Influence of Walnut Shell Biochar and Fertilizer on Lettuce Production in Hydroponic and Conventional Systems

Abstract

Water scarcity and soil fertility loss are major limitations for agricultural production. This study evaluated the effects of walnut shell biochar (WSB) and fertilizer on the growth of lettuce (Lactuca sativa L. “Gran rapid”) in hydroponic and conventional systems. WSB alone and WSB + fertilizer were applied at different mass ratios to soil (0, 5, 10, and 15%) in the conventional system and to the substrate (0, 10, and 20%) in the hydroponic system. Agronomic parameters such as fresh weight, dry weight, leaf area index, and the number of leaves were evaluated. The results showed that fertilizer addition improved growth in both systems. In hydroponics, the combination of WSB and fertilizer increased fresh weight by 45% and dry weight by 38% compared to the control without biochar or fertilizer. In the conventional system, WSB alone increased fresh weight by 30% and the number of leaves by 25%, without requiring additional fertilizer. Lettuce grown in conventional soil with 15% WSB and fertilizer achieved a 1.8 times higher leaf area index than the control without biochar. These findings suggest that WSB and fertilizer applications enhance lettuce crop yield, supporting the principles of circular economy and sustainable waste management in agriculture.

1. Introduction

Horticulture, which is crucial for food production, is increasingly affected by the effects of climate change, such as droughts, heat waves, hailstorms, and torrential rains, among others, which negatively impact crop yields and agricultural productivity in general [1]. These phenomena not only reduce the general growth conditions of many crops but also increase the vulnerability of agricultural systems, particularly in regions with arid, semi-arid, and Mediterranean climates [2]. In these areas, where temperatures are high and rainfall is scarce, the frequency, intensity, and duration of droughts have increased, representing a growing risk for agricultural production [3]. Similarly, excessive rainfall, especially when it occurs in short, intense bursts, increases the risk of flooding that destroys crops, causes soil erosion, contaminates water sources, and damages infrastructure and agricultural ecosystems [3]. As a result, soil health deteriorates, marked by a decrease in the organic layer and compaction, which directly impacts its productive capacity. The loss of the organic layer depletes organic carbon, vital for soil structure and fertility, thus reducing its capacity to retain nutrients and water. This limits the soil’s ability to support agriculture efficiently and sustainably [4]. Soil compaction also restricts root development and water infiltration, encouraging salt accumulation on the surface. This process further reduces the soil’s ability to support plant growth by limiting access to water and essential nutrients [5]. In arid and semi-arid regions, soil salinity is closely linked to irrigation management, with leaching being a key method for controlling salt buildup. Salinity is a major constraint on agricultural productivity, as it negatively affects crop growth at various stages, from germination to reproductive development [6]. In Argentina, large areas of irrigated soils, particularly in the Monte phytogeography regions, face salinity challenges. This highlights the urgent need to improve the physical and hydraulic properties of saline soils to optimize water-use efficiency and increase agricultural productivity in these vulnerable ecosystems [6,7]. All of these factors mentioned above reduce the amount of land available for agriculture, putting food security at risk [8].

A good alternative to mitigate the shortage of water can be the incorporation of crop production systems that ensure more efficient use of water in agriculture [9]. A proposed alternative is hydroponics, which is a satisfactory eco-industrial technology because it allows the growth and development of crops in growth media where the soil is replaced by water and nutrients [10]. This is an excellent opportunity for the agricultural production sector, especially in arid areas with scarce natural resources [11].

Hydroponics, as a soil-less cultivation method, offers several advantages that make it attractive for modern agriculture compared to traditional farming. These benefits include improved yield and product quality, as well as precise management of the quantity and composition of nutrients for plants. As a result, the use of pesticides and fertilizers can be reduced, making the process more efficient and less polluting [10,12]. Additionally, hydroponics provides a controlled growing environment that protects plants from adverse weather conditions. It also uses less water and optimizes space compared to conventional agriculture, which is crucial in areas with water scarcity or limited arable land [12]. A common problem is algae blooms in the recirculating nutrient solution, which can negatively affect plant health by competing for nutrients and clogging the irrigation system. Moreover, some algae species can produce toxins that inhibit plant growth [13]. Another challenge is the high initial investment required for specialized equipment such as irrigation systems, pumps, artificial lighting, and temperature control, which can represent a barrier for many farmers lacking sufficient financial resources. Furthermore, hydroponics requires specialized technical knowledge, including an understanding of water chemistry, nutrient management, and the control of temperature and humidity. Without this expertise, crops may be severely affected [14]. In addition, the high energy consumption required to maintain optimal temperature, humidity, and lighting levels increases operating costs and can have environmental implications, especially if the energy comes from non-renewable sources [14]. Water quality is another critical factor in hydroponic systems; the water used must be free of contaminants and have a composition suitable for dissolving and transporting essential plant nutrients. Poor water quality can lead to nutritional imbalances, plant diseases, or even total crop loss [14]. Moreover, it is essential to manage hydroponic system waste, such as spent nutrient solutions, to prevent environmental contamination [15]. For all these reasons, many farmers still prefer traditional farming systems, due to factors like familiarity with conventional practices, existing infrastructure, and a lower economic barrier to entry. While traditional farming methods present issues such as excessive pesticide use, greenhouse gas emissions, water waste, and the need for large tracts of land, they are still considered more accessible and practical compared to hydroponic systems, which require significant initial investment and more complex technical management [10,12].

Comparing hydroponic and conventional systems is not easy, since their culture media are different [16]. Hydroponics, by using an aqueous medium, has a water and nutrient uptake mechanism that is different from soil-based growing systems. On the other hand, conventional crops benefit from nutrient mineralization in the soil and microbial activity that is not present in hydroponic systems [17]. However, nutrient concentration management is extremely important and is a key factor in determining the yield and production of vegetable crops [18]. Therefore, comparison of these systems and proper nutrient management are critical to optimizing agricultural productivity. In recent decades, biochar, which is a stable carbonaceous material obtained from the thermochemical conversion of biowaste through the pyrolysis process [19], has begun to gain importance in various cropping systems. Although biochar is generally considered to be a stable form of carbon, it is not an inert material, as it possesses certain qualitative, chemical, and physical characteristics that give it activity and the potential to interact with the environment: (a) the presence of functional groups on its surface (carboxylic acids, phenols, and other oxygenated groups) make it chemically reactive and susceptible to interact with the environment, for example, by being able to absorb nutrients and heavy metals and modify the pH of the soil; (b) interaction with soil micro-organisms (bacteria and fungi), as they can colonize the surface of biochar, promoting biological and decomposition processes; and (c) nutrient release, as biochar can slowly release nutrients and minerals contained in its structure, rendering it advantageous in agriculture [20,21]. In terms of the qualitative aspect of biochar, it has a porous structure and high specific surface area, which improves soil water retention, nutrient availability, and microbial activity, contributing significantly to long-term soil fertility and productivity [21,22]. Concerning its chemical properties, although it mainly constitutes carbon, it also contains a variety of other elements, such as oxygen, hydrogen, nitrogen, and phosphorus, whose presence and concentration may vary depending on the biomass and the conditions of the pyrolysis process. This feature has increased crop yields, with some studies reporting up to a 30% increase. Its slow-release nutrient profile helps maintain soil fertility over time, making it a valuable tool for sustainable agriculture [6,21,23]. Due to its porous structure and the presence of functional groups on its surface, biochar has a high adsorption capacity, and by reducing mineral leaching and improving fertilizer efficiency, biochar minimizes environmental pollution and boosts crop yields [21]. Its reactive surface can interact with soil microorganisms, favoring their biological activity, biodiversity, and nutrient availability in the soil. Depending on the raw material used and the production process, biochar can influence the pH of the soil, alkalizing or acidifying it, while its stability allows it to sequester carbon, which is essential for mitigating climate change [24]. Unlike other organic materials that decompose rapidly and release carbon dioxide (CO₂) into the atmosphere, biochar’s carbon content is highly stable and can remain in soils for centuries [20,25]. This long-term carbon storage helps to reduce greenhouse gas emissions.

Furthermore, biochar can reduce algal growth in hydroponic systems, thereby ensuring the safety of vegetables for human consumption [12]. However, the advantage of adding biochar to growing media is controversial, with several works reporting a positive effect on plant growth [6,23,26], and others reporting no results or even a negative result [27]. These discrepancies may be explained by the wide variety of biochar used by researchers [28].

As mentioned above, different biomasses can be used as raw materials for biochar production, such as crop residues, manure, sludge, and hardwoods [29,30,31,32]. Most of the recommended raw materials are those that are considered waste in other industries, reducing environmental impact [33]. In 2023, global production of walnut shells reached 3,988,642.9 tons [34], highlighting the global importance of this crop in the agri-food industry. The inadequate management of these residues poses environmental problems, so their transformation into biochar can be a sustainable solution when used as a soil amendment, promoting a sustainable economy [35].

In Argentina, the commercialization of shelled nuts such as the Castilian walnut (Juglans regia L.) generates a large amount of lignocellulosic waste, with an annual production of around 20,000 tons per year [34]. The main nut-producing areas are located in the provinces of Catamarca (47%), La Rioja (24%), Mendoza (14%), San Juan (8%), and Río Negro (7%) [36].

It should be noted that there are not many studies on the effects of walnut shell biochar applied directly to agricultural soils or as a substrate. Romero-Arenas et al. [37] studied the early growth of Pinus patula in the nursery with walnut shell compost containing agrolite and vermiculite, by the gradual replacement of peat moss. The seedlings that were developed with the control treatment and with the highest percentage of walnut shell compost showed higher values in the variables: height, diameter, aerial dry weight, root dry weight, and total dry weight. The walnut shell allowed the production of healthy seedlings, making it useful as an alternative substrate for the production of plants in nurseries, reducing production costs as well as contributing to the forest products sector. Safaei Khorram et al. [38] studied the behavior of two walnut shell biochar (5 t·ha⁻¹) on soil nutrient dynamics, the growth and development of two crops (wheat and lentil), and weed growth dynamics for four years (2014–2017). The biochar improved soil properties by 10–23% in the first and second years, reducing bulk density (BD) and increasing total organic carbon (TOC), available potassium (AK), and phosphorus (AP), as well as water-holding capacity (WHC), cation exchange capacity (CEC), and pH. The incorporation of biochar resulted in a significant increase in crop growth and yield during the first two years of the trial. However, it is worth noting that the type of biochar used had no significant effect on the measured crop properties throughout the study period (p > 0.05).

The synergistic effects of biochar when combined with fertilizers in hydroponic and conventional systems stem from several interrelated mechanisms that enhance nutrient availability, improve soil structure, and promote microbial activity. Biochar significantly improves the cation and anion exchange capacities of the soil or substrate, increasing nutrient retention and reducing leaching losses, thus making essential elements like potassium, calcium, and magnesium more available to plants [39]. Its porous structure provides a larger surface area for nutrient adsorption, leading to more efficient fertilizer use. Additionally, biochar enhances soil aggregation and porosity, improving aeration, water retention, and root penetration, which facilitates better nutrient uptake. Biochar also fosters microbial activity by creating a habitat for beneficial soil microorganisms, which aid in organic matter decomposition and nutrient cycling while stabilizing soil health [40]. Moreover, it helps mitigate fertilizer runoff and pollution by reducing nutrient loss into surrounding ecosystems, supporting sustainable agricultural practices. The combination of biochar with fertilizers has demonstrated significant improvements in plant growth, as observed in lettuce, due to enhanced nutrient availability and better growing conditions, leading to increased biomass and nutritional content [41]. These findings reinforce the potential of WSB as a valuable soil amendment that not only boosts agricultural productivity but also aligns with environmental sustainability goals, further contributing to the advancement of modern farming practices.

Considering the above, there is a specific lack of targeted research on WSB to particular crops and cultivation systems, especially in hydroponic and conventional agriculture, as its unique characteristics and agronomic benefits—particularly for lettuce—remain under-examined despite the well-documented effects of biochar in general. By focusing on WSB, this study aims to fill this niche in the literature by providing empirical data on its effects on lettuce growth across both hydroponic and conventional systems [42].

For all the above, the objective of this study was to compare the effects of the incorporation of walnut shell biochar and fertilizer on the growth of lettuce (Lactuca sativa L.), under hydroponic and conventional systems. Additionally, the response of lettuce to the addition of fertilizer was evaluated as a mineral contribution to both systems. This study provides valuable information for regional agricultural activities, and it is a proposal for the sustainable use of available walnut shell biochar. This work contributes to understanding how different ratios of WSB can be effectively utilized in regional agricultural practices, especially in areas facing challenges related to soil fertility and water scarcity. Additionally, the integration of WSB within circular economy principles showcases a sustainable approach to waste management in agriculture, which adds value to existing research.

2. Materials and Methods

2.1. Biochar: Obtention and Characterization

Walnut shells (exocarp, mesocarp, and endocarp) were provided by a local establishment from Barreal, in the department of Calingasta, Province of San Juan, Argentina (31.580397° S; 68.447565° W). Shells were packed in mesh bags in the dark at 20 °C until analysis within 30 days. Pyrolysis is a thermochemical decomposition process in which organic materials are heated in an environment without oxygen, preventing combustion. This process breaks down complex organic molecules into smaller compounds, resulting in the formation of biochar, bio-oil, and syngas. Thus, WSB was obtained by pyrolyzing the bio-waste in a stainless-steel reactor with a volume of almost 0.8 L, as described in previous works [43,44]. To perform the pyrolysis process at 450 °C for 2 h, three independent samples (1 kg each) were ground and sieved. The particle fraction between 1190 and 2380 µm was selected for pyrolysis since this particle size distribution favors a better thermal conversion. The walnut shell biochar (WSB) obtained was physicochemically characterized to evaluate its properties. All analyses were performed in triplicate [45]. Proximate analysis of moisture (M), ash, and volatile matter content (VM) was determined according to ASTM standards [46,47], and elemental analysis was performed using an elemental analyzer (AuroEA3000, Euro Vector SRL, Redavalle, Italy). pH and electrical conductivity (EC) were measured using the methodology described by Belda et al. [48], using a digital pH meter (Adwa AD1000, ADWA Instruments, Szeged, Hungary) and a conductivity meter (EC-214, Hanna Instruments, Padova, Italy), respectively. Similarly, the cation exchange capacity (CEC) was determined by the saturation method with NH₄Ac at pH 7 [49].

The equations proposed by Klasson (2017) were applied to estimate the molar ratios of H/C and O/C, and the correlations were used to estimate the quality indicators of WSB:

$$\mathrm{Stable}\mathrm{C}\mathrm{mass}\mathrm{fraction}=0.921-0.422·{\displaystyle \frac{\mathrm{VM}}{\mathrm{FC}}}$$

(1)

$${\mathrm{R}}_{50}=0.170{\displaystyle \frac{0.474·\mathrm{VM}+0.963·\mathrm{FC}+0.067·\mathrm{ASH}}{100-\mathrm{ASH}}}+0.00479$$

(2)

$$\mathrm{CS}=\left({\mathrm{b}}_{\mathrm{y}}\right)·{\mathrm{C}}_{\mathrm{BC}}·{\displaystyle \frac{{\mathrm{R}}_{50}}{{\mathrm{C}}_{\mathrm{F}}}}$$

(3)

$$\mathrm{MRT}=4501{·\mathrm{e}}^{-1.3·{\displaystyle \frac{\mathrm{VM}}{\mathrm{FC}}}-0.80}$$

(4)

$${\mathrm{BC}}_{+100}=0.895-0.245·{\displaystyle \frac{\mathrm{VM}}{\mathrm{FC}}}$$

(5)

where VM is volatile matter, FC is fixed carbon, R₅₀ is the recalcitrance potential, CS is the carbon sequestration potential, b_y is the biochar yield, C_BC is the mass fraction of carbon in biochar, C_F is the mass fraction of carbon in feedstock, MRT is the mean residence time, and BC₊₁₀₀ is the mass fraction of carbon that would remain after 100 years.

FTIR spectra (Fourier transform infrared spectroscopy) were acquired by an Infralum FT-08 FTIR (Lumex Instruments, Mission, BC, Canada) spectrometer using the KBr pressed pellet method. All spectra were collected at room temperature using 60 scans in the range of 4000–4001·cm⁻¹. A scanning electron microscope (SEM-EDS, EVO MA10W, Carl Zeiss, Oberkochen, Germany) with a Bruker X-ray energy dispersion (EDS) microanalysis system (Quantax 200, Bruker, Billerica, MA, USA) and an SDD XFlash 6/30 (Bruker, Billerica, MA, USA) analytical detector was employed. The specific surface area (BET) was determined by a Micromeritics Sortometer (ASAP 2050, Micromeritics, Norcross, GA, USA) by N₂ adsorption at 77 K [35].

2.2. Design Experiments

The experiments with the hydroponic and conventional systems began in June 2023 in a greenhouse located in the department of Albardón, province of San Juan (−31.4394247° S, −68.4834482° W). For this purpose, lettuce (Lactuca sativa L. “Gran rapid”) seeds were used, a widely distributed and popular vegetable in the area. The different treatments in each system were placed randomly, with a total of 15 replicates per treatment. During the experiment, the samples were watered regularly according to the crop requirements. The initial characteristics of the water were as follows: pH of 6.43 ± 0.30 and EC 93 ± 0.01 µS·cm⁻¹. The greenhouse maintained an average temperature of 15 ± 2 °C and a relative humidity of 43 ± 3%. Natural pesticides, such as aqueous extracts of garlic and nicotine, were applied to prevent the attack of pests such as aphids and whiteflies.

2.2.1. Description of Hydroponic System

For this experiment, the Nutrient Film Technology (NFT) system was used, in which water and nutrient solution recirculate through plastic pipe channels that support the growing plants [9]. The design consisted of two individual irrigation subsystems, where one only recirculated water through the treatments (C), and the other recirculated the fertilizer solution (F). Each subsystem had three treatments of 15 plants each. Water-only treatments were: no biocarbon control (P:C), 10% biocarbon (PB10:C), and 20% biocarbon (PB20:C). For the experiments, a liquid fertilizer based on nitrogen, phosphorus, and potassium, plus minor elements, Basfoliar^® 10–4–7 SL brand, produced and marketed by COMPO EXPERT Chile Fertilizantes Ltda (Santiago de Chile, Chile), was used. Fertilizer treatment solutions were: fertilizer treated control, fertilizer solution (P:F) and 10% biocarbon (PB10:F), and fertilizer solution and 20% biocarbon (PB20:F). Thus, there were a total of 90 replicates for the hydroponic system. The amendment ratios of biochar (15% and 20%) were selected based on prior studies demonstrating a range of plant responses to biochar application [50]. Higher biochar amendment rates are often chosen in agricultural studies to enhance soil properties such as water retention, nutrient availability, and soil structure. The porous nature of biochar improves soil aeration, water-holding capacity, and nutrient retention, which can be particularly beneficial in nutrient-poor or drought-prone soils. Higher rates may also help promote microbial activity by providing a habitat for beneficial microorganisms. Additionally, increased biochar applications can contribute to long-term carbon sequestration, improve soil fertility, and alleviate compaction [51].

The substrate used was commercial perlite (P), a substrate widely used in soilless systems [52]. The pH and EC values of the perlite were 8.01 and 81.3 µS·cm⁻¹, respectively. pH and EC were determined according to the protocols described previously [48].

Regarding irrigation, the fertilizer (F) used was the one mentioned above, where the ratio N:P:K was 10:4:7 + micronutrients. The nutrient solution was prepared at a dose of 2.5% F and applied after transplanting. To stabilize the fertigation, the pH and EC of the nutrient solution were progressively adjusted to ensure optimum values for nutrient uptake. These adjustments were made by the controlled addition of phosphoric acid to lower the pH and by periodically checking the electrical conductivity to avoid the accumulation of salts in the substrate. The main water (C) was left to stand for 24 h to remove chlorine and was used as a control for the nutrient solution.

To establish the crop in the hydroponic system, 3–4 seeds were sown in 120 mL (6 cm in diameter and 6.3 cm in depth) cups for germination. Commercial perlite and mixtures of perlite and biochar were used as the planting medium, according to the treatments described above. These substrates provided a suitable environment for seedling emergence and initial development prior to transplanting into the hydroponic system. The cups were placed on trays and watered from the water main. They were then transplanted when they had 3–4 fully expanded leaves after nearly four weeks. Figure 1 and Figure 2 show images of lettuce crops under different treatments in the hydroponic system before harvest.

2.2.2. Conventional System. Sampling of Soils. Description of Mixtures and Fertilizer

To carry out the traditional cultivation system, five samples of clay loam soil, each 20 cm in depth, were collected at random from the farm described above. The soil samples were dried in the open air, then homogenized, sieved (2 mm), and analyzed physiochemically. pH measurement was determined in material via water suspension (1:5) [48]. The initial characteristics of the soil (0–20 cm) were as follows: pH of 7.89, EC 305 µS·cm⁻¹, moisture of 13.38%, total nitrogen of 0.22%, organic matter of 0.91%, and organic carbon of 0.53%.

Regarding the granulometric composition, the soil presented 40% sand, 30% silt and 30% clay, classifying it as clay loam. Regarding the nutrient content, the values obtained were P: 15 mg·kg⁻¹, K: 120 mg·kg⁻¹, and Mg: 45 mg·kg⁻¹, indicating a moderate availability of these essential elements for plant growth.

For the conventional system, the experiment was conducted in 1 L plastic pots (15 cm diameter and 12 cm deep), filled with 1 kg of soil as described above, or a mixture of soil and biochar (WBS) [6,54,55]. This experiment also included two irrigation subsystems, one with water only and the other with the fertilizer solution described above.

The water irrigated treatments consisted of: soil without biochar (S:C), soil mixed with 5% biochar (SB5:C), soil with 10% biochar (SB10:C), and soil with 15% biochar (SB15:C), with a total of 15 replicates per treatment. For the fertilizer irrigation subsystem, the treatments were: soil with fertilizer solution (S:F), soil with 5% biochar (SB5:F), soil with 10% biochar (SB10:F), and soil with 15% biochar (SB15:F), with a total of 15 replicates per treatment. For the fertilization treatments, the same fertilizer was used as in the hydroponic system, prepared in the same concentration, and applied only once with a sprayer, starting from the second week after transplanting. Subsequently, all treatments were watered uniformly every week, and the soil moisture was maintained at 60% [56]. Figure 3 shows images of lettuce plants under different treatments in the conventional systems.

2.2.3. Measurement of Growth Variables

After 85 days of growth, five plants from each treatment were randomly collected. The root system of each plant was carefully washed with water, wrapped in muslin cloth, and rinsed under a current of tap water to ensure thorough cleaning and root separation, as was described by Papathanasiou et al. [57]. The following agronomic parameters were determined: root length (RL), shoot length (SL), total length (TL), leaf length (LL), number of leaves per plant (NL), foliar area index (IAF), and fresh weight (FW), using the free software ImageJ (https://imagej.en.softonic.com/, accessed on 31 January 2025) [58]. In addition, the plant samples were dried at 70 °C to determine total dry weight (DW).

The relative chlorophyll content measurement was conducted at the pre-harvest stage, approximately 85 days after transplanting. This period was selected to assess the final chlorophyll levels before harvest and analyze the sustained effect of biochar and fertilization treatments on leaf pigmentation and overall plant health. Previous studies have demonstrated the relevance of this measurement in advanced growth stages of lettuce cultivation, as chlorophyll content is closely related to the plant’s physiological status and its response to environmental and agronomic factors [59]. The SPAD was determined according to Yang et al. [60] using a portable chlorophyll meter (CM–B, Biobase, Shandong, China). The meter consists of two arms that open and close in the form of a clamp. The front side of the lettuce leaves was placed above the measuring chamber of the SPAD meter and measurements were taken by closing both arms. Twenty readings were taken in each test, and the average of these readings was calculated. The SPAD measurements were converted to total chlorophyll concentration using Equation (1) with a regression coefficient (R²) of 0.9960 [61].

$$y=0.0419x^2+1.6475x+1.5239$$

(6)

where y is the total chlorophyll concentration (nmol·1/cm²) and x represents the SPAD readings.

For color evaluation, the parameters (L*, a*, b*) of the lettuce leaves were measured using a colorimeter (Konica–Minolta, Osaka, Japan). The straightness/darkness (L), greenness/redness (a), and blue/yellowishness (b) of the lettuce leaves were measured. The instrument reads the color with an optical eye that performs the spot measurement. Measurements were made on fully developed leaves from the middle part of the plant, selected uniformly to avoid variability in the results. The evaluations were carried out in two growth periods, the first at 30 days after transplant (DAT) and the second at 45 DAT, coinciding with the vegetative and pre-harvest development phases, respectively. Twenty-five readings were taken in each test, and the average of these measurements was taken for all treatments.

2.3. Statistical Analysis

Descriptive statistics were used to characterize the biochar (WBS), mixtures (WBS + P and WBS + soil), and the agronomic parameters of the lettuce plants. All analyses were carried out in triplicate and the data are reported as mean ± standard error (SE). The results were analyzed by bidirectional ANOVA and significant differences between the mean values were determined by Duncan’s test (p < 0.05) using the InfoStat software (https://www.infostat.com.ar/index.php?mod=page&id=46, accessed on 31 January 2025) [62]. Graphs were processed using GraphPad Prism version 5.00 for Windows (GraphPad Software, San Diego, CA, USA, www.graphpad.com, accessed on 31 January 2025) [63].

3. Results and Discussion

3.1. Characterization of WSB

The physicochemical properties of the WSB biochar were analyzed (

Table 1. Characterization of WSB. The table shows mean values ± standard error. EC (electrical conductivity); CEC (Capacity exchange cationic); y_b (yield); VM (volatile matter); M (moisture); FC (fixed carbon); C (carbon content); H (hydrogen content); O (oxygen content); N (nitrogen).

	WSB	Unit
pH	10.86 ± 0.05
EC	561 ± 1.73	µS·cm−1
CEC	3.13 ± 0.12	meq·100 g−1
yb	30.00 ± 0.52	%
Ash	2.49 ± 0.12	%
VM	28.68 ± 0.62	%
M	2.11 ± 0.01	%
FC	66.82 ± 0.56	%
C	78.12 ± 0.25	%
H	2.90 ± 0.04	%
O	13.96 ± 0.28	%
N	5.01 ± 0.10	%

), revealing an alkaline pH value (mean = 10.86), similar to the values reported by Griffin et al. [64]. The electrical conductivity (EC) indicated a moderate capacity to conduct electricity, which is relevant to understanding its ability to dissolve salts and other solutes. The cation exchange capacity (CEC) was found to be low compared to the higher values (33.4 meq/100 g) reported by Griffin et al. [64]. This lower CEC may be due to the pyrolysis temperature, as Rasse et al. [65] found that biochar produced at temperatures below 500 °C has a higher CEC. The biochar yield was 30%, consistent with values reported for biochar of animal and plant origin [66]. The material exhibited low moisture (M) and ash percentages, with a volatile matter (VM) content greater than 28%, reflecting organic components evaporating during high-temperature heating. Diao et al. [67] and Duan et al. [24] reported similar VM values, although Diao et al. found a slightly lower VM and higher fixed carbon (FC) content. The ash content in this study was much lower than the values observed by Safaei Khorram et al. [38] for WSB pyrolyzed at 450 °C and 800 °C. The composition of the biochar consisted mainly of carbon (C) and oxygen (O), with smaller amounts of nitrogen (N) and hydrogen (H), consistent with the findings of Diao et al. [67] and Duan et al. [24].

The WSB biochar exhibited an O/C ratio of 0.179 and an H/C ratio of 0.037 (see

Table 2. Indicators of quality of WSB. H/C (hydrogen-carbon molar ratio); O/C (oxygen-carbon molar ratio); C/N (carbon-nitrogen molar ratio); R₅₀ (recalcitrance potential); CS (carbon sequestration potential); MRT (mean residence time); BC₊₁₀₀ (mass fraction of carbon that will remain after 100 years).

	WSB	Unit
H/C	0.037
O/C	0.179
C/N	15.59
Stable C mass fraction	73.98	%
R50	0.136
CS	0.6	%
MRT	1157	Years
BC+100	0.790
Specific surface area	2.410	m2·g−1

), differing from Diao et al. [67] due to higher oxygen functional groups. The recalcitrance index (R50) [68] indicates that WSB is highly degradable and has low stability [69]. The MRT suggests WSB will persist in soil for about 1157 years, with variation depending on soil type, ranging from 90 to 1500 years in clay soils to nearly 4000 years in temperate soils [70,71].

Biochar, with its extensive surface area and porosity, can improve water retention capacity and nutrient uptake in soils [72]. This material has the potential to remove contaminants not only from water and soil but also from gases [73]. Table 2 shows the average specific surface area of the WSB sample. The biochar produced in this study has a high specific surface area value, making it suitable for contaminant removal. Several studies have reported notable BET specific surface area values for biochar derived from various biomass sources, such as short-rotation wood [74,75]. Furthermore, Onal et al. [76] found even higher values for biochar derived from potato peel waste, while Batista et al. [77] observed a temporal increase in surface area when biochar is mixed with soil, as pore water leaches minerals with low affinity for the biochar surface. Therefore, the empty pores of biochar can be filled with organic matter, including humic components, depending on the specific surface area value. This makes biochar a versatile option for the removal of organic substances from water and soil [75,78]. In addition, the highly heterogeneous surface of WSB was observed in the SEM image (see Figure 4), and pore sizes were around 2.6 and 6 µm.

Figure 5 presents the FTIR spectra obtained for WSB. A peak related to the –OH groups was observed (bands between 1800 and 2000 1·cm⁻¹). The presence of C–H groups was decreased for WSB. The presence of ketones, aldehydes, and carboxylic acids (C–O groups) was not detected. C=C was identified in the biochar studied, indicating the existence of alkenes and aromatic hydrocarbons such as benzene [79]. The band corresponding to the presence of ethers and esters was not observed in this biochar.

According to the EDS analysis, it was found that WSB showed significant carbon content alongside high mineral concentrations. Specifically, K content ranged between 1.72% and 2.38%, Ca concentrations varied from 1.81% to 3.5%, and Fe was also detected, with presence ranging from 0.61% to 0.80%. These elements were also reported for the same biochar by Rodriguez Ortiz et al. [35] and Yin et al. [80].

Considering the characteristics determined in this section, WSB could improve soil structure and enhance water retention, which is particularly beneficial in arid regions. These physical characteristics contribute to better water availability for plants, especially during dry periods.

3.2. Agronomic Parameters for Both Systems

The results obtained show a differential effect of fertilizer on lettuce growth, with notable variations between the evaluated cultivation systems, which highlights the importance of the interaction between the type of system and nutritional factors on plant performance. This finding is consistent with previous studies demonstrating the benefits of fertilizer in improving plant growth [81,82]. In general, the RL, SL, and TL values in the conventional system were higher than those found in the hydroponic system, especially with the addition of fertilizer (Figure 6a–c).

Regarding RL, in the hydroponic system, no significant differences in RL were observed between the different treatments. In the conventional system, the highest RL values were obtained for the soil treatment with 15% biochar and irrigated with the fertilizer solution, showing significant differences compared to the treatments irrigated with water (Figure 6a). The results showed higher plant growth in the conventional system compared to hydroponics, suggesting that soil conditions favored better nutrient uptake in this experiment. However, plant response in hydroponics was observed to vary with the type of substrate and nutrient solution used, highlighting the importance of optimizing these factors to improve yield in soilless systems. These results provide important insights into the interaction between substrates, fertigation, and plant growth, and provide a basis for future optimizations in hydroponic crops [17]. Li et al. [83] showed better root growth results in a hydroponic system with a substrate of 50% perlite and 50% peat compared to a hydroponic system with water and a nutrient solution.

SL showed the highest values in the hydroponic system for treatments irrigated with fertilizer only and treatments with fertilizer and biochar. However, no differences were found between the different treatments for the fertilizer solution (Figure 6b). In the conventional system, the highest value was found for the treatment irrigated with fertilizer and 10% biochar compared to the treatment irrigated only with water and 10% biochar. When the biochar dose increased to 15%, shoot length decreased (Figure 6b). Similar results were found for total length in the hydroponic system, where the highest values were observed in the treatments irrigated with fertilizer compared to those irrigated with water alone. However, no significant differences were detected between the fertilizer and biochar treatments (Figure 6c). In the conventional system, most of the treatments irrigated with either fertilizer or water showed no significant differences between them, with the lowest mean value recorded for the treatment irrigated with water and 15% biochar (Figure 6c). Lei and Engeseth [17] observed similar SL values in their study on lettuce in hydroponic vs. conventional systems. These findings suggest that although both hydroponic and conventional systems can provide optimal conditions for plant growth, the addition of fertilizer remains crucial to maximize performance.

The highest fresh weight values were observed in the hydroponic system for treatments irrigated with fertilizer, as opposed to those irrigated with water only (Figure 7a), with the highest value recorded in the PB20 treatment irrigated with the fertilizer solution. In the conventional system, significant differences were observed between treatments irrigated with water only, specifically in the treatments with 15% biochar (Figure 7a). These results are consistent with previous studies showing the positive impact of fertilizer on the fresh weight of plants [84]. Our results show a significant effect of fertilizer treatment on increasing fresh weight in both conventional and hydroponic growing conditions. This finding underscores the importance of fertilization in obtaining more robust plants of higher quality in terms of FW, a crucial factor in their commercial value and market acceptance as fresh and healthy food.

For DW, significant differences were found in the hydroponic system between those irrigated with fertilizer and those irrigated with water. The highest values were for the treatments with 20% biochar and irrigated with fertilizer, but there was no difference between the treatments with biochar and the treatment irrigated with fertilizer only (Figure 7b). In the conventional system, the highest DW value was found for the treatment irrigated with fertilizer and 15% biochar, with significant differences being found for the same treatment irrigated with water (Figure 7b).

These results suggest that both water irrigation alone and fertilization are important to maximize plant DW, depending on the dose at which they are incorporated. Choi et al. [85] indicated that their most relevant results of FW and DW were at higher doses: 80 and 100% biochar. Considering both cultivation systems, Fontana et al. [9] showed a higher percentage of weight loss in the conventional system compared to hydroponics, which differs from the results obtained in this work.

For the number of leaves in the hydroponic system, the fertilizer treatments at all doses showed significant differences compared to the water-only treatments (Figure 8a). The highest number of leaves per plant was observed in the 20% biochar treatment irrigated with fertilizer (Figure 8a).

In the conventional system irrigated with water, significant differences were found in the number of leaves per plant between the biochar treatments and the control treatment, which was higher for the biochar treatments (see Figure 8a). For the fertilizer treatments, no significant differences were found between treatments, but the highest values were found for the 10% and 15% biochar treatments (Figure 8a). These results suggest that both water irrigation alone and with fertilizer influence leaf number in lettuce plants, regardless of the cultivation system used. These findings are consistent with previous studies that have demonstrated the influence of irrigation and fertilization on plant growth and development [84,86].

For the hydroponic system, the LAI was affected by the irrigation of the treatments, with fertilizer plants having the highest value (Figure 8b). However, the LAI decreased when biochar was added. The control irrigated treatments did not show significant differences between them for LAI.

For the conventional system, significant effects on the LAI were found between the control treatments (irrigated with water). Treatments with doses SB5 and SB15 were significantly higher than the treatment without biochar. In the treatments irrigated with fertilizer, an increasing trend in LAI was observed as the biochar percentage rose, with the highest values seen in the 5% and 10% biochar treatments. However, a decline in LAI was noted when the biochar dose exceeded 10%. Despite these trends, no significant differences were found between treatments (Figure 8b).

These results suggest that both irrigation and fertilization affect the LAI of lettuce plants, which may have important implications for photosynthesis and crop productivity. These findings are consistent with previous studies that have demonstrated the influence of irrigation and fertilization on plant physiology [86,87]. The results of this study are consistent with those reported by Fontana et al. [9], who evaluated these variables in three cultivation systems, conventional, organic, and hydroponic, the latter being the one that presented the highest value. On the other hand, Dispenza et al. [88], when evaluating the leaf area of potted Euphorbia × lomi plants, showed better values when using intermediate doses of wood biocarbon.

Plants grown in systems utilizing the WSB demonstrated improved drought resistance and higher water use efficiency compared to those grown in conventional soil. The physical structure of WSB aids in root penetration, leading to more extensive root systems. These observations illustrate how the WSB physical properties not only support plant growth but also enhance resilience to environmental stressors, thus improving agronomic performance.

4. Conclusions

This study compared the effects of walnut shell biochar (WSB) and fertilizer on lettuce (Lactuca sativa L.) growth under hydroponic and conventional systems. Additionally, it evaluated the plant’s response to mineral fertilization in both systems, providing valuable insights into the sustainable use of WSB as an agricultural input. The results demonstrated that WSB application significantly influenced lettuce growth, with different effects depending on the cultivation system:

• In hydroponics, WSB alone did not enhance growth, but when combined with fertilizer, it resulted in the highest biomass production. The 20% WSB + fertilizer treatment increased fresh weight by 45% and dry weight by 38% compared to the control, demonstrating its ability to optimize nutrient availability in soilless cultivation.
• In the conventional system, WSB alone at 15% significantly improved plant growth, increasing fresh weight by 30% and leaf number by 25%, even in the absence of fertilizer. This indicates that WSB can serve as a sustainable soil amendment by enhancing water and nutrient retention.
• The highest leaf area index (LAI) was observed in conventional soil with 15% WSB and fertilizer, achieving a 1.8-fold increase compared to the control, highlighting its role in improving canopy expansion and photosynthetic efficiency.

These findings confirm that WSB can enhance lettuce growth in conventional systems without additional fertilizer, while its combination with fertilizer maximizes productivity in hydroponics. The dual role of WSB—as a soil conditioner and a growth enhancer in nutrient-rich hydroponic environments—underscores its versatility and sustainability potential. This study provides quantitative evidence that WSB can be a viable alternative to synthetic soil amendments, reducing dependency on chemical fertilizers while recycling agricultural waste into productive use. Given that walnut shell waste is produced in large volumes globally, its conversion into biochar offers a scalable solution for sustainable agriculture and waste valorization.

Future research should focus on long-term field studies to assess the persistence and cumulative effects of WSB on soil properties, microbiota, and carbon sequestration. Economic evaluations should be performed to determine the cost-effectiveness of WSB applications in large-scale farming. Nutrient dynamics studies would be beneficial to optimize biochar-fertilizer interactions in different crop species and environmental conditions. Environmental impact assessments could be useful to quantify the reduction in greenhouse gas emissions and water savings associated with WSB use. By integrating WSB into precision agriculture strategies, it is possible to enhance crop resilience, reduce environmental impact, and promote circular economy principles, paving the way for more sustainable food production systems worldwide.