Biochar Derived from Agro-Industrial Waste: Applications in Agricultural and Environmental Applications

Abstract

The aim of this study was to investigate in vegetation and laboratory experiments the impact of biochars derived from agro-food industry waste (wheat bran and cherry pits) on selected soil chemical properties, maize yield, and chemical composition, as well as their ability to adsorb zinc and copper, thereby reducing their uptake by plants. The obtained results indicate that biochars produced under the same pyrolysis conditions differ in chemical composition. Both biochars significantly increased the total organic carbon (TOC) and total nitrogen (TN) content in the soil, but they did not affect the availability of nutrients in the soil. The tested biochars did not enhance plant yields or increase the uptake of N, P, K, Mg, and Ca by plants. However, both biochars reduced the uptake of Zn and Cu by plants due to the adsorption of these elements by the biochars. The results obtained in the laboratory experiment indicate that biochar from wheat bran adsorbed approximately 438.5 mM(+) kg⁻¹ and 566.8 mM(+) kg⁻¹, while biochar from cherry pits adsorbed approximately 239.4 mM(+) kg⁻¹ and 303.5 mM(+) kg⁻¹ from the solution. The ion exchange contribution to the adsorption of Zn²⁺ and Cu²⁺ by biochar from wheat bran was approximately 65.8% and 65.0%, respectively. In contrast, for biochar from cherry pits, the contributions were approximately 59.4% and 44.7%, respectively.

1. Introduction

A common feature of European agricultural soils is their susceptibility to degradation due to erosion, soil sealing, soil contamination, a decrease in organic carbon content, soil acidification, and reduced soil fertility [1,2]. In all 27 countries covered by the EU monitoring network—the Land Use and Cover Area Frame Survey (LUCAS)—a decline in organic carbon content in agricultural soils has been observed. The loss of organic carbon in soil reduces its capacity to provide ecosystem services: provisioning services related to plant biomass production, regulatory services associated with carbon sequestration and climate stability, and supporting services [3,4,5]. Considering the significance of organic carbon in shaping the physical and chemical properties of soil [6] and reducing CO₂ emissions into the atmosphere [7], increasing its content in agricultural soils is a priority task. Research conducted in long-term field experiments indicates that the most significant factors influencing soil organic carbon content are organic fertilization, crop rotation, and the intensity of soil tillage [8,9,10]. The subject of recent research is the evaluation of biochar’s suitability for enhancing the physical and chemical properties of soil [11]. Biochar is produced through pyrolysis, a thermal process of organic waste under limited oxygen conditions [12,13]. It is characterized by a high carbon content [14], porosity, specific surface area [15,16], and ash content [14,17]. Therefore, the application of biochar to the soil is expected to improve its overall characteristics. The beneficial effect of biochar on soil organic carbon content stems from its chemical composition, which determines its resistance to physical, chemical, and biological transformations, ensuring its persistence in the soil environment for several centuries [18,19]. As a result of the desired changes in soil properties induced by biochar application, an increase in crop yields is expected. Jiang et al. [20] analyzed data from 367 studies and demonstrated that biochar leads to an approximately 8–22% increase in corn yield in the first four years after application, with this increase reaching around 22% in subsequent years. Naeem et al. [21] achieved an even greater increase in corn yields (up to 26%), an increase in relative plant growth, and plant protein content as a result of biochar application to the clay soil. In these studies, a lower dose of biochar resulted in a higher yield increase. However, studies conducted on sandy soils with low yield potential indicate that the increase in crop yields resulting from biochar application is minimal [22].

Another aspect of biochar’s agricultural use, widely explored in research, is its ability to adsorb contaminants [23,24]. Various mechanisms of heavy metal adsorption on biochar are well understood [25], and their effectiveness depends on the properties of biochar [26,27], the type of heavy metal ions [28], and soil characteristics [29]. Studies on the sorption of heavy metals by biochar reveal that different mechanisms are responsible for the sorption of various metals. For instance, Li et al. [30] found that complexation is a significant or dominant process for most metals. Additionally, the sorption is influenced by electrostatic interactions; chromium (Cr) sorption involves both electrostatic interactions and reduction, while cadmium (Cd) and lead (Pb) sorption occurs through cation exchange and precipitation. In contrast, mercury (Hg) sorption primarily involves reduction. Furthermore, Rodriguez-Villa et al. [31] reported that the sorption capacity of biochar for copper (Cu) and zinc (Zn) relies on the electrostatic interaction between the positively charged cations of these metals and the delocalized pi-electrons on the surface of the biochar. Therefore, the suitability of different biochars for reducing the transfer of heavy metals from soil to the human food chain may vary and should be verified on a case-by-case basis.

The uncertainties in the literature regarding the tangible effects of biochar application on sandy soils have led to a key question: what justifies the series of actions and investments, including emission-intensive pyrolysis, aimed at applying biochar? The authors of this study hypothesized that biochar produced under identical pyrolysis conditions from different plant-based feedstocks (cherry pits and wheat bran) exhibits (i) different chemical properties, (ii) varying effects on selected properties of sandy soil, crop yield, and plant chemical composition, and (iii) different capacities for heavy metal adsorption. These research hypotheses were tested in two experiments: a vegetation experiment and a laboratory experiment. The vegetation experiment aimed to assess the impact of different biochars on crop yield, nutrient uptake (including Zn and Cu) by plants, and soil properties. The laboratory experiment aimed to determine the extent of Cu and Zn adsorption from solution by different biochars and to identify the primary adsorption mechanism for these elements in biochar. Copper (Cu²⁺) and zinc (Zn²⁺) were selected for the sorption experiment based on their environmental relevance, particularly in soils impacted by livestock farming, fertilization, and agrochemical use.

2. Materials and Methods

2.1. Production and Evaluation of Biochar Properties

For this study, biochar was produced from two agricultural and food industry by-products, wheat bran (BIO1) and cherry pits (BIO2), using the Slow Thermal Biomass Refining (STBR)—InnEco Method, developed by InnEco (Kielce, Poland). In this method, the biomass temperature in the pyrolysis chamber was gradually increased by 20 °C increments until reaching 400 °C. At each stage of the process, different fractions and quantities of liquid products were obtained. The transition to the next heating cycle occurred after the pressure stabilized at a level similar to atmospheric pressure and the release of liquid substances had ceased. The total duration of the pyrolysis process was 90 min.

In this study, the following physical and chemical properties of BIO1 and BIO2 were measured: bulk density, determined by the gravimetric method; pH in water (pH meter Schott, Mainz, Germany); total nitrogen (TN) and organic carbon content (TOC), analyzed by the combustion method (Vario MacroCube, Elementar, Langenselbold, Germany); and calcium, magnesium, potassium, sodium, zinc, and copper content, determined using atomic spectrometry (Thermo Scientific iCE 3000 Series, AA Spectrometer, Cambridge, UK) after sample mineralization in HNO₃ (Velp Scientifica, DK 20 Heating Digester, Usmate (MB), Italy).

2.2. Vegetation Experiment

The vegetation experiment with biochar was conducted at the Experimental Station of the Institute of Agriculture in Skierniewice (51°94′41″ N, 20°16′74″ E, altitude 123.0 m), Central Poland. The Experimental Station hosts a microplot experiment. A microplot consists of a stoneware pot, 1.2 m in length and 0.4 m in diameter, embedded in the ground and filled with soil while maintaining its natural horizontal structure. The pots were filled with soil classified according to the International Soil Classification System [32] as Albic Gleyic Luvisols (EndoLoamic Ochric). The zinc and copper content in the topsoil layer (0–30 cm) before the experiment was 296.5 ± 13.85 mg Zn and 57.3 ± 6.33 mg Cu kg⁻¹ soil (in aqua regia), and the soil pH was 4.7.

BIO1 and BIO2 were applied to the soil at a dose of 250 g C per microplot, equivalent to 377.1 and 336.0 g per microplot (corresponding to 30.1 and 26.8 t ha⁻¹ BIO1 and BIO2, respectively). Considering the chemical composition of the biochars (

Table 1. Chemical properties of BIO1 and BIO2.

Biochar	pHH2O	TOC	TN	P	K	Ca	Mg	Zn	Cu
		%	g kg−1					mg kg−1
BIO1	7.97	66.31 ± 0.9	43.45 ± 1.38	2.41 ± 0.09	3.87 ± 1.15	1.31 ± 0.63	6.11 ± 0.28	265.72 ± 29.26	16.60 ± 0.59
BIO2	7.54	74.40 ± 2.5	23.59 ± 0.27	1.70 ± 0.10	3.58 ± 0.20	6.00 ± 1.70	1.74 ± 0.14	66.53 ± 28.74	21.80 ± 1.17

Mean value ± standard deviation (SD).

), the amounts of N, P, K, Ca, and Mg introduced with BIO1 and BIO2 were 16.4 and 7.9 g N, 0.9 and 0.6 g P, 1.5 and 1.2 g K, 0.5 and 2.0 g K, and 2.3 and 0.5 g Mg per microplot. The results obtained from the BIO1- and BIO2-treated microplots were compared to those from the control microplot. The experiment was conducted in three replications in 2022. On 15 May, 10 maize seeds were sown per microplot. During emergence, the number of plants was reduced to 3 in each microplot. At the 2-leaf stage (12 BBCH), 2.0 g N was applied to all microplots in the form of an ammonium nitrate solution. During emergence, the number of plants was reduced to 3 in each microplot. At the 2-leaf stage (12 BBCH), 2.0 g N was applied to all microplots in the form of an ammonium nitrate solution. The plants were harvested at the 59 BBCH stage, characterized by the full emergence of the tassels in maize. This stage holds significant importance in cereal crops. At this stage, cereals are transitioning from vegetative to reproductive growth, starting with pollination, which is essential for yield. During the 59 BBCH, cereals have high nutrient and water demands to develop reproductive structures. The aboveground parts of cultivated plants were collected from each microplot. Plants collected from each microplot were dried at 105 °C, weighed, and homogenized.

After the experiment ended, soil samples were collected from each microplot. The samples were analyzed for total organic carbon (TOC) and total nitrogen (TN) content using a combustion method (Vario MacroCube, Elementar, Langenselbold, Germany); plant-available phosphorus and potassium were determined by the Egner DL method (measured with a Genesys 10 UV-VIS and a Thermo Scientific iCE 3000 Series, AA Spectrometer, Cambridge, UK, respectively); and plant-available magnesium was determined by the Schachtschabel method at a 10:1 extractant (0.0125 M CaCl₂)/soil ratio (measured with a Thermo Scientific iCE 3000 Series, AA Spectrometer, Cambridge, UK); soil pH was measured in 1M KCl at an extractant/soil ratio of 1:2,5 (pHmeter Schott, Mainz, Germany).

In the plant samples collected from each microplot, the TN content was measured (Vapodest VAP 30 model, Gerhardt, Bonn, Germany), and after sample mineralization in HNO₃ (Velp Scientifica, DK 20 Heating Digester, Usmate (MB), Italy), the content of potassium, calcium, magnesium, zinc, and copper (Thermo Scientific iCE 3000 Series, AA Spectrometer, Cambridge, UK), as well as phosphorus, was measured by the vanadomolybdophosphoric method (Genesys 10 UV-VIS). Based on the obtained results, the uptake (U) of zinc and copper by plants was calculated according to Formula (1):

U = Y × Nc

(1)

where

Y is a crop yield,

and Nc is nutrient content.

Based on the differences in zinc and copper accumulation in plants grown in the control microplot and those treated with BIO1 and BIO2, the adsorption capacity of these heavy metals by the tested biochars was determined according to Formula (2):

A_Me = ((U_{Me cont} × eq(+)) − (U_{Me BIO} × eq(+))/D_BIO × 1000

(2)

where

A_Me is the adsorption of the heavy metal on biochar [mq(+) kg⁻¹],

U_{Me cont} is the uptake of the heavy metal by plants in the control microplot,

U_{Me BIO} is the uptake of heavy metal by plants in the biochar-treated microplot,

eq(+) is the conversion factor expressing the ratio of the molar mass to the valency of the heavy metal ion,

D_BIO is the biochar dose applied to the microplot, and 1000 is the conversion factor for 1 kg of biochar.

2.3. Laboratory Experiment

In the laboratory studies on the ability of biochars to adsorb copper and zinc from aqueous solutions of their salts, 1.00 g of each type of biochar was added to plastic containers. Aqueous solutions of CuSO₄·5H₂O and ZnSO₄·7H₂O were prepared, containing 500 mg of Cu²⁺ per dm³ (15.74 mM(+) dm⁻³) and 500 mg of Zn²⁺ per dm³ (15.29 mM(+) dm⁻³), respectively. A quantity of 100 cm³ of copper sulfate solution or 100 cm³ of zinc sulfate solution was added to the biochar sample containers. The amount of elementary charge accumulated in the extraction solution in the form of zinc and copper cations acting on 1 kg of biochar was 1529.52 mM(+) and 1529.52 mM(+) kg⁻¹, respectively. After filling the containers with the solutions, they were sealed and maintained at room temperature for 7 days. Although the equilibrium point of heavy metal adsorption on biochar is usually reached after 24 h of extraction, and 77–83% of the final adsorption occurs within the first 2 h of extraction [29], in our study, the extraction time was set to 7 days due to the possibility of delayed precipitation of insoluble heavy metal compounds, identified as a mechanism of their complexation by biochar [27,33,34]. After 7 days of extraction, the chemical composition of the extraction solutions from both biochars was analyzed. The contents of calcium, magnesium, potassium, and sodium were measured in all solutions, and, additionally, sodium was measured in all solutions. Additionally, on the objects with copper sulphate solution, the copper content was measured, while on the objects with zinc sulphate solution, the zinc content was measured. The analysis was performed using atomic absorption spectrometry (Thermo Scientific iCE 3000 Series, AA Spectrometer, Cambridge, UK). The measured element’s concentrations in the solution were converted into the amount of elementary charge of metal ions involved in the exchange process and expressed per 1 kg of biochar.

Based on the obtained results, the ability of the analyzed biochars to adsorb copper and zinc ions was determined, along with the contribution of ion exchange in the adsorption process of these elements by different biochars. The total heavy metal adsorption capacity of the biochar was equivalent to the decrease in the element concentration in the solution after 7 days of extraction. In contrast, the contribution of ion exchange (CIE) in metal adsorption on biochar was calculated as the ratio of the total elementary charge of calcium, magnesium, potassium, and sodium cations appearing in the solution after 7 days of biochar extraction (Δ_7d mM⁺ kg⁻¹(Σ_Ca,Mg,K,Na)) to the elementary charge of zinc or copper cations removed from the solution after 7 days of biochar extraction (Δ_7d mM⁺ kg⁻¹ (HM_{Zn or Cu})), according to Formula (3):

CIE = [Δ_7d mM⁺ kg⁻¹(Σ_Ca,Mg,K,Na)]/[Δ_7d mM⁺ kg⁻¹ (HM_{Zn or Cu})] × 100%

(3)

2.4. Statistical Analysis

Basic statistics, such as the mean and standard deviation, were calculated using Statistica PL 13.3 software (Tulsa, OK, USA). Additionally, the results underwent a one-way analysis of variance (ANOVA), and confidence intervals for the means were determined using Tukey’s test, with a significance level of p < 0.05.

3. Results

3.1. Physical and Chemical Properties of Biochar

Biochar BIO1 consisted of loose fragments of dry, carbonized material of varying sizes, generally not exceeding 1 mm in diameter (Figure 1). In contrast, BIO2 was a natural granulate that retained the shape and size of the plant substrate—cherry pits—and was relatively resistant to crumbling. The bulk density of BIO2 averaged 376.6 kg m⁻³, which was approximately 37% higher than that of BIO1, averaging 237.07 kg m⁻³.

The analyzed biochars exhibited an alkaline reaction (Table 1). The total organic carbon (TOC) content in BIO2 was approximately 12.2% higher, and the total nitrogen (TN) content was 84.2% higher than in BIO1. The magnesium content in BIO1 was about 3.5 times higher, while the potassium content was only about 8.0% higher than in BIO2. In contrast, the calcium content in BIO2 was nearly 4.6 times higher than in BIO1. Despite these differences, the total content of key nutrients (P, K, Ca, Mg) was similar in BIO1 and BIO2 (respectively, 13.7 and 13.0 mg kg⁻¹). The zinc content in both biochars was higher than the copper content. The zinc content in BIO1 was approximately four times higher than in BIO2, whereas BIO2 contained about 31.3% more copper than BIO1.

3.2. Vegetation Experiment

The application of BIO1 and BIO2 affected soil properties in different ways (

Table 2. Effect of BIO1 and BIO2 application on soil chemical properties.

Treatment	pHKCl	TOC	TN	P	K	Mg
		g kg−1		mg kg−1
Control	4.68	10.02 a ± 0.22	1.10 a ± 0.02	30.99 a ± 0.23	115.47 a ± 2.19	24.29 a ± 1.23
BIO1	4.92	14.29 b ± 0.41	1.30 c ± 0.03	31.79 a ± 0.67	120.09 a ± 2.27	24.10 a ± 0.23
BIO2	4.97	14.19 b ± 0.44	1.19 b ± 0.03	31.49 a ± 0.47	119.38 a ± 1.56	23.78 a ± 0.73

Mean value ± standard deviation (SD). Means marked with different letters in the columns differ significantly at p < 0.05.

). The soil remained acidic in both the biochar-treated plots and the control plot; however, a slight alkalizing effect of the biochars on the soil was observed. The TOC content in the soil of the BIO1 and BIO2 plots was similar and significantly higher (by approximately 42.6 and 41.6%, respectively) than in the control plot. The application of BIO1 significantly increased the TN content in the soil, compared to both the control plot and the BIO2 plot, by approximately 18.2% and 9.2%, respectively. However, no statistically significant effect of BIO1 or BIO2 on the content of available nutrient forms (P, K, and Mg) in the soil was demonstrated (Table 2).

Table 3. Yield and nutrient content in corn.

Treatment	Yields	N	P	K	Mg	Ca	Zn	Cu
	g d.m. microplot−1	g kg−1					mg kg−1
Control	133.18 a ± 13.37	9.30 a ± 0.34	3.00 a ± 0.24	17.90 a ± 0.96	1.81 a ± 0.23	4.78 a ± 0.38	143.11 b ± 8.26	9.78 b ± 1.07
BIO1	133.53 a ± 10.08	9.31 a ± 0.85	2.81 a ± 0.25	16.59 a ± 1.66	1.81 a ± 0.21	4.60 a ± 0.28	114.81 a ± 8.56	7.00 a ± 0.29
BIO2	138.21 a ± 13.38	9.51 a ± 0.28	2.90 a ± 0.56	17.80 a ± 1.26	1.91 a ± 0.13	4.60 a ± 0.41	117.62 a ± 11.45	7.45 a ± 0.41

Mean value ± standard deviation (SD). Means marked with different letters in the columns differ significantly at p < 0.05.

presents the yields and chemical composition of plants. The plant yield obtained in the control plot, as well as in the BIO1 and BIO2 plots, did not differ significantly. Similarly, no significant effect of biochar application on the content of N, P, K, Mg, and Ca in plants was observed. However, the application of BIO1 and BIO2 significantly reduced the Zn and Cu content in plants. On average, the zinc (Zn) and copper (Cu) content in plants grown under the control treatment was 24.6% and 39.7% higher, respectively, compared to plants fertilized with BIO1. The differences in Zn and Cu content between plants from the control and BIO2 treatments were 21.7% and 31.3%, respectively (Table 3).

Soil application of BIO1 and BIO2 had no significant effect on the uptake of N, P, K, Mg, and Ca by the plants (

Table 4. Uptake of macro- and micronutrients by plants.

Treatment	N	P	K	Mg	Ca	Zn	Cu
	g microplot−1					mg microplot−1
Control	1.24 a ± 0.08	0.40 a ± 0.07	2.38 a ± 0.24	0.24 a ± 0.03	0.64 a ± 0.10	18.99 b ± 0.83	1.29 b ± 0.02
BIO1	1.25 a ± 0.20	0.37 a ± 0.03	2.21 a ± 0.13	0.24 a ± 0.04	0.61 a ± 0.02	15.28 a ± 0.69	0.94 a ± 0.11
BIO2	1.32 a ± 0.17	0.40 a ± 0.12	2.46 a ± 0.26	0.26 a ± 0.04	0.64 a ± 0.12	16.16 a ± 0.56	1.03 a ± 0.16

Mean value ± standard deviation (SD). Means marked with different letters in the columns differ significantly at p < 0.05.

). The uptake of Zn and Cu by plants in the control plot was approximately 17.5–24.28% and 25.2–37.2% significantly higher, respectively, compared to the plots treated with BIO1 and BIO2.

The reduced uptake of Zn and Cu by plants from the soil treated with BIO1 and BIO2 can be attributed to the adsorption of these metal ions on the studied biochars. Therefore, Figure 2 shows the differences in the amount of elemental charge adsorption of Zn²⁺ and Cu²⁺ ions, expressed per 1 kg of BIO1 and BIO2 applied to the soil. The amount of positive charge from Zn adsorbed by BIO1 and BIO2 was more than ten times greater than that from Cu. This was most likely due to the Zn content in the soil (296.5 mg Zn kg⁻¹) being approximately 5.2 times higher than the Cu content (57.3 mg Cu kg⁻¹), as well as the different activity levels of both metals in the soil. The results show that the ability of BIO1 to adsorb Zn²⁺ ions was approximately 16.7% higher, and for Cu²⁺ ions about 22.2% higher, than BIO2.

3.3. Laboratory Experiment

Changes in Zn²⁺, Cu²⁺, Ca²⁺, Mg²⁺, K⁺, and Na⁺ Content in the Extraction Solution

Table 5. Amount of ion charge remaining in solution at the beginning of the experiment and after 7 days of BIO1 and BIO2 extraction (per 1 kg of biochar).

Biochar	Treatment		Concentration on the Seventh Day of the Experiment
	Concentration at the Beginning of the Experiment		Zn2+	Cu2+	Ca2+	Mg2+	K+	Na+	Sum of Base
	mM(+) kg−1
BIO1	ZnSO4·7H2O	1529.52 (Zn2+)	1091.06 ± 15.71	-	11.35 a ± 1.64	20.78 b ± 0.56	239.77 b ± 1.11	45.61 b ± 2.12	317.51 b ± 3.07
BIO2			1290.15 ± 5.77	-	21.92 b ± 2.76	16.77 a ± 0.43	96.76 a ± 6.04	32.98 a ± 1.74	168.43 a ± 6.80
BIO1	CuSO4·5H2O	1573.66 (Cu2+)	-	1006.81 ± 58.40	25.63 B ± 3.42	28.00 B ± 1.88	257.03 B ± 6.10	57.92 A ± 7.64	368.58 B ± 26.79
BIO2			-	1270.13 ± 7.05	15.10 A ± 1.06	2.24 A ± 0.17	68.44 A ± 9.81	49.81 A ± 5.98	135.59 A ± 14.41

Mean value ± standard deviation (SD). Means marked with different letters (separately for ZnSO₄·7H₂O (lowercase letters) and CuSO₄·5H₂O (uppercase letters)) in the columns differ significantly at p < 0.05.

presents the amount of elemental charge from zinc, copper, calcium, magnesium, potassium, and sodium cations remaining in the solutions after 7 days of extraction with ZnSO₄·7H₂O and CuSO₄·5H₂O solutions for BIO1 and BIO2. After 7 days of extraction, the amount of elemental charge from Zn²⁺ and Cu²⁺ remaining in the solutions with BIO2 was approximately 18.2% and 26.2% higher, respectively, compared to the analogous solutions with BIO1. The difference between the initial and final amount of charge from the ions of both metals in the solution indicates that BIO1 adsorbed approximately 438.46 mM(+) kg⁻¹ (28.7% of the charge from Zn²⁺ present in the solution) and approximately 566.85 mM(+) kg⁻¹ (36.0% of the charge from Cu²⁺ present in the solution), while BIO2 adsorbed only about 239.37 mM(+) kg⁻¹ (15.7% of the charge from Zn²⁺ present in the solution) and about 303.53 mM(+) kg⁻¹ (19.3% of the charge from Cu²⁺ present in the solution) from the solution. The total amount of elemental base charge released into the solutions from BIO1 indicates that the contribution of ion exchange in the adsorption process of Zn²⁺ and Cu²⁺ by BIO1 was approximately 65.8% and 65.0%, respectively. In contrast, the ion exchange contribution in the adsorption of Zn²⁺ by BIO2 was approximately 59.4%, and for Cu²⁺, it was approximately 44.7%. The contribution of basic cations in the ion exchange process depended on the type of biochar. The series of base contributions from BIO1 in ion exchange was as follows: K⁺ > Na⁺ > Mg²⁺ > Ca²⁺, and in the case of BIO2: K⁺ > Na⁺ > Ca²⁺ > Mg²⁺.

4. Discussion

4.1. Impact of Biochar on Soil Properties

Both tested biochars exhibited an alkaline reaction (Table 1), and their application resulted in a slight decrease in soil acidity (Table 2). This was due to the similar total alkali content of BIO1 and BIO2, although the Ca and Mg content of the biochars tested differed. However, the dose of alkali (Ca and Mg) introduced from BIO1 and BIO2 was too low relative to the accumulated hydronium cation in the soil to change the soil reaction class relative to the control. Soil acidification could, therefore, have been the reason for the limited effect of biochars on crop yields (Table 3). In contrast, BIO1 and BIO2 significantly increased soil TOC and TN content (Table 2). This corresponds to numerous research results [35,36]. Yang et al. [35] report that biochar mineralization is more intense in sandy soil than in clay soil. In our study, however, the degree of biochar mineralization was probably very low. This is evidenced by the limited distribution of nutrients brought in from BIO1 and BIO2, resulting in differences in plant yields and nutrient uptake (Table 3 and Table 4). As a result, the accumulation of more TOC and TN in the soil did not guarantee an increase in plant yields and, therefore, did not affect the level of supply service provided by the soil. This corresponds to the results of Sosulski et al. [37], who found lower rye grain and straw yields on an annual manure-fertilized site, where the soil total organic carbon (TOC) content was approximately twice as high as on a mineral-fertilized site. According to Gross et al. [36], the rapid increase in soil TOC content under the influence of biochar is due to the resistance of the aromatic structure to mineralization. However, the findings of these authors indicate that the effect of TOC accumulation in the soil disappears 4–9 years after biochar application. Therefore, the long-term effect of biochar on improving the soil’s provision of the ecosystem regulatory service of carbon sequestration and climate stability remains debatable. In light of the results of research conducted on long-term experiments, the supply of organic matter to the soil in the form of manure and faba bean crop residues may have better effects on improving soil function and providing supply, regulatory, and support services shaping soil health than biochar [38]. It appears that a long-term increase in the soil’s total organic carbon (TOC) content under biochar is possible under conservation tillage [39].

4.2. Effect of Biochar on Yield and Macronutrient Content of Plants

There are reports in the literature about the beneficial effects of biochar on crop yields and chemical composition [20]. It is well known that nitrogen inputs, in both mineral and organic forms, increase plant yields and soil microbial activity [38]. The results of our study indicate that, despite a rather high TN content, BIO1 and BIO2 did not increase plant yields compared to the control (Table 3). Although the TN content and nitrogen application rate of BIO1 were approximately 84.2% higher and more than twice that of BIO2, respectively (Table 1), the plant yield, nitrogen content, and plant uptake (Table 3 and Table 4) on both biochar-treated sites were similar. This means that in the first year after biochar application to the soil, regardless of the amount of nitrogen input, there is no agriculturally relevant fertilizing effect. Similar conclusions can be drawn regarding the yield-forming effect of phosphorus in the biochars tested. Although the content of this nutrient in BIO1 was approximately 41.3% higher (Table 1), and the dose was about 58.5% higher than in BIO2, plant yield, P content, and plant uptake did not differ significantly (Table 3 and Table 4). Potassium is a nutrient of great importance in the production of plant vegetative parts. It occurs in plants as K⁺ cations [40]. The potassium content of BIO1 and BIO2 was similar (Table 3). With the BIO1 application rate, an amount of potassium was introduced into the soil corresponding to 116.5 kg ha⁻¹, and with the BIO2 application rate, approximately 96.0 kg K ha⁻¹. Admittedly, these amounts were insufficient in relation to the nutritional needs of high-yielding maize [41,42], but the differences in the amount of potassium introduced may have contributed to the variation in plant yields. The lack of differences in plant yields, potassium content, and uptake observed in our study (Table 3 and Table 4) can only be attributed to the influence of another minimum limiting factor on plant responses (e.g., insufficient nitrogen availability to plants). The limiting factor of potassium may also be caused by K–Ca and K–Mg antagonism resulting from insufficient calcium and magnesium in the soil [41,43]. Different amounts of Ca and Mg were introduced from BIO1 and BIO2 (Table 1). However, the molar ratio of K:(Ca + Mg) in both biochars (1:2.89 and 1:2.42, respectively) suggests that the distribution of calcium and magnesium in the biochars should not limit crop yields. Meanwhile, the content and uptake of Ca and Mg by plants growing on acidified sandy soil, whether treated or untreated with biochar, were similar (Table 3 and Table 4). This indicates that biochar was not a good source of these nutrients for plants.

It is noteworthy that the Zn content in BIO1 was nearly four times higher than in BIO2. In contrast, the Cu content in BIO2 was more than 31% higher than in BIO1. Despite this, the application of biochars with such different Zn and Cu contents did not differentiate between the content or uptake of these elements by plants at the BIO1 and BIO2 plots. In contrast, compared to the control, the content and uptake of Zn and Cu by plants on the objects with biochars was significantly lower (Table 3 and Table 4). For Zn and Cu uptake by plants growing on the control site, the values were 17.5–24.3% and approximately 25.2% and 37.2% higher than those on the BIO2 and BIO1 sites, respectively. This was most likely due to the adsorption of bioavailable forms of these metals onto the tested biochars in the soil. Biochar can chemically and physically adsorb heavy metals due to its porous, amorphous-crystalline structure [16,44].

4.3. Adsorption of Zn and Cu onto Biochar

As mentioned, the results obtained in the vegetation experiment suggested the possibility of Zn and Cu adsorption on BIO1 and BIO2. The plant adsorption rate values of Zn and Cu on the analyzed biochars, as shown in the vegetation experiment, indicate that the sorption potential of BIO1 was 300.4 and 29.8 mM(+) kg⁻¹ of Zn²⁺ and Cu²⁺, respectively, and that of BIO2 was 257.4 and 24.3 mM(+) kg⁻¹ (Figure 2). Laboratory results estimated the sorption potential of Zn²⁺ and Cu²⁺ ions on BIO1 to be approximately 438.46 mM(+) kg⁻¹ and approximately 566.85 mM(+) kg⁻¹, respectively, while BIO2 was estimated to be approximately 239.37 mM(+) kg⁻¹ and approximately 303.53 mM(+) kg⁻¹ from the extraction solution. Campos and De la Rosa [45] report that the adsorption of heavy metals by biochar is dependent on the concentration of the absorbate. Therefore, the fundamental differences in the amount of adsorbed Cu on the two biochars in the vegetation and laboratory experiment were probably due to the difference in the concentration of bioavailable Cu in the soil and Cu²⁺ in the soil solution. The zinc content in the soil was more than five times higher than that of copper, while the amount of charge from both ions in solution acting on BIO1 and BIO2 in the laboratory experiment was similar (Table 5).

The results obtained in both experiments confirmed the view known from the literature that biochar is more capable of selectively adsorbing Cu than Zn [46]. The differences in the magnitude of Zn and Cu sorption observed between the vegetation and laboratory experiments were attributed to variations in experimental conditions, including soil and solution pH, as well as heavy metal ion concentrations. Soil acidification may have affected the sorption capacity of Zn and Cu on BIO1 and BIO2 differently due to their different chemical properties. Generally, heavy metals are most strongly adsorbed by biochar at a soil pH of 5.5–6.5. In contrast, in a strongly acidic environment, heavy metal adsorption is negligible [29,47]. In the vegetation experiment, despite the demonstrated alkalizing effect of both biochars, the soil reaction was acidic (Table 2).

In both experiments, the adsorption capacity of the metals tested by BIO1 was found to be higher than that of BIO2 (Figure 2, Table 5). This was due to fundamental differences in the properties of the biochars tested. The mechanisms for the adsorption of heavy metals onto biochar are ion exchange, the formation of covalent bonds between adsorbate and adsorbent, and the formation of weak electrostatic interactions [44,48]. Ion exchange is the predominant mechanism of heavy metal adsorption on biochar, characterized by a high content of exchangeable K+, Ca²⁺, and Mg²⁺ [26,34]. Alkaline cations retained on the biochar surface can be exchanged by heavy metal ions [27,49]. The cations are sorbed due to the negative charge of the phenolic, amine, and carboxyl functional groups present in the biochar [50]. Therefore, the sorption potential of the biochar is determined by the amount of these groups on the surface of the carbonate [14,17] and increases with increasing specific surface area and its alkalinization [27,34]. Oxygen functional groups are also involved in the formation of coordination bonds with heavy metals and the formation of ligands [44]. The mechanism of heavy metal complexation plays a crucial role in their adsorption by biochar, characterized primarily by low ash content, and, therefore, biochar formed at lower pyrolysis temperatures [27]. The intensity of the complexation of heavy metals by biochar can change over time [46]. As a result of the adsorption of heavy metals by the biochar, anions (OH⁻, CO₃²⁻ and PO₄³⁻) are released, which, with the metals, yield insoluble hydroxides, carbonates, and phosphates [27,33,34,51]. The biochars we tested differed in their ability to exchange bases (K⁺, Ca²⁺, Mg⁺, Na⁺) (Table 4). The amount of bases exchanged between BIO1 and the ZnSO₄ and CuSO₄ solutions was approximately 88.5%, which is about 2.7 times higher than the amount exchanged between BIO2 and these solutions, respectively. In contrast, despite major differences in the total Ca and Mg content of BIO1 and BIO2, the total alkali content (K, Ca, Mg, Na) of BIO1 and BIO2 (respectively, 11.29 and 11.32 mg kg⁻¹) was similar (Table 1). Therefore, the differences in Zn and Cu sorption on BIO1 and BIO2 observed in our experiments were probably mainly due to the different ion exchange and heavy metal complexation capacities of the two biochars. According to Alsawy et al. [25], approximately 55% of the heavy metal ions adsorbed by the biochar undergo complexation and ion exchange. In contrast, the other sorption mechanisms (precipitation reactions, electrostatic attraction, and cation–π interaction) are less important in the adsorption of heavy metals by biochar. Our results (Table 5) indicate that the ion exchange and complexation activities in the adsorption of Zn²⁺ and Cu²⁺ by BIO1 were about 65.8% and about 65.0%, respectively, and by BIO2, about 59.4% and 44.7%, respectively. The ion exchange capacity could have been the primary reason for the differences in Zn and Cu adsorption capacity by BIO1 and BIO2.

5. Conclusions

This study demonstrated that biochars produced from wheat bran and cherry pits differ significantly in their bulk density and chemical characteristics, influencing their environmental and agricultural performance. Although applying both biochars to sandy soil increased total organic carbon and total nitrogen content, it did not significantly affect the availability of macronutrients (N, P, K, Mg, and Ca) or improve crop yields within the first year. Interestingly, the uptake of zinc and copper by plants was markedly reduced in biochar-amended soils, suggesting strong metal-binding properties of the tested materials. Bran-derived biochar exhibited notably higher sorption capacities for Zn²⁺ and Cu²⁺ (438.46–566.85 mM(+) kg⁻¹) compared to cherry pit biochar (239.37–303.53 mM(+) kg⁻¹). Moreover, ion exchange was identified as a major mechanism of metal retention, accounting for over 65% of sorption in the case of bran biochar. The findings confirm the potential of agro-waste biochars—particularly wheat bran biochar—as effective sorbents for the remediation of soils contaminated with heavy metals such as zinc and copper, offering a promising tool for sustainable soil management and waste valorization.