Integration of Biochar with Vermicompost and Compost Improves Agro-Physiological Properties and Nutritional Quality of Greenhouse Sweet Pepper

Abstract

Applying organic fertilizers is an issue that is acquiring high attention in modern agriculture. This study aims to evaluate the impact of the co-application of vermicompost and biochar on the growth performance and productivity of sweet pepper plants grown under greenhouse conditions. The applied treatments were as follows: 100% vermicompost (T1), 75% vermicompost + 25% biochar (T2), 50% vermicompost + 50% biochar (T3), 50% biochar + 50% compost (T4), 75% compost + 25% biochar (T5), and 100% compost (control-T6). All applied treatments were distributed randomly, and each treatment was repeated three times over two seasons. The data analysis revealed that the application of vermicompost—alone or in combination with biochar—significantly increased the plant growth measurements (plant height, SPAD value, leaf area, No. of leaves, and No. of branches), leaf nutrient content (N, P, K, and Ca), and total yield in both seasons. The application of vermicompost—alone (T1) or in combination with biochar (T2 and T3)—on average over both seasons significantly increased the total yield by 31.12%, 26.47%, and 22.53%, respectively, compared with the control treatment (T6). Furthermore, the aforementioned treatments also increased the physical quality (fresh fruit weight, fruit length, fruit diameter, and flesh thickness) and chemical quality of sweet pepper fruits [total phenol content (TPC), total soluble solids (TSS), ascorbic acid (AsA), β-carotenoids (β-Carot), and titratable acidity (TA)]. In addition, the co-application of biochar with vermicompost and compost caused a significant reduction in the fruit nitrate concentration compared with the control (T6) over two seasons. In conclusion, the simultaneous application of biochar with vermicompost and compost is a promising strategy to improve the growth performance, nutrition status, total yield, and fruit quality of pepper plants, as well as to reduce the nitrate concentration in the fruits.

1. Introduction

The increasing world population and continuing global demand for food have led to the intensification of agriculture and the overuse of chemical inputs to reduce the food gap [1]. These activities significantly reduce soil fertility and crop productivity [2], whereas the overuse of synthetic fertilizers can lead to soil degradation and cause environmental problems [1,3]. In addition, monoculture intensification of horticultural crops significantly affects soil fertility due to increased planting disease and reduced SOM content [3,4]. Several scientists have reported that agricultural intensification also causes a significant shift in soil microbial diversity and a reduction in soil organic matter and nutrient content [5,6]. In addition, the continued long-term cultivation of cash crops such as vegetable crops will not only hinder the growth of beneficial microbes and stimulate the increase of plant disease but also lead to a decline in crop quantity and quality. Meanwhile, Zhang et al. [7] stated that the cultivation of sweet pepper for the long term has led to the frequent occurrence of pests and diseases, which have caused a continuous reduction in pepper production and fruit quality, extremely limiting the healthy and sustainable development of pepper. Therefore, enhancing the sustainable productivity of marginal land is necessary to meet global crop demands while reducing ecological impacts. The application of organic amendments offers a sustainable way to manage organic waste, enhance plant nutrition, and improve soil quality [2,5], in addition to their efficiency in reducing chemical inputs (fuel, fertilizers, and pesticides), which decreases production costs and detrimental effects on the environment [3,8,9].

Therefore, soil application of organic amendments is fundamental to enhancing organic matter and nutrient content in the soil [2,9]. Hence, preserving suitable levels of SOM and guaranteeing effective biological nutrient cycling is important to soil conservation and agricultural productivity strategies [2]. Therefore, combined application of organic and inorganic fertilizers is a promising strategy to reduce nutrient losses, improve nutrient use efficiency and utilization, and enhance crop yield and quality [2,5]. Furthermore, a single application of organic amendments, such as compost, animal byproducts, and green manure, is relatively low in plant nutrients, and large amounts must be used to supply enough nitrogen to succeeding vegetable crops [8,9,10]. In addition, timing nutrient release to coincide with the demands of the vegetable crop is a great challenge for organic farmers [10]. Therefore, researchers are still searching for suitable combinations among organic amendments that support the specific needs of vegetable production systems to improve crop yield and quality.

Biochar is a carbon-rich solid material produced by the slow pyrolysis of organic wastes at high temperatures under specific conditions. This substance can enhance the physicochemical characteristics of soil, including soil aggregation, Ec, pH, water retention porosity, organic carbon content, nutrient elements, and cation exchange capacity [9,11,12]. Likewise, the addition of these organic materials can significantly stimulate the diversity of microorganism populations by decreasing the account of harmful soil microbes, which can inhibit the growth and development of plant pathogens [4,8,12]. Nevertheless, the application of biochar only to soil is not certain to enhance soil quality and crop yields [13]. For this reason, the co-application of biochar with other organic amendments, such as vermicompost and compost, has acquired attention recently due to its favorable effects on plant growth and production under open field and greenhouse conditions [14]. Lacomino et al. [15]. stated that co-application of biochar with compost had a positive effect on SOM, nutrient content, and total yield of eggplant, fennel, lettuce, onion, rape, and tomato. A similar finding was observed by Liu et al. [16], who found that biochar mixed with compost improved SOM, nutrient content, and water retention capacity and reduced chemical fertilizer use under open field conditions.

At the same time, many experiments have also been carried out on vermicompost applications. Vermicompost is an organic product resulting from earthworm digestion and aerobic breakdown using the activities of microorganisms and macroorganisms at room temperature. This earthworm product has a higher capacity for nutrient absorption and retention and is excellent for water-holding capacity [11,12]. Vermicompost consists of a high number of microorganism communities that can improve the disease tolerance of plants by forming an inhibitory impact against many soil-borne pathogens. Currently, biochar and vermicompost are commonly used in horticultural production as vital organic materials and have shown significant benefits in improving soil fertility, increasing crop yield, and enhancing the quality of agricultural products [10,12]. Therefore, the utilization of organic amendments, in the long term, is considered one of the most important contributors to sustaining food production due to their ability to improve soil fertility and crop productivity [8].

Several studies confirmed that the supplementation of vermicompost and biochar improved the quantity and quality of cash crops [9,10,11]. Zhang et al. [12] confirmed that the incorporation of biochar and vermicompost not only enhanced SOM, nutrient absorption, and plant growth but also increased pepper’s yield and economic benefits. Other studies reported that biochar and vermicompost also reduce the accumulation of heavy metals on the soil surface, which can diminish toxicity to plants by decreasing the movement of heavy metal ions [17,18]. Various studies have revealed that the co-addition of biochar with vermicompost and compost—alone or in combination—altered the physicochemical traits of soil and microbial properties, which affected the physiological and biochemical processes of plants; it has been documented that their application also enhances water use efficacy, photosynthetic activity, nutrient use efficiency, chlorophyll content, plant hormones, and the accumulation of primary and secondary metabolic compounds in different plants [19,20]. Therefore, this study aimed to explore the impacts of vermicompost and compost—alone and in combination with biochar—on the growth performance, nutrient absorption, fruit quality, and total yield of pepper plants in clay soil.

2. Materials and Methods

2.1. Site Location, Experimental Design, and Data Analysis

Two experiments were performed in a plastic greenhouse at a private farm located at El-Ayate, Giza (29°41′58.4″ N 31°13′04.8″ E), during the winter seasons of 2020–2021 and 2021–2022 (Figure 1). The study location is a productive area cultivated with different vegetable crops in the open fields and greenhouses located at the Giza governorate. The study location has a desert climate, with an average annual temperature of 22.2 °C and 23.88 °C, and average monthly temperatures ranging from 25.9 °C in September to 33.1 °C in April. Total rainfall is quite low, ranging from 18.31 to 22.84 mm per year, most of which fell in the wet seasons during 2021/2022 and 2022/2023, with dry conditions for most of the year.

Before transplanting sweet pepper seedlings (hybrid cv. 7158), the physiochemical characteristics of the soil (

Table 1. Physical and chemical properties of the experimental soil.

Physical properties	Silt	%	12.5
	Clay		86.5
	Sand		1
	Texture		Clay
Chemical properties	pH		7.99
	EC	dS/m	2.9
	Ca++	meq/L	6.2
	Mg++		3.8
	K+		2.3
	Na+		9.435
	HCO3		4.02
	Cl−		17.2
	SO4		15.1

) and the chemical properties of the used compost, vermicompost, and biochar were determined (

Table 2. Chemical analyses of the used compost, vermicompost, and biochar. OC* = organic matter.

Chemical Characteristics	Unit	Biochar	Compost	Vermicompost
pH	-	8.5 ± 0.0	7.6 ± 0.2	6.5 ± 0.1
EC	dS/m	2.5 ± 0.3	3.2 ± 0.5	3.5 ± 0.3
OC*	%	70.22 ± 0.1	27.62 ± 0.2	25.21 ± 1.4
Total nitrogen	%	1.55 ± 0.3	1.47 ± 0.3	2.91 ± 0.2
Phosphorus	%	6.99 ± 0.2	0.86 ± 0.1	0.88 ± 0.1
Potassium	%	1.99 ± 0.02	0.80 ± 0.2	0.9 ± 0.0
Calcium	%	0.66 ± 0.2	0.45 ± 0.2	2.55 ± 0.2
Magnesium	%	0.40 ± 0.1	0.96 ± 0.1	2.11 ± 0.0
C:N	-	45.2 ± 2.3	21.57 ± 2.5	14.5 ± 1.4

). Furthermore, all the rows were covered by black plastic mulch after applying the aforementioned treatments.

The organic amendments used—biochar, vermicompost, and compost—were purchased from Miegos (https://miegos.com). These amendments were applied—alone or combined—to the soil before sweet pepper cultivation. The treatments were the following: 100% vermicompost (T1), 75% vermicompost + 25% biochar (T2), 50% vermicompost + 50% biochar (T3), 50% biochar + 50% compost (T4), 75% compost + 25% biochar (T5), and 100% compost (control, T6). The quantity of applied treatments in experimental soil is shown in

Table 3. Quantity of vermicompost, compost, and biochar used during the experimentation (T.ha⁻¹).

Treatments	Vermicompost	Biochar	Compost
Vemicompost 100% (T1)	8	-	-
Vermicompost 75% + 25% biochar (T2)	6	2	-
Vermicompost 50% + 50% biochar (T3)	4	4	-
Compost 50% + biochar 50% (T4)	-	4	4
Compost 75% + biochar 25% (T5)	-	2	6
Compost 100% (control-T6)	-	-	8

. All applied treatments were planned as a randomized complete block design (RCBD), with three replicates for each treatment. The soil of the experimental land was ploughed well and divided into 18 experimental units (5 m²). The pepper seedlings were transplanted on 2nd of September at a space of 40 cm between plants and 1 m between rows (0.4 m × 1 m). A drip irrigation system was used to irrigate the pepper plants. After 15 days of sweet pepper cultivation, supplementary organic fertilizers (Liquid CMS and Mestopower FP101) were sprayed once every two weeks for the plants grown in all treatments to correct the plant nutrition (Supplementary Table S1). Furthermore, the organic insecticides (anti-insect and fly-free) and fungicides (blight stop) were used to control the pests (white fly, aphid, and red mite) and diseases (powdery mildew), respectively, during pepper growth. These pesticides were purchased from the Laboratory of Organic Agriculture of the Agricultural Research Centre, Giza, Egypt. After 60 days of pepper cultivation, the plant samples were collected to determine the morphological, chemical, and physiological characteristics of pepper plants. The life cycle of pepper plants in this experiment was finished on 30 April 2021 and 2022.

2.2. Analyzed Traits

Three pepper plants were chosen randomly from each replicate to measure the morphological, chemical, and physiological traits. Furthermore, the sweet pepper yield was determined over the season. For the determination of fruit quality, five fruits were collected from each replicate to assess the average weight, flesh thickness, fruit diameter, and fruit length in each harvest and then throughout the whole season.

-

Plant height: measured from cotyledon level to the terminal bud 90 days after transplanting.

-

Number of branches and number of leaves: average number of leaves and average number of branches were counted 90 days after transplanting.

-

Total chlorophyll: ten mature leaves from terminal bud from each experimental plot were selected and measured 90 days after transplanting of the two seasons using Minolta Chlorophyll Meter (SPAD 502 Minolta Co., Osaka, Japan).

-

Leaf nutrient content: The fifth leaf from the top of the plant was collected from three plants in each replicate for chemical analysis 90 days after transplanting in both seasons. The collected samples were dried at 70 °C before chemical analysis using a forced-air oven. The concentrations of N, P, K, and Ca in the dry leaves were assessed. Nitrogen, phosphorous, and potassium were estimated in the acid-digested solution using Micro-Kjeldahle for N determination, colorimetric method (ammonium molybdate) using a spectrophotometer (Model 6300, Jenway, UK) for P determination, and flame photometer to determine potassium and calcium (Model PFP7, Jenway, UK) according to methods labelled by Chapman and Pratt [21].

-

Total yield: cumulative yield and total fruit weight was recorded throughout the whole season (number and weight/m²).

-

Physicochemical contents of fruits: A sample of three fruits was taken 120 days after transplanting to assess the chemical contents. In this respect, ascorbic acid, total phenol, total soluble solids (TSS), and β-carotene were determined in fresh fruits. However, total N, P, K, and Ca were evaluated in dried fruits using the oven, as follows. Fresh fruit ascorbic acid (vitamin C) was determined using the 2, 6 Dichlorophenol indophenol technique, as defined by A. O. A. C. [22]. TSS% was determined using a hand refractometer (Model PAL-1, Atago, Tokyo, Japan), total phenols were evaluated using the Folin–Ciocalteu method described by Hernández et al. [23], and β-carotene was analyzed by the technique of Mejia et al. [24]. The dry weight of the fruit was estimated using an air-forced oven at 70 °C. The concentration of N, P, and K in the dry fruits was determined using the techniques mentioned previously for the leaves. Furthermore, the titratable acidity of fruit was determined using the technique stated, by using 5 g of pepper samples mixed with 50 mL of distilled water, then the mixture was filtrated. The filtrate was titrated with NaOH (0.1 N) utilizing phenolphthalein as an indicator until the pH of the filtrate reached to 8.1; values are reported as g L⁻¹ of citric acid fresh weight using citric acid as a control [25]. The fruit diameter and flesh thickness were assessed using digital feet, while the fruit length was determined using a graded ruler.

2.3. Data Analysis

The current study’s data were subjected to a one-way analysis of variance using the Statistical 7 program after performing a normality distribution test according to the Kolmogorov–Smirnov test [26]. The Duncan test was used to assess the significant differences among treatments at a level of 5%. Furthermore, heatmap correlations were conducted using Science and Research online plot (SRplot), an online platform for data analysis [27].

3. Results

3.1. Morphological Traits of Sweet Pepper Plants

The data in

Table 4. Impacts of compost, biochar, and vermicompost and their interactions on vegetative growth measurements of pepper plants. Means having similar small letter(s) in the same column are not statistically different (p ≤ 0.05) according to Duncan’s multiple range test (p ≤ 0.05). ±Values indicate to standard error. T1 = 100% vermicompost, T2 = 75% vermicompost + 25% biochar, T3 = 50% vermicompost + 50% biochar, T4 = 50% biochar + 50% compost, T5 = 75% compost + 25% biochar, and T6 = 100% compost (control).

Treatment	Plant Height (cm)	Number of Branches	Number of Leaves	Leaf Area (cm2)	SPAD Value	Plant Height (cm)	Number of. Branch	Number of Leaves	Leaf Area (cm2)	SPAD Value
	Season 1					Season 2
T1	63.33 ± 2.1 a	2.11 ± 0.11 a	118.2 ± 7.36 a	78.41 ± 3.20 a	59.01 ± 3.01 a	70.96 ± 3.5 a	1.96 ± 0.09 a	119.4 ± 6.1 a	79.77 ± 3.9 a	56.95 ± 2.8 a
T2	62.41 ± 3.1 a	2.07 ± 0.13 a	110.6 ± 4.99 b	71.29 ± 2.9 b	54.89 ± 2.96 ab	62.59 ± 2.9 b	2.11 ± 0.11 a	113.7 ± 5.6 ab	73.4 ± 3.70 b	55.26 ± 3.1 ab
T3	58.78 ± 2.9 b	2.00 ± 0.10 a	105.6 ± 5.23 b	67.81 ± 3.39 c	53.9 ± 2.65 bc	61.63 ± 3.1 b	2.00 ± 0.10 a	111.6 ± 5.61 b	68.17 ± 1.9 c	53.89 ± 2.06 bc
T4	56.04 ± 2.1 c	1.56 ± 0.09 b	104.11 ± 3.24 c	66.31 ± 2.67 c	53.1 ± 2.09 bc	62.15 ± 3.1 b	1.41 ± 0.08 b	104.9 ± 4.92 c	66.41 ± 4.1 c	52.22 ± 2.66 c
T5	52.70 ± 2.3 d	1.52 ± 0.07 b	100.11 ± 4.71 c	63.8 ± 3.01 cd	51.853.01 ± c	57.70 ± 2.6 c	1.44 ± 0.71 b	102.1 ± 5.11 c	64.90 ± 3.2 c	52.20 ± 3.06 c
T6	47.41 ± 3.4 e	1.44 ± 0.076 b	94.221 ± 501 d	61.96 ± 0.9 d	46.53 ± 2.99 d	48.22 ± 2.1 d	1.41 ± 0.08 b	93.93 ± 4.7 d	61.4 ± 3.07 d	46.47 ± 1.99 d

show the vegetative growth measurements—including plant height, number of branches, number of leaves, leaf area, and SPAD values—significantly affected by using vermicompost and biochar application. Hence, the pepper plants amended with vermicompost—alone or in combination with biochar at different rates—showed higher plant height, number of leaves, leaf area, and SPAD values compared to the control (T6) in both seasons. Furthermore, the pepper plants amended with vermicompost alone (T1) showed the highest value of plant height, number of branches, number of leaves, leaf area, and SPAD value. Compared with the control treatment (T6), the application of vermicompost alone (T1), 75% vermicompost + 25% biochar (T2), and 50% vermicompost + 50% biochar (T3) increased the plant height by 25.15%, 24.05%, and 19.36% in the first season and 32.05%, 22.96%, and 21.76% in the second season, respectively. A similar trend was observed in the number of branches, number of leaves, leaf area, and SPAD value. The application of T1, T2, and T3 caused a significant enhancement in the number of branches by 31.75%, 30.4%, and 28% in the first season and 28.06%, 33.18%, and 29.5% in the second season, respectively. Also, the aforementioned treatments (T1, T2, and T3) raised the leaf area by 20.7%, 13.1%, and 8.63 in the first year and 23.03%, 16.35%, and 9.93% in the second year, respectively, compared to the control treatment. Compared with the T6 treatment, the addition of T1, T2, and T3 treatments into soil increased the SPAD value of cucumber leaves by 21.15%, 15.23%, and 13.67% in the first year and 18.40%, 15.91%, and 13.77 in the second year, respectively. Likewise, co-application of biochar with compost (T4 and T5) also significantly increased the plant height, number of leaves, leaf area, and SPAD values of cucumber plants, but the values were lower than with vermicompost compared with the compost treatment (T6).

3.2. Fruit Yield

Co-application of biochar with compost and vermicompost significantly increased the pepper yield compared with the control treatment (T6), as presented in Figure 2. The highest value of total yield was observed in the pepper plants amended with vermicompost alone, and the lowest value of total yield was recorded in the pepper plants amended with T6 treatment. The enhancement ratio in total yield of pepper plants amended with T1 reached 32.37%, 26.89%, 25.09%, 13.46% and 5.54% in the first season and 29.87%, 26.05%, 19.97%, 12.70%, and 5.21% in the second season, respectively. Additionally, the co-application of biochar with compost (T4 and T5) also increased the quantity of pepper fruits, but the values of total yield were lower than vermicompost treatments (T1, T2, and T3) compared with the control treatment (T6) in both seasons.

3.3. Nutrient Content in Leaves and Fruits of Pepper Plants

The concentration of nutrients in the leaves and fruits of cucumber plants is significantly affected by the use of organic materials (N, P, K, and Ca), as shown in

Table 5. Impacts of compost, biochar, and vermicompost and their interactions on nutrient content in leaves and fruits of pepper plants. Means having similar small letter(s) in the same column are not statistically different (p ≤ 0.05) according to Duncan’s multiple range test. ±Values indicate to standard error. T1 = 100% vermicompost, T2 = 75% vermicompost + 25% biochar, T3 = 50% vermicompost + 50% biochar, T4 = 50% biochar + 50% compost, T5 = 75% compost + 25% biochar, and T6 = 100% compost (control).

Treatment	N%	P%	K%	Ca%	N%	P%	K%	Ca%
	Season 1				Season 2
Leaves
T1	4.23 ± 0.21 b	0.357 ± 0.017 a	4.160 ± 0.21 b	1.240 ± 0.062 ab	4.367 ± 0.42 b	0.346 ± 0.017 a	4.358 ± 0.22 b	1.270 ± 0.02 a
T2	4.35 ± 0.22 a	0.367 ± 0.018 a	4.343 ± 0.19 a	1.267 ± 0.49 a	4.536 ± 0.76 a	0.35 ± 0.017 a	4.532 ± 0.78 a	1.28 ± 0.063 a
T3	4.08 ± 0.19 c	0.350 ± 0.015 a	4.047 ± 0.17 c	1.23 ± 0.59 bc	4.224 ± 0.63 c	0.332 ± 0.026 a	4.112 ± 0.93 c	1.254 ± 0.048 a
T4	3.88 ± 0.19 d	0.313 ± 0.006 b	3.52 ± 0.18 d	1.183 ± 0.60 d	4.031 ± 0.81 d	0.30 ± 0.014 b	3.623 ± 0.88 d	1.176 ± 0.10 b
T5	3.74 ± 0.21 e	0.307 ± 0.008 b	3.533 ± 0.18 d	1.203 ± 0.56 cd	3.586 ± 0.71 e	0.293 ± 0.012 b	3.659 ± 0.23 d	1.193 ± 0.09 b
T6	3.30 ± 0.18 f	0.260 ± 0.012 c	3.51 ± 0.13 d	1.127 ± 0.42 e	3.422 ± 0.75 f	0.246 ± 0.016 c	3.354 ± 0.27 e	1.112 ± 0.024 c
Fruits
T1	2.433 ± 0.12 b	0.287 ± 0.014 a	2.467 ± 0.23 a	0.012 ± 0.001 a	2.49 ± 0.13 b	0.281 ± 0.014 a	2.5 ± 0.15 a	0.014 ± 0.0007 a
T2	2.72 ± 0.054 a	0.260 ± 0.019 b	2.373 ± 012 b	0.010 ± 0.0005 b	2.76 ± 0.14 a	0.281 ± 0.013 a	2.44 ± 0.11 b	0.011 ± 0.0005 b
T3	2.05 ± 0.10 d	0.24 ± 0.09 c	2.30 ± 0.18 c	0.009 ± 0.0004 c	2.09 ± 0.104 c	0.252 ± 0.009 b	2.36 ± 0.09 c	0.015 ± 0.0008 a
T4	2.12 ± 0.11 cd	0.25 ± 0.097 bc	2.190 ± 0.51 d	0.0076 ± 0.0004 cd	2.14 ± 0.11 c	0.244 ± 0.008 bc	2.24 ± 0.14 de	0.009 ± 0.0004 cd
T5	2.191 ± 0.09 c	0.237 ± 0.08 c	2.22 ± 0.044 d	0.009 ± 0.00044 c	2.16 ± 0.076 c	0.231 ± 0.011 c	2.27 ± 0.31 d	0.010 ± 0.0004 cb
T6	1.95 ± 0.14 d	0.213 ± 0.101 d	2.137 ± 0.52 e	0.007 ± 0.00034 d	1.83 ± 0.098 d	0.23 ± 0.012 c	2.17 ± 0.12 e	0.008 ± 0.00039 d

. Over two seasons, the highest concentration of N, P, K, and Ca was recorded in the leaves of pepper plants treated with T2, followed by T1 and T3, compared to the control (T6). Also, the application of T4 and T5 showed higher concentrations of P and Ca in the leaves of pepper plants than the control treatment in both seasons. A similar finding was noted in the fruits of pepper plants. Higher concentrations of N, P, K, and Ca were recorded in the fruits of pepper plants treated with T1, T2, T3, T4, and T5 compared to T6 treatment in both seasons.

3.4. Fruit Nitrate Content of Sweet Pepper

Simultaneous application of biochar with vermicompost and compost significantly influenced nitrate concentration in the pepper fruit (Figure 3). The highest concentration of nitrate (NO₃) was noted in the fruits of pepper plants amended with T1 and T2, followed by T3, and the lowest concentration was observed in T4 and T5, compared to the control treatment (T6) in both seasons. This result indicates that the integration of biochar with vermicompost and compost reduced the nitrate concentration in the fruits. Compared with the T1 treatment (vermicompost), the reduction ratio in leaf NO₃ reached −14.5%, −4.86, −21.60%, −20.26%, and −14.50% in the first season and −22.43%, −11.57%, −27.30%, −26.45, and −22.43% in the second season for T2, T3, T4, and T5, respectively. In fact, the mean nitrate concentrations of pepper treated with vermicompost—alone or combined with biochar—were below the maximum level of 2500 mg/kg specified by European Union and 100 mg/Kg identified by WHO [28].

3.5. Fruit Quality of Sweet Pepper

The application of vermicompost, compost, and biochar and their interactions caused a significant variation in the physiochemical quality of pepper fruits (

Table 6. Impacts of compost, biochar, and vermicompost and their interactions on physical quality of pepper fruits. Means having similar small letter(s) in the same column are not statistically different (p ≤ 0.05) according to Duncan’s multiple range test. ±Values indicate to standard error. T1 = 100% vermicompost, T2 = 75% vermicompost + 25% biochar, T3 = 50% vermicompost + 50% biochar, T4 = 50% biochar + 50% compost, T5 = 75% compost + 25% biochar, and T6 = 100% compost (control).

Treatment	Fresh Fruit Weight	Flesh Thickness	Fruit Length	Fruit Diameter	Dry Fruit Weight	Fresh Fruit Weight	Flesh Thickness	Fruit Length	Fruit Diameter	Dry Fruit Weight
	(g)	(mm)	(cm)	(cm)	(g)	(g)	(mm)	(cm)	cm	(g)
	Season 1					Season 2
T1	175.35 ± 9.01 a	5.92 ± 0.30 a	8.96 ± 0.45 a	6.49 ± 0.23 a	22.42 ± 1.12 a	174.9 ± 9.05 a	5.99 ± 0.30 a	8.86 ± 0.44 a	6.47 ± 0.32 a	22.24 ± 1.12 a
T2	168.94 ± 8.44 b	5.48 ± 0.27 b	8.57 ± 0.85 b	6.44 ± 0.41 a	21.60 ± 1.08 a	168.6 ± 8.43 b	5.54 ± 0.28 b	8.62 ± 0.43 b	6.52 ± 0.35 a	21.68 ± 1.04 a
T3	165.74 ± 8.23 c	5.50 ± 0.28 b	8.02 ± 0.41 c	6.46 ± 0.33 a	21.52 ± 1.07 a	164.9 ± 7.98 c	5.53 ± 0.21 b	8.02 ± 0.37 c	6.53 ± 0.24 a	21.76 ± 1.09 a
T4	159.88 ± 8.01 d	5.03 ± 0.25 c	7.50 ± 0.52 d	6.10 ± 0.31 b	19.67 ± 0.98 b	160.3 ± 8.21 d	5.03 ± 0.22 c	7.55 ± 0.23 d	6.07 ± 0.31 b	19.39 ± 0.96 b
T5	150.31 ± 7.51 e	4.54 ± 0.43 d	6.49 ± 0.32 e	5.51 ± 0.27 bc	18.87 ± 0.85 b	152.1 ± 7.605 e	4.43 ± 0.19 d	6.45 ± 0.11 e	5.50 ± 0.27 c	17.52 ± 0.86 c
T6	142.84 ± 7.14 f	4.00 ± 0.19 e	6.51 ± 0.42 e	5.07 ± 0.43 c	15.88 ± 0.78 c	144.44 ± 7.22 f	4.04 ± 0.32 e	6.37 ± 0.24 e	5.09 ± 0.20 d	16.10 ± 0.79 d

and

Table 7. Impacts of compost, biochar, and vermicompost and their interactions on chemical quality of pepper fruits. Means having similar small letter(s) in the same column are not statistically different (p ≤ 0.05) according to Duncan’s multiple range test. TPC = total phenol compounds, T.S.S = total soluble solids, β-carto = β-carotenoids, AsA = Ascorbic acid, TA = titratable acidity. ±Values indicate to standard error. T1 = 100% vermicompost, T2 = 75% vermicompost + 25% biochar, T3 = 50% vermicompost + 50% biochar, T4 = 50% biochar + 50% compost, T5 = 75% compost + 25% biochar, and T6 = 100% compost (control).

Treatment	TPC	β-carot.	AsA	TA	Proline	T.S.S	TPC	β-carot.	AsA	TA	Proline	T.S.S
	(mg/gGAE D.M)	(mg/Kg)	(mg/100)	(%)	(µmol/g FW)	(%)	(mg/gGAE D.M)	(mg/Kg)	(mg/100 g f.w)	(%)	(µmol/g FW)	(%)
	Season 1						Season 2
T1	14.19 ± 0.71 a	73.27 ± 4.02 a	140.52 ± 7.02 a	0.380 ± 0.201 a	0.188 ± 0.009 a	8.06 ± 0.40 a	14.43 ± 0.80 a	73.47 ± 4.01 a	140.86 ± 7.1 a	0.381 ± 0.019 a	0.191 ± 0.01 a	8.08 ± 0.404 a
T2	13.87 ± 0.69 b	68.49 ± 4.34 b	141.47 ± 7.10 a	0.360 ± 0.018 a	0.176 ± 0.008 b	7.90 ± 0.39 a	14.13 ± 0.72 b	68.59 ± 3.34 b	141.52 ± 7.1 a	0.362 ± 0.018 a	0.179 ± 0.009 b	7.82 ± 0.3091 a
T3	12.91 ± 0.55 c	67.29 ± 2.99 c	137.98 ± 6.89 b	0.259 ± 0.012 b	0.173 ± 0.0086 c	7.49 ± 0.37 b	13.05 ± 0.70 c	67.45 ± 3.93 c	138.57 ± 6.9 b	0.261 ± 0.013 b	0.165 ± 0.0082 d	7.57 ± 0.379 b
T4	13.10 ± 0.50 c	63.97 ± 3.12 d	134.68 ± 6.87 c	0.153 ± 0.008 c	0.166 ± 0.007 d	7.43 ± 0.38 b	13.01 ± 0.65 c	63.87 ± 2.98 d	134.62 ± 6.76 c	0.152 ± 0.007 c	0.168 ± 0.0084 c	7.34 ± 0.367 b
T5	12.31 ± 0.62 d	63.53 ± 3.75 d	135.13 ± 5.99 c	0.147 ± 0.007 c	0.166 ± 0.005 d	7.19 ± 0.39 bc	12.21 ± 0.61 d	63.58 ± 2.88 d	135.03 ± 6.80 c	0.145 ± 0.0072 c	0.168 ± 0.011 c	6.99 ± 0.349 c
T6	11.65 ± 0.56 e	62.52 ± 2.34 e	130.99 ± 4.98 d	0.130 ± 0.0065 d	0.162 ± 0.004 e	6.92 ± 0.29 c	11.85 ± 0.59 e	62.34 ± 2.87 e	131.23 ± 5.96 d	0.131 ± 0.0066 c	0.165 ± 0.0012 d	6.89 ± 0.344 c

). In both seasons, the application of vermicompost alone (T1) showed the highest values of dry fruit weight, fruit length, fruit diameter, flesh thickness, and fresh fruit weight of pepper plants compared to all the other treatments (Table 6). Compared with T6 treatment, application of vermicompost alone (T1) improved fresh weight, fruit thickness, fruit length, fruit diameter, and dry weight of cucumber fruits by 18.54%, 32.43%, 27.34%, 17.26%, and 29.17% in the first season and 17.42%, 32.55%, 28.10%, 18.24%, and 27.61% in the second season, respectively. Furthermore, the application of T2, T3, T4, and T5 treatments also increased the aforementioned parameters of pepper fruits compared with compost alone (T6). These results indicated that vermicompost—alone or in combination with biochar—positively improved the fruit physical quality of pepper plants compared to compost alone.

Similar trends were observed in the chemical compositions of pepper fruits (Table 7). The highest concentrations of total phenol content (TPC), β-carotenoids (β-Carot), ascorbic acid (AsA), titratable acidity (TA), total soluble solids (TSS), and proline were observed in the fruit of pepper plants treated with T1, followed by T2, T3, T4, and T5 treatments, in comparison to T6 (control) in both seasons. Moreover, application of vermicompost alone (T1) enhanced the TPC, β-Carot, AsA, TA, and TSS by 17.98%, 14.67%, 6.78%, 65.78%, 13.83%, and 14.14% in the first season and 18%, 15.15%, 6.84%, 65.62%,13.62%, and 14.73% in the second season, respectively, compared with the T6 treatment.

3.6. Correlation Study

Analysis of matrix correlation and cluster heatmap display the variations in agro-physiological and biochemical characteristics of sweet pepper plants treated with compost, vermicompost, and biochar and their interactions (Figure 4 and Figure 5). The cluster heatmap based on the 23 parameters divided them into dual clusters (A and B). Cluster A contains T1, T2, and T3, and Cluster B consists of T3, T4, and T5 (Figure 3). The heatmap correlation also points out that vermicompost (T1) has a great positive impact on the studied measurements, except for plant height. A similar effect was observed in vermicompost plus biochar (T2). This treatment also has a positive effect on the studied measurements, except for leaf calcium content, and it has a negative effect on the fruit’s nitrate content. Conversely, the T6 treatment has a negative impact on all studied parameters.

Similarly, matrix correlation analysis was utilized to identify the positive and negative relationships between the determined parameters in this study. A positive association (blue color) and a negative relationship (red color) are presented in Figure 5. This correlation analysis exhibited that fruit yield positively correlated with leaf area, number of leaves, plant height, and nutrient content (Ca, K, and P) in leaves and fruits. A similar correlation also was noted between the fruit yield of pepper plants and antioxidant compounds (TPC and ASA).

4. Discussion

The current study has shown that the growth and production of sweet pepper plants improved when these plants were treated with compost and vermicompost integrated with biochar, whereas the co-application of vermicompost and compost with biochar significantly augmented plant height, number of branches, number of leaves, leaf area, and SPAD value (Table 4). Similar findings were detected by Zhang et al. [12], who mentioned that soil implementation of vermicompost—alone or in combination with biochar—enhanced the growth and production of pepper plants. Other researchers confirmed that the combined application of biochar with compost and vermicompost increased the plant height, leaf area, number of leaves, chlorophyll content, and nitrate leaf content of the Swiss chard plant [29]. Manzoor et al. [30] also stated that the co-application of biochar with organic amendments or chemical fertilizers significantly upsurges the root growth, nutrient uptake, and yield of tea plants.

In this study, the improvement in the growth performance of sweet pepper plants amended with vermicompost—alone or combined with biochar—might be due to a higher release of nutrients, particularly nitrogen [30,31]. Furthermore, the vital significance of nitrogen, potassium, and phosphorus in improving plant growth, cell division, and elongation could be another possible explanation. Matrix correlation in this study confirmed the previous result, where leaf nutrient content (N, P, K, and Ca) correlated positively with plant height, number of leaves, number of branches, chlorophyll content (SPAD value), and leaf area (Figure 4 and Figure 5). This finding indicates that integrated biochar with compost and vermicompost supplied sufficient nutrients to plants and soil over the plant growth cycle [29]. Other studies also stated that vermicompost activates the enzymes associated with chlorophyll synthesis, plant growth, and fruit development and increases the availability of nutrients in the soil [32]. Arancon et al. [33] reported that vermicompost consists of a suitable quantity of humic acid, growth hormones, amino acids, cytokinins, and available nutrients, which stimulate plant development and production. Other studies reported that the co-application of biochar with vermicompost and compost enhanced the biomass and activity of beneficial bacteria and fungi in the soil [4,7,18,19]. These microorganisms can promote plant growth by increasing nutrient availability, enhancing nutrient uptake by plant roots, producing plant-growth-regulating hormones and enzymes, reducing the mobility of heavy metals, and inhibiting soil-borne diseases [17,18,20,30,33,34,35]. Moreover, vermicompost and biochar addition can stimulate the growth and activity of plant growth-promoting fungi, enhancing plant nutrient uptake and tolerance to environmental stresses such as salinity [36]. It can be summarized that co-applying compost and vermicompost with biochar can improve soil fertility and beneficial microbial biomasses, consequently enhancing pepper plants’ growth performance.

Many publications have revealed that organic materials are rich in elements and small organic compounds that are essential for crop growth and development. These compounds are easily transformed into active components that might be absorbed and used by plants with the assistance of microorganisms, facilitating the accumulation of nutrients in plants [2,4,6,8,32]. Application of organic amendments to soil can increase the activity of soil microorganisms and soil nutrient content, which is also beneficial for promoting nutrient decomposition and release, as well as plant uptake and utilization of elements [34]. In this study, higher nutrient accumulation (N, P, K, and Ca) was detected in the leaves and fruits of sweet pepper plants treated with vermicompost—alone or in combination with biochar (Table 5). This accumulation of nutrients in different parts of plants is not only related directly to available nutrients supplied by vermicompost or biochar to plants but also associated indirectly with an increase in the availability of endogenous nutrients in the soil by promoting the activity of soil microorganisms. Several publications have shown that biochar can be utilized as a substitute for traditional phosphorus fertilizers because of its essentially highly effective phosphorus content, which can also change the cycling and effectiveness of phosphorus in soils through the adsorption and desorption of phosphorus and regulation of the soil microbial community structure [12,35,36,37,38,39,40]. Likewise, biochar can also increase the effective soil potassium content and improve potassium fertilizer use efficiency by stimulating microbial activity. In addition, vermicompost consists of essential elements in forms that are ready for uptake by crop roots, such as nitrogen (nitrates), available phosphorus, soluble potassium, calcium, and magnesium. Vermicompost also stimulates phosphate-potassium organisms and nitrogen-fixing bacteria in the root rhizosphere, which improves N, P, and K availability in the soil [41,42].

Furthermore, the heatmap correlation confirmed that vermicompost alone (T1) and vermicompost plus biochar (T2) have positive effects on the N, P, K, and Ca accumulation in the leaves and fruits of sweet peppers (Figure 4). This result indicated that the addition of vermicompost and biochar increased the nutrient availability and nutrient uptake more than the application of vermicompost alone. This could also be associated with increasing the microbial communities supplied by vermicompost. Wu et al. [43] also suggested that the co-application of vermicompost with biochar into soil cultivated by tomato significantly increased bacteria and fungi populations such as actinomycetes, acidobacteria, ascomycetes, and aspergillus, and reduced the abundance of aspergillus. Meanwhile, it increased the activities of urease and alkaline phosphatase. These enzymes play a vital role in increasing the available nitrogen and phosphorus in soil [43,44]. The co-application of vermicompost and biochar also improves soil aggregation, porosity, soil aeration, water retention, and water hold capacity, as well as enhances cation exchange capacity, which plays an indirect role in the translocation of nutrients from the soil into the plant through plant roots [44,45]. It can be concluded that the co-application of vermicompost and biochar improves the physiochemical and microbial properties of soil, which enhances nutrient availability, reduces nutrient losses, and facilitates nutrient uptake during the growing season according to crop needs [33]. Similar findings were observed by Zhang et al. [12], who reported that the co-application of vermicompost with biochar increased nutrient accumulation in pepper plants more than vermicompost alone.

Remarkably, the co-application of biochar with vermicompost and compost in the present study resulted in a better upsurge in fruit yield than compost alone (control), as shown in Figure 2. The higher fruit yield was achieved in pepper plants amended with vermicompost alone, followed by vermicompost plus biochar, and compost plus biochar, compared to compost alone (Figure 2). This result was also documented by previous publications [12,46,47]. The enhancements in sweet pepper yield supplied with vermicompost—alone and in combination with biochar—and compost plus biochar might be associated with the nutrient availability and microbial environment of soils, mainly by preventing the development of soil-borne diseases [4,12,43,48,49]. This finding is in agreement with numerous investigations that stated that the co-application of biochar with vermicompost and compost increased the production of pepper, tomato, Swiss chard, eggplant, and cucumber [12,32,38,48,50]. This study exhibited that the addition to soil of vermicompost—alone or in combination with biochar—and compost plus biochar increased the diameter, length, flesh thickness, and fresh fruit weight compared to control plants. These findings are in agreement with those of Ebrahimi et al. [51], who observed the addition of vermicompost—alone or in combination with biochar—increases the fruit size, fruit length, fruit width, and wall thickness of sweet pepper. Likewise, the application of biochar with compost and vermicompost in this study also improved the concentration of ascorbic acid (AsA), β-carotenoids (β-Carot.), proline, total phenol compounds (TPC) and titratable acidity (TA) in the fresh pepper fruits. Additionally, matrix correlation showed the nutrient concentration in leaves and fruits (N, P, K, and Ca) correlated positively with fruit quality (proline, AsA, β-Carot, and TPC), as shown in Table 5, Table 6 and Table 7. Similar findings were observed by several researchers who stated that the co-application of vermicompost, compost, and biochar improves the quantity and quality of fruits, such as reducing sugars, TSS, total phenol, ascorbic acid, amino acids, and nutrient content [12,32,52,53].

The improvement in fruit quality of plants treated by vermicompost—alone or in combination with biochar—might be due to the ability of these amendments to improve soil structure and increase nutrient availability, water retention, and soil aeration. This could increase the growth of plant roots, which can absorb water and nutrients from different layers of soil, consequently leading to improved chlorophyll content and photosynthesis rate in plant leaves [12]. Other scientists confirmed that soil application of vermicompost and biochar together augmented the nutrient uptake and enhanced the chlorophyll content and photosynthesis rate of plant leaves and improved the transmission of photosynthetic metabolites to the fresh fruit, which ultimately increased the fruit quantity and quality [53,54]. Furthermore, the accumulation of secondary metabolites, such as β-carotenoids, AsA, TPC, total carbohydrate, TSS, polyphenols, and antioxidants, in broccoli plants increased due to improvements in the photosynthesis system of the plants [55,56]. Similarly, the positive effect of vermicompost might be described by the fact that vermicompost increases essential amino acids in plants, which may contribute to the soluble sugar metabolic process and endorse the improvement of soluble sugar content in fresh fruits [56,57].

In this study, the highest accumulation of nitrate (NO₃) was found in the fruits of pepper plants treated with vermicompost alone, but the NO₃⁻ concentration was under the threshold identified by the European Union. On the contrary, the combined utilization of biochar with compost and vermicompost reduced the concentration of nitrate in the fresh pepper fruits (Figure 2). Several authors reported that biochar application upsurges the absorption of ammonium (NH₄⁺) and lessens the transformation of NH₄⁺ to NO₃⁻, which assists the reduction of nitrate content in the fruits. Nevertheless, the input of amino acid nitrogen into vermicompost significantly reduced in the nitrate content of fruits through the inhibition of nitrate reductase, which may be responsible for the reduction in the nitrate content of fruits [12,35,46,58]. Such findings were noted by Zhang et al. [12], who confirmed that the co-application of biochar and vermicompost decreased the NO₃⁻ content in sweet pepper fruits. According to the above, it can be summarized that the co-application of biochar with vermicompost and compost improves plant growth, fruit yield, and fruit quality due to enhancing the nutrition status and physiological processes, consequently increasing plant growth performance and yield quality.

5. Conclusions

This study suggests that the application of vermicompost—alone or in combination with biochar—was beneficial for increasing growth performance and improving crop yield and fruit quality of sweet pepper plants grown under greenhouse conditions. Furthermore, the co-application of biochar with vermicompost and compost increased the nutrient concentration of the plant leaves. At the same time, the aforementioned organic mixtures improved the nutritional value of pepper fruits. This could be associated with chemical composition and molecular compositions in terms of the elemental content of biochar and the two composts. The co-application of vermicompost with biochar also caused a significant reduction in the NO₃ concentration in pepper fruits. Overall, the simultaneous application of biochar and vermicompost could be recommended to greenhouse growers aiming to enhance fruit yield and quality while minimizing nitrate content. Further experiments are required to evaluate the effectiveness of the biochar and vermicompost mixture on the chemical and biological properties of soil and its relation to plant growth and productivity.