Effects of Biochar on Biointensive Horticultural Crops and Its Economic Viability in the Mediterranean Climate

Abstract

The effects of biochar on different horticultural crops (lettuce, tomato, sweet pepper, and radish) were evaluated in the Mediterranean climate. Biochar was produced by pyrolysis of Pinus pinaster wood chips at 550 °C and used at 1 (B1) and 2 (B2) kg/m² application rates on six 3.5 m² plots in each treatment, with two control plots (B0). No fertilizer was used. Treatment B1 led to a significant increase (p < 0.01) of 35.4%, 98.1%, 28.4%, and 35.2% in the mean fresh weight of radishes, lettuce, tomatoes, and sweet peppers, respectively. Treatment B2 resulted in an improvement of 70.7% in radishes, 126.1% in lettuce, 38.4% in tomatoes, and 95.0% in sweet peppers (p < 0.01). Significant differences between treatments B1 and B2 were observed in the radish, tomato, and sweet pepper crops but not in lettuce. The profitability of biochar application to these crops was studied by considering a biochar price of 800 EUR/t and applying a CO₂ fixation subsidy, assuming the updated February 2022 price (90 EUR/t). In lettuce, tomato, and sweet pepper crops, the investment payback period was approximately one year. Application of biochar generated economic benefit either from the first harvest or in the second year. In radish, this period was longer than two years; however, an increase in the annual frequency of cultivation should be studied to optimize the benefit. The dose that provided the greatest benefit was B1 (for all crops, except for sweet pepper). Biochar considerably improved fruit and vegetable yield under the Mediterranean climate; however, further studies are needed to assess the effects of biochar on soil properties and yield to estimate long-term environmental and economic benefits.

1. Introduction

Currently, climate change problems are one of the main global concerns. Consequently, since the United Nations Framework Convention on Climate Change in 1992, actions were taken to reduce greenhouse gas (GHG) emissions and mitigate climate change effects, producing major agreements such as the Kyoto Protocol in 1997, the Copenhagen Accord in 2009, and the Paris Agreement in 2015.

Despite these agreements, estimates in the latest Intergovernmental Panel on Climate Change (IPCC) report indicate that current decarbonization policies are insufficient to achieve the target of a 1.5 °C global temperature increase by 2050 from pre-industrial levels, instead, reaching an increase of 2.7 °C in the same year [1].

In 2021, during the United Nations Climate Change Conference (commonly referred to as Conference of the Parties 26—COP26), the Glasgow Climate Pact was signed [2] in which the European Union, the United States, the United Kingdom, Japan, and South Korea committed to achieving carbon neutrality by 2050, and China by 2060.

Achieving the Sustainable Development Goals, mitigating global warming, and reaching carbon neutrality not only requires a drastic reduction in GHG emissions, but also the promotion of different carbon capture and storage (CCS) techniques to remove CO₂ from the atmosphere or avoid its emission.

Accordingly, in 2001, a report by the Food and Agriculture Organization (FAO) recognized the potential of agricultural soils as sinks for carbon sequestration and storage and the existence of agricultural practices that increase their retention capacity while improving crop yield [3]. One of the most promising techniques is amendment with biochar, a solid carbonaceous material derived from biomass pyrolysis [4]. This material has a high degree of porosity and surface area, which vary with the pyrolysis conditions and biomass used [5]. The use of biochar as a carbon sink in overcoming climate change has been extensively studied in different fields, such as agriculture or construction [6,7,8,9], with disparate, albeit favorable, results in all cases. As such, biochar is a promising material for reducing GHG emissions in the present and future.

The use of biochar also promotes the expansion of pyrolysis as a profitable lignocellulosic waste recovery method [10]. Usually, this waste is incinerated, with the consequent emission of GHGs and loss of materials. Now, the pyrolysis method could favor the closure of the material cycles, reinforcing the strategy established in the European Union Circular Economy Action Plans in 2015 [11] and 2020 [12].

To transform lignocellulosic waste into bioproducts, the profits from all products generated, including wood vinegar, bio-oil, syngas, and biochar, must cover the transformation and application costs of the products [13,14,15].

Much research has been conducted recently on the use of biochar for applications such as manufacturing carbon nanomaterials [16,17,18,19] and biofilters [20], and on its use as an organic soil amendment [21,22,23]. In the latter application, the surface properties of biochar improve the retention and exchange of water and nutrients between the soil and plants in addition to reducing GHG emissions in the soil [24,25,26,27]. Therefore, biochar must have low density and high porosity [28].

In recent years, many studies have assessed biochar effects on soil fertility and on the yield and quality of different horticultural crops, especially under water stress conditions. These studies have shown the efficacy of biochar in pumpkin [29], onion [30], pea [31], lettuce [32], corn [33], cucumber [34], sweet pepper [35], watermelon [36], and tomato [37] crops, among others. Conversely, other studies have not reported significant improvements in yield when supplying biochar [38], usually on soils fertile enough to meet all plant requirements, even under water stress conditions.

Furthermore, despite the environmental benefits of biochar, it is important that the amendment is economically profitable as well. Some studies have evaluated the viability of biochar in several crops [39,40,41,42,43], and the results depend on many factors such as the dose, the crop, the climate, the market price of biochar, and the bonuses for CO₂ fixation.

The main objective of this study was to assess biochar effects on different horticultural crops in the Mediterranean region, which has specific climatic conditions given severe water shortage during the warmer months. The biochar used in this study was produced by pyrolysis of Pinus pinaster wood chips, as described by Aguirre et al. [33].

This study was conducted with different types of fruits and vegetables to provide a broad view of the potential benefits of biochar. The radish (Raphanus sativus) was chosen as a root vegetable because of its short cycle and use as an indicator plant in evaluating compost and agricultural fertilizers [44]; Romaine lettuce (Lactuca sativa var. longifolia), as a leaf vegetable, given its sensitivity to soil nutrient concentrations [45]; tomato (Solanum lycopersicum), as a climacteric fruit because of its high yield in small spaces; and sweet pepper (Capsicum annuum), as a non-climacteric and one of the most cultivated fruits in the Mediterranean region [46].

2. Materials and Methods

2.1. Experimental Site

The experimental site is located in the Cisnerian Orchards of the Royal Botanical Gardens (Huertos Cisnerianos del Real Jardín Botánico) at the University of Alcalá (Universidad de Alcalá—UAH; Alcalá de Henares, Community of Madrid, Spain). In this location, the climate is typically Mediterranean (Csa in the Köppen climate classification), with hot and dry summers and cold and rainy winters.

The study was conducted from March to October 2021. Temperature and precipitation data were collected during that period at the State Meteorological Agency (Agencia Estatal de Meteorología—AEMET) weather station in Alcalá de Henares (Figure 1). The absolute minimum temperature was −4.2 °C, and the absolute maximum temperature was 42.4 °C, with an average monthly temperature ranging from 10 to 25 °C.

The soil amended with biochar where the different fruits and vegetables were grown is a sandy clay loam soil. The soil was analyzed by an external laboratory following internal methods, and the results are shown in

Table 1. Textural, physicochemical, and chemical properties of the soil (N = 8). EC = Electrical Conductivity; TN = Total Nitrogen; CEC = Cation Exchange Capacity.

Parameter	Unit	Value
Textural analysis
Sand	%	61.2 ± 3.5
Silt	%	8.9 ± 3.5
Clay	%	29.9 ± 1.2
Texture	-	Sandy clay loam
Moisture parameters
Field capacity	%	21.9 ± 1.0
Wilting point	%	11.2 ± 0.7
Available moisture interval	mm·m−1	106.9 ± 2.6
Physicochemical and nutrients
pH	pH unit	7.80 ± 0.06
EC	µS·cm−1	264 ± 49
TN	%	0.17 ± 0.05
K assimilable	mg·kg−1	603 ± 246
P assimilable (Olsen)	mg·kg−1	50.8 ± 31.8
Organic matter	%	3.5 ± 1.0
C/N	–	12.3 ± 1.1
Sulfate	mg·L−1	8.1 ± 3.4
Magnesium	mg·L−1	6.4 ± 2.4
Saturation percentage	%	29.1 ± 1.4
Phosphate	mg·L−1	5.6 ± 2.8
Nitrate	mg·L−1	28 ± 18
Micronutrients
Fe available	mg·kg−1	31 ± 15
Cu available	mg·kg−1	1.9 ± 0.7
B available	mg·kg−1	0.5 ± 0.2
Mn available	mg·kg−1	108 ± 44
Zn	mg·kg−1	4.4 ± 1.4
Exchangeable cations
Na+	meq·100 g−1	0.6 ± 0.1
K+	meq·100 g−1	0.7 ± 0.1
Ca2+	meq·100 g−1	16.8 ± 1.1
Mg2+	meq·100 g−1	5.8 ± 0.6
CEC	meq·100 g−1	23.9 ± 1.6

. The measurement of the parameters was carried out by the following methodologies: textural analysis by particle size analysis, pH by potentiometry, EC by conductivity, TN by sum of organic, inorganic and ammonia, ions such as sulfates and phosphates by ionic chromatography, and the rest of nutrients and metals by ICP-OES.

2.2. Materials

The biochar was produced via thermal pyrolysis of pine (Pinus pinaster) wood chips in a continuous screw reactor at 600 °C with a residence time of 20 min. It was performed at a semi-industrial plant of the company, Neoliquid Advanced Biofuels and Biochemicals S.L., whose development is integrated into the program LIFE CCM/ES/000051 (LignoBioLife): Development of high value-added bioproducts from forest waste through microwave technology.

The biochar used in this study was characterized, and its properties are outlined in

Table 2. Composition and characteristics of Pinus pinaster biochar. HHV = High Heating Value.

Parameter	Unit	Value
Ultimate analysis (N = 4)
Carbon	wt.%	72.00 ± 0.03
Hydrogen	wt.%	4.45 ± 0.11
Nitrogen	wt.%	0.13 ± 0.01
Sulfur	wt.%	n/a
Oxygen	wt.%	23.42 ± 0.62
Proximate analysis (N = 3)
Moisture	wt.%	1.70 ± 0.52
Volatile matter	wt.%	38.73 ± 3.33
Fixed carbon	wt.%	58.10 ± 3.93
Ash content	wt.%	1.47 ± 0.50
HHV	MJ·kg−1	27.92 ± 0.01
BET surface	m2·g−1	210
Physicochemical properties
pH	pH unit	8.60
Electrical conductivity (25 °C)	dS/m	0.51
Particle size	mm	2–7
C/N ratio	-	553.85
Humic acid	wt.%	0.90
Total humic extract	wt.%	20.30
Fulvic acid	wt.%	19.40

. Proximate analysis was performed by Thermogravimetric analysis (TGA), using a Netzsch STA 449 F3 Jupiter^®. The analysis was carried out in Ar, increasing the temperature by 10 K/min to 900 °C and recording 10 min isotherms at 100 and 900 °C and a 10 min isotherm at 900 °C in air (O₂/N₂ 80:20) (Figure 2). Three replicates were performed to ensure the reproducibility of the results.

In addition, the characterization also included ultimate analysis (N = 4) and other parameters of interest. Physicochemical parameters were measured by an external laboratory with their internal methods. The High Heating Value (HHV) was theoretically calculated using data from the proximal analysis and from the elemental analysis.

The species radish (R. sativus), lettuce (L. sativa), tomato (S. lycopersicum) and sweet pepper (C. annuum) were selected for the study to encompass different types of fruits and vegetables and assess the effects of biochar on some of the crops most commonly grown in Spain according to data from the Ministry of Agriculture, Fisheries and Food (Ministerio de Agricultura, Pesca y Alimentación—MAPA) in 2020 [46].

The following varieties were chosen: a vining tomato of the variety “óptima”, Romaine lettuce, sparkler white tip radish, and Italian sweet pepper. The tomato, lettuce, and pepper seedlings were purchased from the Provincial Association of Farmers and Ranchers (Asociación Provincial de Agricultores y Ganaderos—APAG Group-Coagral; Guadalajara, Spain), and radish seeds at Sanchez Nurseries (Viveros Sánchez; Cabanillas del Campo, Guadalajara, Spain).

2.3. Experimental Setup

The experimental setup consisted of two 1.3 × 8.0 m (total = 20.8 m²) contiguous and homogeneous terraces. The surface of each terrace was divided proportionally into three sections, resulting in 6 plots of approximately 3.5 m².

The soil was tilled to a depth of 30–40 cm for biointensive farming and optimal land use. Subsequently, the biochar was administered, stirring to a depth of approximately 15–20 cm. Application rates of 1 kg/m² = 10 t/ha (B1) and 2 kg/m² = 20 t/ha (B2) were evaluated, and two control plots were left without biochar (B0).

A drip irrigation system was used to supply a flow of up to 4 L/h, as well as a solenoid valve programmed to irrigate 3 days a week for 1.5 h starting in May.

The vegetables were arranged considering the principles of biointensive farming [39] and the potential benefits of intercropping.

The radish seeds were sown using the method of sowing by blows and separated by approximately 5–10 cm between each position. Lettuce and tomato seedlings were planted by burying each root ball in the terraces following a staggered arrangement to optimize the available space and separating by 30 and 45 cm, respectively. Sweet pepper seedlings were planted to replace lettuce and radish after the harvest.

In addition, following the principles of biointensive agriculture, calendula and basil were planted on the sides of both terraces to take advantage of their allelopathic effect as an insect repellent.

2.4. Harvesting and Measurement

In this study, two parameters were used to measure biochar effects on production: (1) the individual fresh weight of all fruits and vegetables and (2) their quantity. The fruits and vegetables were weighed on a Mettler Toledo PL3001-S precision weighing balance, with 3100 g maximum capacity and 0.1 g accuracy.

Given their variety, the fruits and vegetables were harvested in stages: radishes were harvested in April, lettuces were harvested in June, and tomatoes and sweet peppers were harvested in several batches between July and October.

The tomatoes and sweet peppers were harvested based on their apparent ripening, without pre-established dates, but as regularly as possible. In October, with the arrival of rain and cold weather, the harvest was interrupted because of the evident decay of the plants.

2.5. Statistical Analysis

Statistical analysis was performed using the software Statplus 7.1. (AnalystSoft Inc., Walnut, CA, USA). Data analysis consisted of calculating confidence intervals (p = 0.05), performing analysis of variance (ANOVA), and applying a Fisher’s Least Significant Difference (LSD) test to identify significant differences between means.

2.6. Estimation of Economic Viability

Different models were developed considering numerous factors that affect the costs and savings of amending with biochar. Aguirre et al. followed a simplified model for calculating the benefit of improving corn production and for estimating the time required to cover the cost of biochar, considering the price of CO₂ fixation, to which they assigned a value of 30 EUR/t [33]. The price of CO₂ is constantly increasing after COP26, surpassing 90 EUR/t in February 2022 [47].

Following Aguirre’s method, this work aims to give a simplified vision of the economics of different horticultural crops and in two doses, in order to assess the profitability of biochar in agriculture and whether the investment made can be recovered within a reasonable time to provide farmers with both environmental and economic benefits.

The simplified model consists in considering the price received by farmers for a ton of a certain vegetable and its mean productivity per hectare. Through Equation (1), the estimated crop income improvement can be calculated.

Once the income is calculated, the cost of biochar for the amendment must be considered in order to estimate the payback period for each vegetable and each dose. At this point, the value of CO₂ fixation may be calculated (Equation (2)) and considered for the estimation of payback period (Equation (3)).

$$Income(€/ha)=Price(€/t)\ast Productivity(t/ha)\ast Improvement$$

(1)

$${CO}_{2}value(€/ha)=\frac{{CO}_{2}Price(€/t)}{{CO}_{2}Fixation}\ast Dose(t/ha)$$

(2)

$$Payback(years)=\frac{Biocharcos\mathrm{t}(€/ha)}{Income(€/ha)+{CO}_{2}value(€/ha)}$$

(3)

3. Results and Discussion

3.1. Biochar Effect on Radish

Adding biochar significantly improves the mean fresh weight of radish (

Table 3. Mean radish weight in each treatment, expressed as mean ± standard deviation, with 95% CI standing for the confidence interval at p = 0.05. * Significant difference p < 001.

Treatment	Mean Weight (g)	95% CI (g)	Improvement (%)
B0 (79 plants)	21.9 ± 7.4 *	[20.3–23.6]	-
B1 (101 plants)	29.7 ± 9.6 *	[27.5–31.9]	35.4
B2 (105 plants)	37.4 ± 14.9 *	[34.3–40.6]	70.7

, Figure 3) at both application rates (Fisher LSD; p < 0.01), with 35.4% and 70.7% increases in B1 and B2, respectively. In addition, the improvement in B2 is also significantly higher than in B1 (Fisher LSD; p < 0.01).

The biochar effect on radish growth was extensively evaluated; however, most studies were conducted under controlled conditions of humidity, temperature, and radiation, among other factors. In turn, most field studies on biochar were conducted in tropical regions, and its use in the Mediterranean region has not been evaluated in depth.

Van Zwieten et al. performed a comparative analysis of the effect of adding two types of biochar on the yield of different crops in two types of agricultural soils. In both types of soil (ferrosol and calcarosol), adding biochar improved the radish yield [48]. Sousa et al. assessed the effects of different biochar doses on plant biomass, concluding that biochar can be effectively used as an amendment in short-cycle crops such as radish [49]. Conversely, Adekiya et al. conducted a study assessing biochar effects on the soil and on radish yield. After applying doses of 25 and 50 t/ha, significant improvements were observed in both treatments in comparison with the control, albeit without a large difference between doses [50]. The climate of the study region is characterized by a bimodal precipitation regime and an average temperature of 30 °C [51]. In turn, Dahal et al. studied the effect of adding different biochar mixtures with organic fertilizers on radish yield. When using a biochar dose of 10 t/ha, biomass production increased by 92% and marketable yield by 122% [52].

The findings of the present study corroborate these previous studies. In this case, radish biomass significantly increased in both biochar treatments in relation to the control, particularly with the B2 dose.

3.2. Biochar Effect on Lettuce

Lettuce biomass (shoots and roots) was significantly higher in plots B1 and B2 than in plot B0 (Fisher LSD; p < 0.01). The B2 treatment did not significantly improve the yield in comparison with the B1 treatment (Fisher LSD; p > 0.05) (

Table 4. Mean lettuce weight in each treatment, indicated as mean ± standard deviation, with 95% CI standing for confidence interval at p = 0.05. *: Significant difference p < 0.01.

Treatment	Mean Weight (g)	95% CI (g)	Improvement (%)
B0 (11 plants)	175.6 ± 80.9	[121.2–229.9]	-
B1 (14 plants)	347.8 ± 109.8 *	[284.4–411.2]	98.1
B2 (10 plants)	396.9 ± 87.9 *	[334.0–459.8]	126.1

, Figure 4).

Adding biochar at dose B1 increased the mean lettuce weight by 98.1%, whereas dose B2 increased this parameter by 126.1%.

Most studies on lettuce were conducted under controlled conditions. Matos et al. assessed the effect of adding different doses of biochar (10, 20, 30 t/ha) on lettuce yield in pots, concluding that the dose of 30 t/ha improved plant height, leaf number, and fresh weight [53]. Upadhyay et al. also assessed biochar effects at different application rates, 10, 30, 50, and 100 t/ha, and found a significant increase in fresh weight compared with the control in all doses, reporting an 87% increase at 30 t/ha. In the present study, the increase was 126.1% in dose B2. Furthermore, those authors reported a slight decrease from the application rate of 30 t/ha, possibly because, as excess nutrients are retained in the soil, they are no longer available for plants [45]. Conversely, a high amount of biochar in the soil can increase its salinity, which harms lettuce crops because they are sensitive to salinity above 1.3 dSm⁻¹ [32], thereby explaining the high increases at low doses.

In the Mediterranean region, Meddeb et al. did not report significant differences in the fresh weight of lettuce when applying biochar from the germination stage [54], perhaps because the doses were too low, and the sample was very small. In addition, as explained by Upadhyay et al., the biochar effect becomes important in the growth stage with the increase in plant nutrient requirements [45].

3.3. Biochar Effect on Tomato

As outlined in

Table 5. Mean tomato weight in each treatment, expressed as mean ± standard deviation, with 95% CI standing for confidence interval at p = 0.05. * Significant difference p < 0.01.

Treatment	Mean Weight (g)	95% CI (g)	Improvement (%)	Total Weight Improvement (%)
B0 (234 tomatoes)	186.0 ± 61.6	[178.1–193.9]	-	-
B1 (306 tomatoes)	236.2 ± 76.1 *	[223.6–240.7]	24.8	63.3
B2 (233 tomatoes)	257.5 ± 84.1 *	[246.6–268.3]	38.4	37.8

, adding biochar significantly increases the mean tomato weight in plots B1 and B2 with respect to the tomatoes of plot B0 (Fisher LSD; p < 0.01). In addition, B2 also shows a significant increase when compared with B1 (Fisher LSD; p < 0.01).

The B1 and B2 doses improved the mean tomato weight by 24.8% and 38.4%, respectively. Therefore, adding a moderate dose of biochar significantly improved tomato yield (Figure 5).

The total sum of the tomatoes collected in each plot was 43,525 g in plot B0, 71,043 g in plot B1, and 59.988 g in plot B2. The improvement in total weight with respect to B0 was 63.3% in B1 and 37.8% in B2; that is, although the mean tomato weight was higher in B2 than in B1, expressed as total weight, the highest value was that of B1.

Biochar’s effects on tomato plant growth and yield were evaluated worldwide. Most studies were performed in tropical and temperate zones of America, Asia, and Oceania, with no shortage of rain and even with a risk of flooding. Nevertheless, Guo et al. reported yield improvements of up to 60% when using application rates of 30, 50, and 70 t/ha, fundamentally due to biochar interference in the fixation of supplemented nutrients and nutrient exchange in the soil–plant system [55]. In these temperate zones, other authors have reported results ranging from no improvement to slight improvements of 10% in tomato yield [56,57].

In arid or semi-arid climates, where the absence of rainfall is a limiting factor in agriculture, especially in summer, different studies have reported the beneficial effect of biochar on tomato growth and yield [58,59]. Li et al. conducted a study of different biochar application rates, assessing significant improvements in tomato yield and in the physicochemical properties of the soil; after their economic analysis, the authors concluded that the optimal dose was 30 t/ha [60].

In the Mediterranean climate, tomato is one of the most commonly grown fruits. In this climate with hot and dry summer, biochar optimizes soil–plant water exchange and reduces irrigation water consumption. In addition, soil variability in this climate account for the highly uneven amendment effect. Tartaglia et al. assessed biochar effects on a typical tomato ecotype from Italy and reported a 12% increase in mean tomato weight [61]. In addition, Ronga et al. reported significant improvements in different tomato yield parameters in two consecutive years, with an increase in yield and mean fruit weight of approximately 25% [62]. The biochar effect differs between the two articles because of several factors, such as the specific climate of the area, its physicochemical properties, and the type of soil. The soil used by Tartaglia et al. had a higher proportion of sand, which implies higher porosity and lightness and lower nutrient concentrations; therefore, a 5% amendment was insufficient to induce a significant improvement. In contrast, a study by Cavoski et al. reported no significant differences in tomato yield or quality, which may be related to the low application rate of biochar (5 t/ha) [63].

The results from this study are generally in line with findings reported by other authors in areas with a Mediterranean climate. The increase achieved in the present study (63.3% total weight improvement in B1 and 37% in B2) was higher than that in other studies (reaching almost 40% mean tomato weight improvement). In other words, both the mean and total fruit weight were higher. The total weight improvement was higher in B1 than in B2 owing to the higher number of tomatoes in B1 plots, which may have resulted from issues unrelated to the experiment.

3.4. Biochar Effect on Sweet Peppers

Table 6. Mean sweet pepper weight in each treatment, expressed as mean ± standard deviation, with 95% CI denoting the confidence interval at p = 0.05. * Significant difference p < 0.01.

Treatment	Mean Weight (g)	95% CI (g)	Improvement (%)	Total Weight Improvement (%)
B0 (91 peppers)	37.4 ± 11.2 *	[35.1–39.8]	-	-
B1 (111 peppers)	50.6 ± 15.2 *	[47.8–53.5]	35.2	39.4
B2 (118 peppers)	73.0 ± 20.6 *	[69.3–76.7]	95.0	92.7

and Figure 6 show that both biochar doses significantly improved sweet pepper yield (Fisher LSD; p < 0.01), especially in the amendment with B2, where the mean sweet pepper weight was almost double that in the section without biochar. The results of the sweet peppers grown in B2 were significantly higher than the values of the peppers grown in B1 (Fisher LSD; p < 0.01).

The total sum of the sweet peppers collected in both plots was 3406 g in plot B0, 5620 g in plot B1, and 8613 g in plot B2. Compared to B0, the total weight improvement was 39.4% in B1 and 92.7% in B2. The results are quite similar to those of mean weight because the number of fruits picked was highly similar.

As with other vegetables, few studies have assessed biochar effects in Mediterranean regions. The findings of this study are in line with other studies [64,65]. Graber et al. assessed biochar effects on sweet pepper plant growth and yield, identifying a significant improvement in fruit weight of 16% with a dose of 1% in weight and of 21% with a dose of 3% in weight [64]. The increases are lower than those found in this study, possibly because those authors did not conduct an actual field study with soil, but instead used soilless media.

Applying biochar significantly improves sweet pepper yield, which may be due to the increase in soil water retention and the improvement in nutrient availability resulting from the reduction of leaching processes [22]. However, its effectiveness may vary with the type of soil [66] and its water and nutrient retention capacity.

3.5. Overall Biochar Effect on Fruits and Vegetables

In summary, the results indicate that amendment with biochar significantly increases the average weight of all fruits and vegetables assessed in this study. Similarly, tomatoes and sweet peppers showed a significant improvement in B2 with respect to B1. Individually, these results generally agree with other studies conducted in the Mediterranean region, reaching an even greater improvement.

Disparities between results from different studies result from heterogeneous parameters such as the origin of the biochar, type of soil, conditions of application (in the greenhouse, in pots or in the field), and dose or its use combined with fertilizers and other compounds to enhance its effect [67].

Although low doses of biochar (<40 t/ha) positively affect yield, higher amounts (>40 t/ha) lead to a lower yield [55,60]. This is possibly because high biochar adsorption decreases plant nutrient availability. Therefore, the nutrient retention capacity, especially nitrogen, must be considered when planning the tests [60,65].

In addition, different studies show biochar benefits several months after adding it to the soil [32,35,50,65,66] because this period is necessary for biochar leaching, oxidation, and degradation processes to occur. Accordingly, nitrogen is released gradually, and the effects are delayed and prolonged [32].

Several studies concluded that biochar effects are enhanced when adding nitrogenous compounds, compensating for the retention produced during the first months [22,50,65]. Furthermore, Trupiano et al. suggested that the benefits of biochar could also increase after its oxidation and bioactivation with soil microorganisms after a few months, thus also suggesting that the effect can be amplified over time [68]. In radish crops, biomass increases when using biochar produced from green waste, which increases exchangeable phosphorus and potassium [44], and adding nitrogenous fertilizers [45,50].

Biochar increases bacterial diversity and the number of beneficial microorganisms for the plants in the rhizosphere, which can reduce their vulnerability to pathogens [22,62,63,66].

3.6. Economic Study of Biochar Application

The results show that applying biochar to soils considerably improves fruit and vegetable yield. Nevertheless, we must analyze whether its use is profitable and whether the investment made can be recovered within a reasonable time to provide farmers with both environmental and economic benefits.

For the calculation of the improvement of crop income, the average price received by farmers in Spain in the last three years was 496.8 EUR/t for tomato, 920.1 EUR/t for sweet pepper, and 269.9 EUR/t for lettuce [69]. Because tomatoes show large price differences between the optimal production season and the rest of the year, an average value was calculated. There is no consistent data on radishes given their low production; nevertheless, this value was estimated, based on wholesale trade data, at 600 EUR/t.

Due to the nature of the fruits and vegetables chosen in this study, mean crop productivities are each widely different: 16.4 tons of radish, 28.3 tons of lettuce, 36.7 tons of sweet pepper, and 71.0 tons of tomato are produced per hectare of land in Spain, according to data from the Ministry of Agriculture, Fisheries and Food (Ministerio de Agricultura, Pesca y Alimentación—MAPA) [70].

Assuming that the improvement in yield is equivalent to the improvement in mean fresh weight, a different benefit was assessed for each fruit and vegetable and for each application rate (

Table 7. Calculation of the crop benefit for each vegetable and biochar dose.

	Price Received by Farmers (EUR/t)	Mean Productivity (t/ha)	Dose (t/ha)	Improvement (%)	Crop Income (EUR/ha)
Radish	600.0	16.407	10	35.4	3485
			20	70.7	6960
Lettuce	269.9	28.264	10	98.1	7484
			20	126.1	9619
Tomato	496.8	70.962	10	24.8	8743
			20	38.4	13,538
Pepper	920.1	36.687	10	35.2	11,882
			20	95.0	32,068

), which should be higher than the cost of the biochar used in each case.

After conducting a comparative study with different biochar suppliers in Spain, a cost of approximately 800 EUR/t was estimated to calculate the total investment costs for both application rates: 8000 EUR/ha for B1 and 16,000 EUR/ha for B2. Because experiments of the Lignobiolife (www.lignobiolife.com, accessed on 2 February 2022) project have demonstrated that standard fertilizer machinery can effectively apply the product, no application costs are required from farmers. The calculated payback period is the period after which the estimated benefits equal the estimated biochar costs, considering one vegetable crop per year and one hectare of land. In this study, we worked with the expected income, which is lower than the expected benefits since they disregard fruit picking expenses.

In addition, the subsidized price of using biochar as a CO₂ sink, which falls within the framework of climate change mitigation, can be considered to calculate the period of recovery of the investment (

Table 8. Calculation of the period of recovery of the investment and contribution of the price of CO₂.

	Dose (t/ha)	Biochar Cost (EUR/ha)	CO2 Value (EUR/ha)	Crop Income (EUR/ha)	Payback (Years)	Payback with CO2 (years)
Radish	10	8000	1854	3485	2.3	1.5
	20	16,000	3708	6960	2.3	1.5
Lettuce	10	8000	1854	7484	1.1	0.9
	20	16,000	3708	9619	1.7	1.2
Tomato	10	8000	1854	8743	0.9	0.7
	20	16,000	3708	13,538	1.2	0.9
Pepper	10	8000	1854	11,882	0.7	0.6
	20	16,000	3708	32,068	0.5	0.4

). Biochar can fix 2.06 t of CO₂ per ton [39]; therefore, 20.6 t/ha could be fixed with the B1 dose and 41.2 t/ha with the B2 dose.

Applying biochar is associated with savings in water and fertilizer consumption, which implies a reduction in expenses. Accordingly, a more detailed economic study considering these and other factors, such as those related to biochar application machinery, should be conducted to determine those savings.

As estimated for radish, the payback period is greater than 2 years with a single annual harvest; however, radish is a very short-cycle vegetable; therefore, the number of harvests per year can be increased, whilst considering the need for crop rotation to recover the investment in a significantly shorter period. The payback period for lettuce is estimated to be longer than 1 year; as such, in the second year, assuming a similar improvement in yield, the investment would be recovered. Tomato requires assessing biochar effects on long-term yield and throughout the year given the high variability of prices with the season. With the annual average price, the payback period is estimated at approximately 1 year. For sweet peppers, the benefit to the crop is so high that the investment is recovered in less than 1 year, that is, in a single crop.

The results showed no improvement in radish, lettuce, and tomato profitability when comparing the doses B2 and B1 during the first year. Therefore, applying the B1 dose would be more appropriate to optimize the benefits during the study period.

In addition, the data were collected in the absence of any type of external fertilization, with which the economic scheme would be more positive, if possible, for the farmer.

The results indicate that applying biochar in soils to grow fruits and vegetables in the Mediterranean region can become highly economically profitable. Furthermore, given the lack of consistent data, long-term biochar effects on the soil should be studied, which could further increase the future economic benefit to crops, especially with the B2 dose.

4. Conclusions

The results of this study demonstrate that applying a dose of 1 kg/m² biochar increased the mean fresh weight of tomato, radish, lettuce, and sweet pepper by 24.8%, 35.4%, 98.1%, and 35.2%, respectively. In turn, when applying a dose of 2 kg/m², the mean fresh weight of tomato, radish, lettuce, and sweet pepper increased by 38.4%, 70.7%, 126.1%, and 95.0%, respectively. The total weight of the fruits picked improved by 63.3% and 37.8% in B1 and B2, respectively, for tomato and by 39.9% and 92.6% in B1 and B2, respectively, for sweet pepper. The improvement with respect to the control was significant in all cases, and significant differences were observed between B1 and B2 for tomato, sweet pepper, and radish.

This improvement also brought high economic benefits, ranging from 3500 to 32,000 EUR/ha of income, with which the investment made in biochar acquisition and supply is recovered in periods close to one year for all crops, except for radish, whose cultivation frequency may be increased to optimize the benefit. When considering the possible subsidy for CO₂ fixation, the economic benefit increased significantly, and the investment recovery period decreased. During the first year, the B1 dose provided greater profit than B2, except for sweet peppers (in this crop, the B2 dose generated higher profitability in the first year).

These findings demonstrate that biochar is an excellent resource in biointensive orchards in the Mediterranean region to increase horticultural production in an economically viable way while optimizing water consumption and mitigating the effects of agriculture on climate change. In addition, this combination is within the framework of organic farming practices, sustainable development, and combating climate change.

The profitability of biochar can make the transformation of agricultural and forestry residues profitable, favoring the management of these residues through pyrolysis.

Further studies on the use of biochar as an amendment of agricultural soils should be conducted because this is not a well-established field, and the results vary considerably with the parameters, especially in the Mediterranean region.