Effects of Biochar on Soil Properties and Tomato Growth

Abstract

The paper aimed at evaluating the effects of biochar (BC) produced by slow pyrolysis of vine pruning residue on soil physicochemical properties and tomato plant growth. A greenhouse experiment was conducted for 66 days, applying different treatments for 3 soil types, i.e., foliar fertilizer, BC (at a volumetric ratio between BC and soil of 20/80), BC + foliar fertilizer, and no treatments. Strongly alkaline BC (pH = 9.89 ± 0.01) had a significant beneficial effect on the growth performance of tomato plants sown in a strongly acidic soil (pH = 5.40 ± 0.02). The mean values of height, number of leaves, and collar diameter of plants grown in BC-amended soil without foliar treatment were up to 50% higher than those of plants grown in soil with the other treatments. This positive effect of BC on plant growth is due to the changes in the soil properties. The addition of BC led to increased values of electrical conductivity, pH, soluble and available nutrient concentration. Moreover, BC reduced soil bulk density by about 50%, resulting in improved plant root development and thus enhanced water and nutrient uptake. Accordingly, BC derived from vine pruning residues can improve soil quality and tomato plant growth, as well as reduce biomass residues.

1. Introduction

Biochar (BC) is a low cost and environmentally friendly amendment, usually produced by slow pyrolysis of biomass residues [1,2,3,4,5]. Its application to agricultural soils is a promising strategy to improve soil quality, enhance plant growth and production, reduce biomass residues, and mitigate climate change [1,2,3,4,5,6,7,8,9,10,11].

BC can enhance the soil structure, increase its porosity, pH value, electrical conductivity (EC), water-holding capacity (WHC), cation exchange capacity (CEC), nutrient retention capacity, organic carbon concentration, available phosphorus concentration, and total carbon and nitrogen concentration, as well as reduce the soil bulk density (BD), nutrient loss, and bioavailability of heavy metals [1,2,3,4,5,6,7,8,11,12,13,14,15,16,17,18,19,20,21]. Moreover, it can suppress soil pathogens, and improve the growth and activity of beneficial microbial populations as well as the activity of soil enzymes [3,6,14,19,22,23].

These positive effects of BC on the physicochemical and biological properties of the soil enhance the availability of plant nutrients, generally leading to improved crop performance [3,5,24]. Several studies assessed that BC applied with inorganic and organic fertilizers enhanced soil fertility and plant performance [1,3,5,6,11,13,17,19,20,25,26,27]. Increased soil fertility results in lower fertilizer inputs as well as higher crop productivity and therefore additional CO₂ consumption, leading to agronomic, environmental, and economic benefits [16,28]. The effects of BC on the soil-plant system depend on various factors, e.g., physicochemical properties and dosage of BC, type and dosage of fertilizer, plant species, soil characteristics, climate [1,3,4,5,6,8,10,11,16,20,25,26]. The physicochemical properties of BC depend on the type and pretreatment of the feedstock, pyrolysis conditions (heating rate, duration, temperature, type and flow rate of sweeping gas), and post-pyrolysis treatment [1,5,6,20,21,28,29,30,31,32]. BC produced at a higher temperature is generally more alkaline, has higher values of WHC, EC, specific surface area (SSA), fixed carbon and ash concentrations, higher aromaticity, fewer surface functional groups, and lower values of CEC than that obtained at a lower temperature [1,4,6,20]. The agronomic benefits of BC amendment are more obvious in sandy soils, followed by sandy loam, and then silty soils, as well as in acidic rather than neutral and alkaline soils, and in greenhouse rather than in field experiments [6,11,13,16,17,19,20].

Moreover, BC applied as an amendment increases soil carbon sequestration and reduces greenhouse gas (GHG) emissions, contributing to climate change mitigation [1,4,5,6,9,12,15,16,23]. The emissions of GHGs (CO₂, CH₄, and N₂O) can be reduced directly (by improving soil aeration) or indirectly (by diminishing fertilizer inputs and energy supply for irrigation of soils with enhanced WHC) [16].

This paper focuses on the production of BC from Vitis vinifera branches left over from winter pruning and its application as a soil amendment. The effects of the addition of BC on soil physicochemical properties and tomato plant growth were evaluated. Nowadays, vine pruning residue is usually burnt in situ or chopped and incorporated into the soil. On the one hand, its burning causes serious environmental concerns due to the emission of GHGs as well as loss of nutrients and organic matter, which are very valuable for the soil [33]. On the other hand, vine pruning residue incorporated into the soil represents a substrate and shelter for pathogenic fungi living in the soil. Moreover, the C/N ratio of vine pruning residue is higher than 50 [34,35], indicating that it is not suitable for direct use as soil fertilizer without a prior treatment. Accordingly, the pyrolysis of vine pruning residue to obtain BC, bio-oil, and pyrolytic gases could be an effective solution for disposal of this residue. In addition to its application as a soil amendment, BC is useful as a renewable fuel, sorbent of environmental contaminants, and precursor of activated carbon and catalysts, while bio-oil and pyrolytic gases are valuable sources of chemicals and renewable energy [28,34,36].

2. Materials and Methods

2.1. Materials

2.1.1. Production of BC

BC was obtained by slow pyrolysis of vine pruning residue. The vine branches were cut with pruning shears resulting in cylindrical pieces having a mean volume (V) of 2.3 cm³ (0.7 cm diameter and 6 cm length) (Figure 1a). Carbon dioxide (CO₂) (purity >99.9%) was used as a sweeping gas and reactant in the slow pyrolysis process.

2.1.2. Preparation of BC-Amended Soils

Three soil types, i.e., Fluvisol (S1), Chernozem (S2), and Luvisol (S3), classified as medium loam soils [37], were collected from agriculture fields in Romania. Soil samples were taken from a depth of 0–20 cm, according to the methodology described by Mușat et al. [38], aerated for 3 days, and then ground to a diameter ≤2 mm. BC (Figure 1b) was ground with a soil mill to a diameter ≤2 mm and soil-BC mixtures were prepared at a volumetric ratio between BC and soil (S) of 20/80.

2.2. Equipment, Procedures, and Variables

2.2.1. Production of BC

Slow pyrolysis of vine pruning residue was performed at University POLITEHNICA of Bucharest (UPB) in the setup shown in Figure 2. The chopped vine residue was fed into a ceramic cylindrical reactor (1), 27 cm height, 15.5 cm internal diameter (D), and 3.5 cm wall thickness, which was set into an autoclaved cellular concrete (ACC) support (2). The pyrolysis reactor was placed on a precision balance (3), put on a metal support (4). The inner wall of the reactor was heated by a resistor, 8.4 m length and 110 Ω electrical resistance (R). The resistor was powered (220 V) by a transformer (5) (Electrotehnica, Romania), which was powered (380 V) by another transformer (6) (Electrotehnica, Romania).

The sweeping gas (CO₂) from a gas tank (7) was introduced into the reactor through a pipe (8), passed upwards through the fixed bed biomass, and discharged through a collecting pipe (9) along with the volatile products (VPs) obtained during the process, i.e., vapour and non-condensable gases. The mixture of VPs was condensed in a double pipe heat exchanger (10), resulting in a pyrolysis liquid (bio-oil) and VPs. The bio-oil was collected in a triple neck round-bottom flask (11), set into a polyurethane (PU) support (12). The VPs entered a double pipe heat exchanger (13), resulting in a bio-oil, which returned to the collector (11), and pyrolysis gases, which were discharged through an exhaust hood (14). The cold water from a coolant supply/discharge system (15) was used as a cooling fluid in both heat exchangers (10) and (13).

The volumetric flow rate of CO₂, G_V (cm³/s), was measured with a flow meter (16) and the voltage, U (V), with a voltmeter (17). The masses of solid (m) and liquid (m_L) were monitored using the precision balances (3) and (18) (Kern, Germany). The temperature inside the fixed bed of vegetal material was measured using the thermocouples (19) and (20) attached to the thermometers (21) and (22) (TFA Dostmann, Germany). The values of mass and temperature were recorded and processed using a KernSoft/HuaweiMate software/hardware system (23). The experiments of slow pyrolysis were conducted for 1 h at 1 atm.

The power of the resistor, P = U²/R, CO₂ superficial velocity, w = 4G_V/(πD²), and mean volume of vegetal material particles, V, were selected as process independent variables (factors). According to the results obtained in our previous study [28], slow pyrolysis was performed at the following levels of process factors: P = 110 W, w = 1.5 cm/s, and V = 2.3 cm³. The final values (at 1 h) of specific masses of biochar and bio-oil, m_f/m₀ = 0.31 ± 0.01 and m_Lf/m₀ = 0.37 ± 0.01, where m₀ was the initial mass of vine waste, and of mean temperature of fixed bed, t_mf = 517 ± 16 °C, were process dependent variables (responses).

2.2.2. Characterization of Non-Amended Soils, BC Amendment, and BC-Amended Soils

The humidity (HU) was determined gravimetrically at 105 °C, pH was measured using the potentiometric method (solid/liquid ratio of 1/2.5 g/cm³ for non-amended and BC-amended soils and of 1/10 g/cm³ for BC), and electrical conductivity (EC) using the conductometric method (solid/liquid ratio of 1/5 g/cm³ for non-amended and BC-amended soils and of 1/10 g/cm³ for BC). The bulk density (BD) was calculated by dividing the mass of a sample of dried substrate (non-amended soil, BC amendment, and BC-amended soil) by its volume, measured using a graduated glass cylinder. The concentrations of soluble nitrate nitrogen and ammonium nitrogen (N-NO₃ and N-NH₄) were determined at 420 nm (solid/liquid ratio of 1/5 g/cm³) using a CECIL 2041 Spectrophotometer (Buck Scientific, Norwalk, CT, USA). The concentration of soluble phosphorus (P) was determined spectrophotometrically at 720 nm (solid/liquid ratio of 1/5 g/cm³) using a CECIL 2041 Spectrophotometer (Buck Scientific, USA). The concentration of soluble potassium (K) was measured using a Sherwood 410 Flame Photometer (Sherwood Scientific, Cambridge, UK) (solid/liquid ratio of 1/5 g/cm³). The concentrations of available phosphorus and potassium (P-AL and K-AL) were determined using Egner–Riehm–Domingo extraction method (solid/liquid ratio of 1/20 g/cm³, where the liquid phase was a solution of ammonium acetate-lactate (AL)) followed by spectrophotometric and flame photometric methods, respectively, applied to determine P and K. The concentrations of available calcium, magnesium, and sodium (Ca, Mg, and Na) (extractable in ammonium acetate) were determined using an Optima 8300 ICP-OES (PerkinElmer, Waltham, MA, USA) (solid/liquid ratio of 1/50 g/cm³). The total carbon concentration (C) was measured using an EA3100 Elemental Analyser (Eurovector, Pavia, Italy). All measurements were performed in triplicate.

2.2.3. Effects of BC Amendment on Tomato Growth

A greenhouse experiment was conducted for 66 days (15 November 2021–20 January 2022) to assess the effects of BC on tomato plant growth. The plants were cultivated in the greenhouse of the Research Center for Studies of Food Quality and Agricultural Products, part of University of Agronomic Sciences and Veterinary Medicine of Bucharest (USAMV), solely for experimental purposes. Experimental research and greenhouse studies on plants, including the collection of plant material, complied with relevant institutional, national, and international guidelines and legislation. The tomato seeds (Cœur de Bœuf variety) were purchased from the local market.

According to the experimental scheme presented in

Table 1. Experimental scheme for the greenhouse experiment.

No.	Treatment	Code
1	No BC and foliar fertilizer treatments	S
2	BC (volumetric ratio between BC and soil of 20/80)	S + BC
3	Foliar fertilizer (0.2%)	S + F
4	BC + foliar fertilizer (0.2%)	S + BC + F
5	BC + foliar fertilizer (0.1%)	S + BC + F/2

, 5 treatments for each soil type (a total of 15 treatments and 10 replicates per treatment) were used for tomato growth. A commercial foliar fertilizer was applied to the plants every 10 days, 25 days after sowing (a total of 4 foliar treatments), at concentrations of 0.2% (F) and 0.1% (F/2), respectively, where the percentages represent mL fertilizer/100 mL water. The total amounts of nutrients per plant applied with 4 foliar treatments were as follows: 160 μg P, 80 μg N, 22 μg Mg, 8 μg K, 8 μg Fe, 2.4 μg B, 2.2 μg Mn, 2 μg Zn, and 1.2 μg Cu. The foliar fertilizer also contained amino acids, multivitamins, enzymes, growth stimulators (auxin, cytokinin, and giberalin), and other micronutrients.

Non-amended and BC-amended soils (S and S + BC) were placed in seedling trays with 32 cells (a total of 150 cells). The plants were irrigated with tap water taking into account their humidity needs (usually every 2 days). The indoor temperature was measured using the greenhouse compartment sensors, while the outdoor temperature was measured using the greenhouse weather station. All environmental data were collected using PRIVA software. Plant height (H), number of leaves (NL), collar (transition zone between the root and stem) diameter (CD), and root volume (RV), evaluated 66 days after sowing, were selected as dependent variables.

2.3. Statistical Analysis

A single factor analysis of variance (ANOVA) was applied to assess whether the addition of BC had a significant effect (p < 0.05) on soil properties and whether the type of treatment had a significant effect on plant growth parameters. The Pearson correlation coefficient (r) was used to reveal the strength of linear correlations between the physicochemical properties of non-amended soils, BC amendment, and BC-amended soils. A data matrix with 7 rows (number of samples, i.e., 3 non-amended soils, BC amendment, and 3 BC-amended soils) and 14 columns (number of variables, i.e., HU, BD, pH, EC, C, N-NO₃, N-NH₄, P, K, P-AL, K-AL, Ca, Mg, Na) was used in principal component analysis (PCA). Univariate and multivariate analyses were performed using XLSTAT Version 2019.1 (Addinsoft, New York, NY, USA).

3. Results

3.1. Characterization of Non-Amended Soils, the BC Amendment, and BC-Amended Soils

The mean values of physicochemical properties of non-amended soils (S1, S2, and S3), the BC amendment, and BC-amended soils (S1 + BC, S2 + BC, and S3 + BC) are summarized in

Table 2. Mean values of physicochemical properties of soils (S), BC, and BC-amended soils (S + BC).

No.	Property	S1	S2	S3	BC	S1 + BC	S2 + BC	S3 + BC
1	HU (%)	14.69	10.43	13.10	3.51	12.33	9.60	12.36
2	BD (g/cm3)	1.054	0.986	1.314	0.319	0.799	0.851	0.873
3	pH	7.99	6.26	5.40	9.89	8.33	7.10	6.45
4	EC (dS/m)	0.22	0.10	0.09	2.04	0.35	0.21	0.16
5	C (%)	2.89	2.01	1.98	76.01	10.99	10.26	14.35
6	N-NO3 (mg/kg)	31.2	14.9	9.2	5.4	26.2	11.3	8.0
7	N-NH4 (mg/kg)	11.9	11.3	3.0	15.6	13.9	12.4	3.7
8	P (mg/kg)	6.3	2.3	0.8	16.0	13.6	14.7	2.9
9	K (mg/kg)	23.9	5.5	6.4	3131	132.0	130.1	51.1
10	P-AL (mg/kg)	103.6	16.4	12.7	1615	226.7	168.7	86.7
11	K-AL (mg/kg)	360	200	240	8200	1040	860	700
12	Ca (mg/kg)	2990	2940	2366	2044	2763	2443	2202
13	Mg (mg/kg)	371.9	308.3	605.9	472.2	386.6	334.4	582.7
14	Na (mg/kg)	31.1	31.0	44.9	87.9	35.3	34.7	50.5

(HU) humidity; (BD) bulk density; (EC) electrical conductivity; (C) total carbon concentration; (N-NO₃) soluble nitrate nitrogen concentration; (N-NH₄) soluble ammonium nitrogen concentration; (P) soluble phosphorus concentration; (K) soluble potassium concentration; (P-AL) available phosphorus concentration; (K-AL) available potassium concentration; (Ca) available calcium concentration; (Mg) available magnesium concentration; (Na) available sodium concentration; (S1) Fluvisol soil; (S2) Chernozem soil; (S3) Luvisol soil.

. A single factor ANOVA assessed that the addition of BC amendment had a significant effect (p < 0.05) on each soil property.

The data presented in Table 2 indicate the following aspects:

• the values of humidity (HU), bulk density (BD), soluble nitrate nitrogen concentration (N-NO₃), and available calcium concentration (Ca) are higher for non-amended soils than for BC (3–4 times for HU and BD, 2–6 times for N-NO₃, 16–46% for Ca) and BC-amended soils (6–19% for HU, 16–50% for BD, 15–32% for N-NO₃, 7–20% for Ca);
• the values of pH, electrical conductivity (EC), total carbon concentration (C), concentrations of soluble ammonium nitrogen (N-NH₄), soluble phosphorus and potassium (P and K), available phosphorus and potassium (P-AL and K-AL), and available sodium (Na) are lower for non-amended soils than for BC (24–83% for pH, 9–22 times for EC, 26–38 times for C, up to 5 times for N-NH₄, 3–21 times for P, 131–567 times for K, 16–128 times for P-AL, 23–41 times for K-AL, and 2–3 times for Na) and BC-amended soils (4–19% for pH, about 2 times for EC, 4–7 times for C, 10–26% for N-NH₄, 2–7 times for P, 6–24 times for K, 2–10 times for P-AL, 3–4 times for K-AL, and up to 13% for Na);
• the values of available magnesium concentration (Mg) are higher for non-amended soil 3 than for BC (by 28%) and BC-amended soil 3 (by 4%); compared to non-amended soils S1 and S2, the values of Mg are higher for BC (up to 53%) as well as for BC-amended soils S1 and S2 (up to 8%).

The physicochemical properties of non-amended soils (S1, S2, and S3), BC amendment, and BC-amended soils (S1 + BC, S2 + BC, and S3 + BC) were processed using PCA [22,39,40]. The PCA results referring to eigenvalues highlighted 2 eigenvalues that were >1, i.e., those corresponding to PC1 (7.85) and PC2 (3.09). These first two PCs explain 78.1% (56.0% + 22.1%) of the total variance. The factor loadings (coordinates of variables on the factor-plane PC1−PC2) are specified in Table S1, where their significant levels are highlighted in bold. The factor scores (projections of cases on the factor-plane PC1−PC2) are summarized in Table S2.

The PCA bi-plot, i.e., projections of 14 variables (HU, BD, pH, EC, C, N-NO₃, N-NH₄, P, K, P-AL, K-AL, Ca, Mg, and Na) and 7 samples (S1, S2, S3, BC, S1 + BC, S2 + BC, and S3 + BC) on the factor-plane PC1−PC2, is shown in Figure 3. The correlation matrix is presented in

Table 3. Correlation matrix.

Variables	HU	BD	pH	EC	C	N-NO3	N-NH4	P	K	P-AL	K-AL	Ca	Mg	Na
HU	1
BD	0.69	1
pH	−0.49	−0.85	1
EC	−0.68	−0.86	0.81	1
C	−0.73	−0.89	0.77	0.97	1
N-NO3	0.30	0.14	0.25	−0.25	−0.34	1
N-NH4	−0.33	−0.59	0.70	0.42	0.37	0.31	1
P	−0.53	−0.72	0.70	0.55	0.61	0.00	0.62	1
K	−0.49	−0.72	0.63	0.81	0.82	−0.37	0.49	0.51	1
P-AL	−0.45	−0.75	0.71	0.83	0.83	−0.16	0.29	0.57	0.56	1
K-AL	−0.87	−0.73	0.67	0.80	0.82	−0.20	0.34	0.49	0.52	0.55	1
Ca	0.50	0.32	−0.03	−0.36	−0.52	0.64	0.42	−0.21	−0.23	−0.32	−0.51	1
Mg	0.09	0.18	−0.25	0.07	0.17	−0.45	−0.72	−0.23	0.09	0.11	0.02	−0.65	1
Na	−0.66	−0.72	0.56	0.88	0.94	−0.50	0.09	0.42	0.79	0.69	0.74	−0.67	0.45	1

(HU) humidity; (BD) bulk density; (EC) electrical conductivity; (C) total carbon concentration; (N-NO₃) soluble nitrate nitrogen concentration; (N-NH₄) soluble ammonium nitrogen concentration; (P) soluble phosphorus concentration; (K) soluble potassium concentration; (P-AL) available phosphorus concentration; (K-AL) available potassium concentration; (Ca) available calcium concentration; (Mg) available magnesium concentration; (Na) available sodium concentration; the significant values of correlation coefficients (α = 0.05) are highlighted in bold.

, where the significant values of correlation coefficients (r) at a significance level α = 0.05 (two-tailed test) are highlighted in bold.

The PCA bi-plot (Figure 3) and correlation matrix (Table 3) indicate the following relevant aspects:

• BC amendment has higher levels of C (76.01 ± 0.68%), EC (2.04 ± 0.07 dS/m), pH (9.89 ± 0.01), K-AL (8200 ± 0 mg/kg), P-AL (1615 ± 7 mg/kg), K (3131 ± 183 mg/kg), P (16.0 ± 0.3 mg/kg), and Na (87.9 ± 7.0 mg/kg), but lower levels of BD (0.319 ± 0.018 g/cm³) and HU (3.51 ± 0.18%) than other samples [discrimination on PC1 between BC (blue circle) and the other samples];
• non-amended soil 3 (S3) and BC-amended soil 3 (S3 + BC) have higher levels of Mg (572.2–607.5 mg/kg) and lower levels of N-NO₃ (8.0–9.7 mg/kg), N-NH₄ (2.8–4.1 mg/kg), and Ca (2182–2373 mg/kg) than the other non-amended and BC-amended soils (Mg = 302.6–388.1 mg/kg, N-NO₃ = 10.6–32.8 mg/kg, N-NH₄ = 11.0–14.1 mg/kg, and Ca = 2415–3035 mg/kg) [discrimination on PC2 between S3 and S3 + BC and the other non-amended and BC-amended soils (green circles)];
• pH, EC, C, N-NH₄, P, K, P-AL, K-AL, and Na are directly correlated and they are inversely correlated with HU, BD, N-NO₃, and Ca;
• there are very strong positive correlations (r ≥ 0.85) between EC and C (r = 0.97), Na and C (r = 0.94), Na and EC (r = 0.88);
• there are very strong negative correlations between BD and C (r = −0.89), HU and K-AL (r = −0.87), BD and EC (r = −0.86), BD and pH (r = −0.85).

3.2. Effects of BC Amendment on Tomato Growth

The levels of temperature inside and outside the greenhouse were 19.0 ± 1.4 °C and 4.0 ± 4.0 °C, respectively. The mean values ± margin of error of tomato plant height (H), number of leaves (NL), collar diameter (CD), and root volume (RV) corresponding to 5 treatments (S, S + BC, S + F, S + BC + F, and S + BC + F/2) for each soil type, 66 days after sowing, are presented in Figure 4.

BC had a beneficial effect on H of plants grown in soil S3 as well as a detrimental effect on H of plants grown in soils S1 and S2 (Figure 4a). Single factor ANOVA assessed the following aspects regarding the mean values of height (H_m) for the plants grown in soil S3: (i) there are no significant differences between the treatments S3 and S3 + F (p = 0.65); (ii) there are no significant differences between the treatments S3 + BC + F and S3 + BC + F/2 (p = 0.16); (iii) there are significant differences between the treatment S3 + BC and the other treatments (p ≤ 0.004). H_m of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC), i.e., 21.3 cm, was about 50% higher than H_m of plants grown in non-amended soil S3 (untreated control S3 and foliar treatment S3 + F) as well as about 30% higher than H_m of plants grown in BC-amended soil S3 with both foliar treatments (S3 + BC + F and S3 + BC + F/2). Plants grown in BC-amended soil S1 (S1 + BC, S1 + BC + F, and S1 + BC + F/2) had similar mean values of height, i.e., H_m ≈ 18 cm (p = 0.99), about 40% and 30% lower (p ≤ 0.002) than H_m of plants grown in non-amended soil S1 (S1 and S1 + F). Plants grown in non-amended soil S2 (S2 and S2 + F) had similar mean values of height, i.e., H_m ≈ 18 cm (p = 0.58), up to about 40% higher (p ≤ 0.004) than those of plants grown in BC-amended soil S2 (S2 + BC, S2 + BC + F, and S2 + BC + F/2).

BC had a beneficial effect on NL of plants grown in soil S3 and no favourable effect on NL of plants grown in soils S1 and S2 (Figure 4b). The mean value of number of leaves (NL_m) of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC), i.e., 7.5, was up to 25% higher (p ≤ 0.005) than NL_m of plants grown in non-amended soil S3 (S3 and S3 + F) as well as 9% higher (p = 0.05) than NL_m (6.9) of plants grown in BC-amended soil S3 with both foliar treatments (S3 + BC + F and S3 + BC + F/2). Moreover, there was no significant difference (p = 0.12) between NL_m corresponding to non-amended soil S3 (untreated control S3 and foliar treatment S3 + F). In the case of plants grown in soil S1, the mean values of number of leaves for all treatments were not significantly different (p = 0.52). Plants grown in BC-amended soil S2 (S2 + BC, S2 + BC + F, and S2 + BC + F/2) had similar NL_m, i.e., 5.4–5.8 (p = 0.29), 20–40% lower (p ≤ 0.0007) than NL_m of plants grown in non-amended soil S2 (S2 and S2 + F).

BC had a favourable effect on CD of plants grown in soil S3 and no beneficial effect on CD of plants grown in soil S1 (Figure 4c). The mean value of collar diameter (CD_m) of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC), i.e., 3.1 mm, was about 20% higher (p ≤ 0.007) than the CD_m corresponding to non-amended soil S3 (untreated control S3 and foliar treatment S3 + F); it was 7% and 12% higher (p = 0.21 and p = 0.04), respectively, than CD_m of plants grown in BC-amended soil S3 with both foliar treatments (S3 + BC + F and S3 + BC + F/2). In the case of plants grown in soils S1 and S2, the mean values of collar diameter corresponding to all treatments (3.0–3.4 mm and 2.5–2.9 mm) were not significantly different (p = 0.06 and p = 0.14).

The results depicted in Figure 4d indicate that BC had a favourable effect on RV of plants grown in all soil types. The mean value of root volume (RV_m) of plants grown in BC-amended soil S3 using the less concentrated fertilizer solution (S3 + BC + F/2), i.e., 0.58 cm³, was up to 2.8 times higher (p ≤ 8.32 × 10⁻⁸) than RV_m of plants grown in non-amended soil S3 (untreated control S3 and foliar treatment S3 + F) as well as up to 41% higher (p ≤ 0.04) than RV_m of plants grown in soil S3 with the other treatments (S3 + BC and S3 + BC + F). RV_m of plants grown in BC-amended soil S1 using the less concentrated fertilizer solution (S1 + BC + F/2), i.e., 0.51 cm³, was 34–76% larger (p ≤ 0.03) than RV_m of plants grown in soil S1 with treatments S1, S1 + BC, and S1 + BC + F. The mean values of root volume corresponding to the treatments S1 + BC + F/2 and S1 + F (0.51 cm³ and 0.44 cm³) were not significantly different (p = 0.38). RV_m of plants grown in BC-amended soil S2 with both foliar treatments (S2 + BC + F and S2 + BC + F/2), i.e., 0.38 cm³ and 0.37 cm³, were not significantly different (p = 0.82) and were 1.4–2.1 times larger (p ≤ 0.03) than those corresponding to the other treatments (S2, S2 + F, and S2 + BC).

The results obtained revealed that H_m, NL_m, and CD_m of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC) were up to 50% higher than those of plants grown in soil S3 with the other treatments (S3, S3 + F, S3 + BC + F, and S3 + BC + F/2). Figure 5 contains images of tomato plants grown for 66 days in non-amended and BC-amended strongly acidic soil S3. RV_m of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC) was 32% lower than RV_m of plants grown in soil BC-amended soil S3 using the less concentrated fertilizer solution (S3 + BC + F/2) and up to 2.1 times higher than RV_m of plants grown in soil S3 with other treatments (S3, S3 + F, and S3 + BC + F).

Data presented in Figure 4a–c suggest that BC had no favourable effect on H_m, NL_m, and CD_m of plants grown in slightly alkaline soil S1 (pH of 7.99 ± 0.01) and slightly acidic soil S2 (pH of 6.26 ± 0.02). Non-amended slightly alkaline soil S1 was a more suitable grown medium than non-amended soil S1 with foliar treatment (S1 + F) and medium alkaline BC-amended soil S1 (pH of 8.33 ± 0.03). Non-amended slightly acidic soil S2 with foliar treatment (S2 + F) was a more appropriate option than non-amended soil S2 and very slightly alkaline BC-amended soil S2 (pH of 7.10 ± 0.03). The mean height (H_m) of plants grown in non-amended soils (untreated control S and foliar treatment S + F) decreased in the order: slightly alkaline soil S1 (25.4 cm and 23.2 cm) >slightly acidic soil S2 (18.1 cm and 18.4 cm) >strongly acidic soil S3 (14.7 cm and 14.4 cm).

4. Discussion

Strongly alkaline BC (pH of 9.89 ± 0.01) was obtained by slow pyrolysis of vine residue at 517 ± 16 °C, using CO₂ as a sweeping gas and reactant in the process. BC was mixed with 3 soil types (20/80 m³/m³ soil, corresponding to 56 t/ha), i.e., slightly alkaline soil S1 (pH of 7.99 ± 0.01), slightly acidic soil S2 (pH of 6.26 ± 0.02), and strongly acidic soil S3 (pH of 5.40 ± 0.02). The effects of BC application on soil physicochemical properties and tomato plant growth were evaluated.

Strongly alkaline BC had a significant beneficial effect on height, number of leaves, collar diameter, and root volume of plants grown in strongly acidic soil S3 (Luvisol). This positive effect of BC on tomato growth was reported in other related studies. Rehman et al. (2021) [3] examined the effects of BC produced by slow pyrolysis of cotton stick, corncob, and rice straw at 450 °C and applied at rates of 1.5% and 3% (w/w), corresponding to 34 t/ha and 68 t/ha, respectively, on tomato growth in a medium alkaline soil (pH = 8.03). Pot experiment results highlighted that plant height as well as root length and mass increased with BC application rate. Field experiments conducted by Mohawesh et al. (2021) [8] revealed that a strongly alkaline BC (pH of 9.5 ± 0.35) obtained by slow pyrolysis of olive pruning residue at 300–350 °C and applied at rates of 8 t/ha and 16 t/ha improved tomato growth in a slightly alkaline soil (pH of 7.7 ± 0.08). However, at application rates at 30 t/ha and 40 t/ha, plant growth declined. Suthar et al. (2018) [21] investigated the effects of pyrolysis temperature (300 °C, 450 °C, and 600 °C) and application rate [1% and 3% (w/w)] of bamboo-derived BC on the performances of tomato grown in a sand medium. Pot experiments demonstrated that BC produced at 300 °C (pH = 6.7) and amended at 3% or produced at 450 °C (pH = 5.2) and amended at 1% improved tomato growth and fruit quality. Graber et al. (2010) [25] studied the influence of slightly alkaline BC (pH = 7.55) derived from citrus wood on tomato growth in a commercial soilless mixture, i.e., coconut fibre and tuff. Pot experiment results proved that tomato plant heights were significantly greater at BC application rates of 1% and 3% (w/w) compared with the control, with no difference between the two levels of BC dose.

The beneficial effect of BC on plant growth is due to the changes in the physicochemical and biological properties of the soil. In general, BC is alkaline, increases soil pH and EC, provides macronutrients and micronutrients, reduces soil BD and nutrient loss, and improves water and nutrient retention capacity [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27].

Strongly alkaline BC used as a soil amendment in this study increased EC and pH values of strongly acidic soil S3 (0.09 ± 0.00 dS/m and 5.40 ± 0.02). The increase in the soil EC (reflecting the concentration of soluble salts) and pH affects the nutrient availability to plants and could be detrimental to tomato growth and yield [3,8]. EC and pH values of BC-amended soil S3 (0.16 ± 0.01 dS/m and 6.45 ± 0.11) were beneficial for plant growth.

BC reduced the BD of soil S3 by about 50% (from 1.314 ± 0.036 g/cm³ to 0.873 ± 0.038 g/cm³). The mean value of BD of BC used in this research, i.e., 0.319 g/cm³, is within the range reported in the literature (0.08–0.43 g/cm³) [14,16,20,21,23,28]. Reducing soil compaction by adding BC enhances plant root development, leading to beneficial effects on the crop growth and yield due to improved water and nutrient uptake [8,9].

In this study, BC significantly increased the concentrations of soluble and available potassium and phosphorus (K, P, K-AL, and P-AL) in all soil types. Moreover, the concentrations of soluble ammonium nitrogen (N-NH₄) and available sodium (Na) were higher. According to the data summarized in Table 2, BC derived from vine pruning residue is an important source of K, P, N-NH₄, and Na. The mean values of K and P concentration in BC-amended soil S3, i.e., K_m = 51.1 mg/kg, P_m = 2.9 mg/kg, (K-AL)_m = 700.0 mg/kg, and (P-AL)_m = 86.7 mg/kg, were 3–8 times higher than those of non-amended soil S3, whereas (N-NH₄)_m = 3.7 mg/kg and Na_m = 50.5 mg/kg were 26% and 13% higher, respectively. These findings are consistent with data reported in the related literature [3,8,9,10].

Strongly alkaline BC obtained in this study by slow pyrolysis of vine pruning residue at 517 ± 16 °C and applied at a rate of 56 t/ha had no beneficial effect on height, number of leaves, and collar diameter of plants grown in slightly alkaline soil S1 (Fluvisol) and slightly acidic soil S2 (Chernozem). Lower levels of application rate and/or pyrolysis temperature could lead to better results [3,8,21].

5. Conclusions

Strongly alkaline BC (pH of 9.89 ± 0.01) obtained by slow pyrolysis of vine pruning residue had a significant beneficial effect on the growth performances of tomato plants sown in strongly acidic soil S3 (pH of 5.40 ± 0.02). The mean values of height, number of leaves, and collar diameter of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC) were up to 50% higher than those of plants grown in soil S3 with the other treatments (S3, S3 + F, S3 + BC + F, and S3 + BC + F/2). The mean value of root volume (RV_m) of plants grown in BC-amended soil S3 without foliar treatment (S3 + BC) was up to 2.1 times higher than RV_m of plants grown in soil S3 with other treatments (S3, S3 + F, and S3 + BC + F). This positive effect of BC on tomato growth performance is due to the changes in the physicochemical and biological properties of the soil.

BC increased electrical conductivity and pH values of soil S3, resulting in a more suitable environment for plant growth. BC significantly increased the concentrations of soluble and available K and P (up to 8 times) as well as those of soluble N-NH₄ and available Na (up to 26%). Moreover, BC reduced the bulk density of soil S3 (by about 50%), resulting in improved plant root development and thus enhanced water and nutrient uptake, leading to beneficial effects on plant growth.

BC had no favourable effect on height, number of leaves, and collar diameter of plants grown in slightly alkaline soil S1 (pH of 7.99 ± 0.01) and slightly acidic soil S2 (pH of 6.26 ± 0.02). Non-amended soil S1 without foliar treatment (S1) and non-amended soil S2 with foliar treatment (S2 + F) were more suitable growth media than the other options.

The results obtained in this study suggest that the application of very strongly alkaline BC produced from vine pruning residue as an organic amendment to strongly acidic soils is a promising strategy for enhancing soil quality, improving tomato plant growth, reducing biomass residues, and mitigating climate change.