of Biochar Amendment on Barley Growth

The effect of different biochar-amended soil treatments on spring barley growth was evaluated in this study. According to plant appearance after the 40-day growth period (Figure 4), the biochars produced at 400 °C had a greater positive impact.

Table 3. Effect of biochar amendments at 1% on the length and weight of shoots and roots of spring barley plants (values are means of four replicates ± standard deviation. For each plant parameter, means followed by the same small letter are not statistically different at p ≤ 0.05).

Treatments	Shoot Length (cm)	Shoot Dry Weight (g)	Root Length (cm)	Root Dry Weight (g)
Control assay	34.0 ± 0.957 a	0.09 ± 0.013 a	14.6 ± 0.520 a	0.07 ± 0.010 a
PM-B-400	46.2 ± 2.872 c	0.39 ± 0.054 d	14.7 ± 0.656 a	0.28 ± 0.032 b
PM-B-400-Fe	47.5 ± 1.000 c	0.36 ± 0.085 d	23.8 ± 0.629 c	0.62 ± 0.224 e
PM-B-400-Mn	46.8 ± 1.315 c	0.34 ± 0.028 d	18.7 ± 0.816 b	0.56 ± 0.169 e
PM-B-600	39.0 ± 1.500 b	0.26 ± 0.047 c	22.0 ± 1.291 c	0.59 ± 0.170 e
PM-B-600-Fe	41.5 ± 2.483 b	0.22 ± 0.050 b	24.7 ± 3.304 c	0.49 ± 0.114 d
PM-B-600-Mn	40.7 ± 2.217 b	0.24 ± 0.056 bc	24.0 ± 0.816 c	0.35 ± 0.105 c
F	27.190	16.462	37.777	7.029
p	7.25 × 10−9	5.832 × 10−7	3.423 × 10−10	3.3 × 10−4

gives the measurements of the root and shoot parts sampled after 40 days of cultivation. Compared to the control assay, the shoot length increased by 38% with PM-B-400-Fe, followed by PM-B-400-Mn (37%), raw PM-B at 400 °C (26%), PM-B-600-Fe (22%), PM-B-600-Mn (19%), and raw PM-B at 600 °C (14%). Root growth showed a similar trend (Table 3). The higher shoot biomass yield was notably observed for the three biochars generated at 400 °C (raw or post-modified with Fe or Mn), which might be attributed to a higher release of minerals needed for the plant growth (Table 3). Based on statistical defined groups (a, b, c, d, e), the biometric parameters were tightly related to the used treatment (Table 3). A similar trend was reported in previous studies [24]. According to Gunes et al. [45], the amendment of a loamy clay soil with 1% of a PM-B enhanced the dry weight of the lettuce shoot by 121% for a biochar produced at 250 °C compared to the one produced at 350 °C. Thus, biochars produced at lower temperatures would combine several properties like adequate water retention, sufficient nutrient availability, and better microbial activity [58]. Additionally, biochars produced at higher temperatures usually exhibit excessive alkalinity (Table 1), low nutrient release capability [59], and reduced functional groups content that may negatively impact crop growth [25,60].

In conclusion, PM-B-400-Fe seems to have promising agronomic properties for enhancing plant growth, as indicated by preliminary spring barley growth experiments. This positive effect is likely due to several physico-chemical improvements resulting from iron modification. Specifically, Fe incorporation increases micronutrient content and potentially improves iron bioavailability, crucial for plant growth processes. Furthermore, Fe-modified biochars may promote iron plaque formation on root surfaces, regulating nutrient uptake and immobilizing toxins [61]. These iron-rich surfaces might also buffer rhizosphere redox and improve soil structure, indirectly aiding root development [62]. Although detailed physiological measurements and soil analysis were not performed, these changes suggest that Fe-modified biochar could function as a slow-release nutrient carrier after soil application. Further research is required to validate these mechanisms in realistic soil–plant environments. Similar results were found by Lu et al. [63], who examined whether magnetic biochars offer advantages over conventional biochars for remediating a multi-contaminated paddy soil near a Pb-Zn mining area in China. Biochars from poultry litter and eucalyptus were produced at 300 °C and 500 °C and applied to this soil polluted with Cd, Cu, Zn, and Pb. They showed that poultry litter-derived biochars reduced the solubility of Cd, Zn, and Cu in the soil, with some differences observed between magnetic and conventional biochars. Moreover, the feedstock selection appeared to be more important than pyrolysis temperature or magnetization in reducing heavy metal mobility. However, the magnetization process showed high potential in enhancing plant growth.

In addition, Fe-modified biochar exhibited a better effect on plant growth in comparison to the Mn-modified material (Table 3), which can be explained by (i) the crucial importance of Fe for plant metabolic processes, (ii) its ability to improve the availability of nutrients, and (iii) its antimicrobial properties [64]. In addition, Fe plays a central role in photosynthesis and root development, and consequently on plant health and growth [65]. A comparable result was also reported for rice growth cultivated in a paddy soil amended with Fe-modified biochar [27]. Therefore, the PM-B-400-Fe amendment of agricultural soil can be regarded as a promising approach.

3.4. Nutrient Content in Plant Biomass

Table 4. Effect of poultry manure-derived biochar amendments on macronutrient and micronutrient content in shoots and roots of spring barley (values are means of four replicates ± standard deviation. For each element, means followed by the same small letter are not statistically different at p ≤ 0.05).

Treatment	Macronutrients Shoots (mg g−1)				Macronutrients Roots (mgg−1)				Micronutrients Shoots (mg g−1)						Micronutrients Roots (mg g−1)
	K	Ca	P	Mg	K	Ca	P	Mg	Si	Fe	Mn	Cu	Zn	As	Si	Fe	Mn	Cu	Zn	As
Control assay	20.57 ± 1.9 a	10.83 ± 1.7 b	1.50 ± 1.1 a	3.14 ± 0.9 d	3.38 ± 1.5 a	0.86 ± 0.6 a	0.22 ± 0.1 a	0.76 ± 0.06 a	38.5 ± 2.1 b	1.8 ± 0.05 c	0.13 ± 0.02 a	0.02 ± 0.001 a	0.02 ± 0.001 a	0.0034 ± 0.0002 b	41.4 ± 0.001 a	3.1 ± 0.001 a	0.21 ± 0.001 a	0.06 ± 0.078 a	0.02 ± 0.001 a	0.0215 ± 0.004 a
PM-B-400	25.76 ± 1.5 ab	11.95 ± 1.5 b	8.50 ± 2 d	3.67 ± 0.8 d	30.86 ± 2.5 c	6.77 ± 1.5 e	3.10 ± 1.0 e	7.16 ± 1.0 c	7.0 ± 1.2 a	0.31 ± 0.2 b	0.07 ± 0.004 a	0.02 ± 0.001 a	0.05 ± 0.005 b	0.0001 ± 0.0001 a	399.6 ± 0.001 c	33.0 ± 0.001 b	1.04 ± 0.001 b	0.04 ± 0.002 a	0.12 ± 0.001 b	0.0178 ± 0.006 a
PM-B-400-Fe	27.05 ± 1.5 b	18.37 ± 1.5 c	4.12 ± 1.9 b	4.06 ± 1.01 e	24.12 ± 1.9 b	4.21 ± 1.0 c	1.59 ± 0.6 b	6.86 ± 1.0 bc	8.4 ± 1.0 a	0.33 ± 0.1 b	0.06 ± 0.002 a	0.02 ± 0.001 a	0.03 ± 0.003 a	0.0005 ± 0.0002 a	356.0 ± 0.001 b	36.9 ± 0.001 b	1.11 ± 0.001 b	0.05 ± 0.001 a	0.08 ± 0.002 b	0.0107 ± 0.004 a
PM-B-400-Mn	37.86 ± 2.1 d	9.62 ± 1.5 b	6.70 ± 1.7 c	2.67 ± 0.6 c	31.03 ± 1.5 c	5.56 ± 1.0 d	2.47 ± 0.6 d	7.19 ± 0.6 c	8.7 ± 1.0 a	0.24± 0.01 a	0.13 ± 0.007 b	0.012 ± 0.001 a	0.03 ± 0.003 a	0.0001 ± 0.0001 a	378.4 ± 0.001 bc	35.9 ± 0.001 b	2.17 ± 0.002 d	0.04 ± 0.003 a	0.11 ± 0.001 b	0.0063 ± 0.002 a
PM-B-600	31.18 ± 1.9 c	6.82 ± 1.0 a	4.46 ± 1.9 b	2.44 ± 0.6 b	29.91 ± 2.0 c	5.68 ± 1.5 d	2.62 ± 0.6 c	6.98 ± 0.59 c	8.6 ± 1.0 a	0.21 ± 0.1 a	0.04 ± 0.004 a	0.011 ± 0.001 a	0.02 ± 0.001 a	0.0004 ± 0.0001 a	373.4 ± 0.002 bc	35.6 ± 0.001 b	1.54 ± 0.001 c	0.04 ± 0.02 a	0.10 ± 0.001 b	0.0143 ± 0.005 a
PM-B-600-Fe	29.43 ± 2.0 bc	8.88 ± 1.0 b	2.47 ± 1.1 a	2.16 ± 0.6 a	25.23 ± 1.5 b	2.61 ± 0.6 b	1.73 ± 0.6 c	6.04 ± 0.59 b	11.3 ± 1.0 a	0.31 ± 0.1 b	0.05 ± 0.002 a	0.02 ± 0.001 a	0.02 ± 0.001 a	0.0002 ± 0.0001 a	385.3 ± 0.001 b	38.0 ± 0.001 b	1.30 ± 0.001 b	0.05 ± 0.001 a	0.09 ± 0.001 b	0.0111 ± 0.008 a
PM-B-600-Mn	34.09 ± 2.0 c	8.19 ± 1.0 b	3.48 ± 1 ab	2.38 ± 0.5 ab	31.18 ± 2.0 c	5.68 ± 0.9 d	2.58 ± 0.6 d	7.60 ± 0.6 d	9.5 ± 1.0 a	0.28 ± 0.1 b	0.12 ± 0.01 b	0.04 ± 0.047 a	0.02 ± 0.001 a	0.0021 ± 0.0001 b	377.5 ± 0.001 bc	37.6 ± 0.001 d	2.31 ± 0.001 d	0.45 ± 0.001 b	0.11 ± 0.001 b	0.0119 ± 0.001 a
F	22.485	12.49	5.914	3.932	84.64	12.34	10.12	37.96	974.1	2030	68.0	0.66	65.02	268.4	10,340	244.12	12,854	80.97	90.99	2.42
p	2 × 10−6	6 × 10−5	3 × 10−3	1.6 × 10−2	3 × 10−10	6 × 10−5	2 × 10−3	7 × 10−8	1 × 10−17	9 × 10−20	1 × 10−9	6.8 × 10−1	2 × 10−9	1.2 × 10−13	1 × 10−24	2.41 × 10−13	2 × 10−25	4 × 10−10	2 × 10−10	8.1 × 10−2

reports the concentration of the main macro- and micro-nutrients in spring barley roots and shoots cultivated in the different soil treatments. It can be clearly seen that all biochar amendments increased P and K in the shoots of spring barley compared to un-amended soil, which is mainly due to the gradual release of nutrients and their uptake by spring barley plantlets. This trend was significantly confirmed based on statistical analysis, and mainly F and p values (Table 4). A similar result was obtained by Deinert et al. [17], who demonstrated that the application of a chicken litter-derived biochar produced at 600 °C promoted the P and K content in barley shoots, respectively, by 4.8 and 4.4 times higher than the control. Moreover, it appears that either the raw or post-modified biochars produced at 400 °C had more positive impact on P, K, Ca, and Mg uptake by the shoot/root parts than the PM-Bs generated at 600 °C. This is attributed to the fact that biochars produced at lower temperatures have higher cation exchange capacities (CEC) and organic carbon content, which promotes microbial activity and therefore release of nutrients and their subsequent uptake by plant roots. In this context, Subedi et al. [25] showed that a biochar produced at 400 °C from poultry litter had more positive impact on microbial activity, nutrient release and ryegrass growth compared to the same biochar produced at 600 °C. In addition, biochar amendments enhanced the availability and assimilation of Si, Fe, Mn, and Zn in spring barley roots, in contrast to Cu and As, which did not vary significantly at p ≤ 0.05 in general. This suggests that the immobilization of As and Cu by the biochars is advantageous as it reduces toxicity risk in the food chain [27]. Indeed, biochar application facilitates the formation of Fe/Mn plaques on the surface of plant roots, which act as active zones for the recovery and immobilization of cadmium in rice roots [61]. This process may not only mitigate arsenic mobility, but also enhances the sequestration of other toxic heavy metals, such as Cd and Pb, reducing their bioavailability [66].

Pyrolysis temperature played a critical role in micronutrient bioavailability in the roots of spring barley plants (Table 4). Accordingly, the PM-B-400 retains higher levels of labile organic compounds and functional groups. However, PM-B-600 exhibited greater thermal stability, a larger surface area, and higher adsorption capacities, making it more effective in immobilizing and stabilizing micronutrients. When modified with Fe and Mn, these biochars further enhanced nutrient availability and the stabilization of toxic elements like arsenic. The use of a higher pyrolysis temperature tends to promote stronger binding of As to Fe/Mn oxides, reducing its mobility and toxicity while also improving the retention of essential micronutrients like Fe, Mn, Cu, and Zn. This temperature-dependent behavior highlights the importance of optimizing pyrolysis conditions to tailor biochar for the improvement of plant growth and the reduction of environmental risks. Similar results were reported by Masocha and Diknya [67], who found that a biochar produced from poultry litter at 350 °C released more nutrients compared to biochar generated at 600 °C, and consequently allowed for better growth of Jatropha curcas plants.

3.5. Correlation Analysis and Principal Component Analysis Application

For a tailored interpretation of the obtained data, a correlation analysis (

Table 5. Pearson’s correlation matrix analyzing agricultural soil, PM-B-400, PM-B-400-Fe, PM-B-400-Mn, PM-B-600, PM-B-600-Fe, and PM-B-600-Mn treatments and different studied parameters.

Treatments	Control Assay	PM-B-400	PM-B-400-Fe	PM-B-400-Mn	PM-B-600	PM-B-600-Fe	PM-B-600-Mn
Parameters
SL	−0.704	0.345	0.417	0.402	−0.273	−0.057	−0.129
RL	−0.559	−0.552	0.339	−0.165	0.160	0.421	0.356	1.000
SDW	−0.700	0.461	0.346	0.283	−0.100	−0.190	−0.100	0.8
RDW	−0.647	−0.255	0.358	0.254	0.309	0.119	−0.138	0.600
S P	−0.539	0.734	−0.062	0.408	0.000	−0.363	−0.179	0.500
S K	0.197	−0.426	−0.327	0.501	−0.011	−0.145	0.211	0.400
S Ca	0.019	0.149	0.896	−0.121	−0.447	−0.208	−0.288	0.300
S Mg	0.129	0.455	0.695	−0.160	−0.304	−0.475	−0.341	0.200
S Si	0.998	−0.203	−0.176	−0.170	−0.173	−0.122	−0.155	0.100
S Fe	0.997	−0.145	−0.125	−0.195	−0.217	−0.145	−0.170	0.000
S Mn	0.482	−0.165	−0.264	0.492	−0.476	−0.439	0.370	−0.100
S Cu	0.014	−0.008	0.068	−0.183	−0.192	−0.113	0.413	−0.200
S Zn	−0.324	0.820	0.161	0.209	−0.155	−0.392	−0.320	−0.300
R P	−0.837	0.483	−0.208	0.197	0.263	−0.144	0.246	−0.400
R K	−0.957	0.254	−0.043	0.261	0.212	0.006	0.268	−0.500
R Ca	−0.768	0.484	−0.058	0.229	0.255	−0.397	0.255	−0.600
R Mg	−0.980	0.198	0.143	0.204	0.165	−0.008	0.279	−0.700
R Si	−0.995	0.239	0.089	0.166	0.149	0.190	0.163	−0.8
R Fe	−0.681	−0.065	0.014	0.679	−0.013	0.037	0.028	−0.900
R Mn	−0.723	−0.207	−0.164	0.482	0.100	−0.060	0.573	−1.000
R Cu	−0.125	−0.173	−0.169	−0.174	−0.175	−0.168	0.985
R Zn	−0.925	0.388	−0.081	0.242	0.156	0.013	0.207

Positive correlations are presented in blue and negative correlations in red. Color intensity is proportional to the correlation coefficient value. Values in bold represent the statistically significant correlations at 0.05. SL: shoot length; RL: root length; SDW: shoot dry weight; RDW: root dry weight; S PC: shoot phosphorus content; S KC: shoot potassium content; S Ca C: shoot calcium content; S Mg C: shoot magnesium content; S Si C: shoot silicon content; S Fe C: shoot iron content; S Mn C: shoot manganese content; S Cu C: shoot copper content; S Zn C: shoot zinc content; R PC: root phosphorus content; R KC: root potassium content; R Ca C: root calcium content; R Mg C: root magnesium content; R Si C: root silicon content; R Fe C: root iron content; R Mn C: root manganese content; R Cu C: root copper content; and R Zn C: root zinc content.

) and Principal Component Analysis (PCA) were performed for the macro- and micronutrient content in shoots and roots of spring barley. Figure 5 shows that the first and second component (F1 and F2) accounts for approximately 69% of the total variability (F1:50.9% and F2: 18.5%). Indeed, the control assay is negatively correlated with plant growth-related traits, namely, length, weight of shoot and root parts, and mineral composition. Moreover, PCA confirms the positive effect of the agricultural soil amendment with biochars generated at 400 °C (raw and post-modified ones) on spring barley growth (Figure 5 and Table 4). In addition, treatments using biochars generated at 600 °C were negatively correlated with most of the parameters (Figure 5 and Table 5). The PM-B-400 had the highest positive correlation with the shoot dry weights, shoot and root phosphorus content, shoot zinc content, root calcium content, and root length (Figure 5 and Table 5). Likewise, for PM-B-400-Fe, positive correlations were found for Ca, S, and Mg contents (Figure 5 and Table 4). Moreover, potassium, iron, and manganese content in roots, as well as manganese content in shoots, were significantly correlated when using PM-B-400-Mn. For the correlation analysis presented in Table 5, N was set to 7, corresponding to the seven treatment groups (agricultural soil, PM-B-400, PM-B-400-Fe, PM-B-400-Mn, PM-B-600, PM-B-600-Fe, and PM-B-600-Mn), and the treatment means were used for calculating Pearson’s correlation coefficients.

Lastly, the obtained results from the PCA and correlation analyses demonstrate a good concordance with the findings of our trait-by-trait analysis, highlighting the consistency of our approach in evaluating the data. Comparable results were reported for various studies dealing with agricultural soil amendments with livestock-derived biochars [68,69,70]. For instance, Gavili et al. [69] showed that both the soil moisture and dose of cattle manure-derived biochar impact the content of iron, zinc, manganese, calcium, magnesium, phosphorus, and nitrogen in soil.

4. Conclusions

This investigation was carried out to assess the properties and the agricultural potential of raw and Fe-, Mn-post modified biochars derived from the pyrolysis of poultry manure at 400 and 600 °C. Our results indicated that the biochar produced at lower temperature and subsequently modified with iron (PM-B-400-Fe) had significant physicochemical properties, suggesting its potential use as a promising organic amendment and soil conditioner. Subsequently, it enhanced nutrient availability as well as spring barley growth, and therefore can be used as a sustainable agricultural amendment instead of synthetic and non-environmentally friendly fertilizers. Such an approach contributes to the management of raw poultry wastes and boosts circular economy and sustainability at the farm level. Beyond their agronomic benefits, these biochars align with circular economy principles by transforming poultry manure, a significant environmental challenge, into high-value products within agricultural systems. This dual-purpose approach supports sustainable agricultural waste management while addressing soil fertility and crop productivity. To validate these promising laboratory results, future open field assays are needed to precisely assess their effects on both soil properties and plant growth for a whole cultivation cycle.