Effect of Biochar and Compost Addition on Mitigating Salinity Stress and Improving Fruit Quality of Tomato

Abstract

To overcome food security, sustainable strategies for reclamation and the subsequent utilization of salt-affected soils for crop production are needed. The aim of the current study was to evaluate the impacts of compost and biochar addition on the growth and fruit quality of tomato under salinity stress. For this purpose, the soil was spiked with analytical grade sodium chloride to achieve a 6 dS m⁻¹ salinity level for a pot experiment. After 30 days of spiking, the compost (2%) and biochar (2%) were added in selected pots. After the seedling transplant, recommended doses of NPK were added to fulfill nutrient requirements of tomato plants. Plants were harvested after 90 days of seedling transplantation. Results revealed that the salinity caused a significant reduction of 28.4% in SPAD value, 23.5% in Ft, 22.6% in MSI, 12.1% in RWC, 18.3% in Chl. a, 13.7% in Chl. b, and 16.5% in T. Chl. as compared to the un-amended non-saline control in physiological attributes of tomato plants. Similarly, a significant decrease of 26.9–44.1% was obtained in growth attributes of tomato as compared to the non-saline control. However, in saline soil, the addition of biochar and compost (alone or together) demonstrated a significant improvement in plant growth (i.e., up 45%) over the respective un-amended control. Moreover, the combined application of compost and biochar significantly reduced the sodium (Na⁺) in shoots and roots of tomato plants by 40% and 47%, respectively, over the respective control. Our findings suggest that the combined application of biochar and compost could be useful to reduce salinity, alleviate salinity-induced phytotoxicity, and subsequently improve plant growth and productivity in salt-affected soil.

1. Introduction

Tomato (Solanum lycopersicum L.), belonging to Solanaceae family, is an important crop and the most consumable and economically attractive crop due to its high yield and short duration [1]. Tomato as a “functional food” is enriched with minerals, vitamins, essential amino acids, lycopene, ascorbic acid, antioxidants, sugars, and dietary fibers. The total area of the world under tomato cultivation is 4.8 million hectares (mha) with the production of 163 million tons per year, and, apart from this, Pakistan produces approximately 439 thousand tons per year on 52,300 hectares area [2]. Destructive climate changes are resulting in the degradation of soil, which eventually lowers the overall crop productivity.

Globally, soil salinization is one of the most important issues that affect almost 836 Mha [3]. In Pakistan, around 6.30 Mha are highly saline [4]. It is estimated that in 2050 more than 50% of the world’s agricultural land would be affected by salinity [5]. Salt-affected soils reduce crop yields by disturbing the physicochemical properties and microbial activities in the soils of semi-arid regions [6]. The usage of low-quality and/or untreated industrial water in agriculture eventually causes soil salinity. In plants, the concentration of Na⁺ and C1⁻ ions disturbs the osmotic potential and transpiration rate which reduces the crop yield [7]. Plant roots absorb these ions from the soil solution and translocate them in stem and leaves. Due to specific ion toxicity, dehydration occurs which reduces the photosynthetic and respiration activities in plant cells [8]. Despite the fact that tomato yield decreases with an increase in salinity, it enhances the fruit quality. Elevated salinity leads the plants to modulate metabolic activity which reallocates the sucrose and organic acids from leaves to fruits, resulting in higher concentration of soluble salts in fruits [9].

Nowadays, different organic amendments such as compost, farmyard manure, biochar, animal manure, and crop residues are being used to mitigate the drastic effects of salinity. Biochar, also referred to as agricultural black gold, enhances crop yield and soil properties and sequesters more carbon (C) in soils [10]. Biochar reduces the salinity stress, Na⁺ adsorption ratio, and electrical conductivity (EC) by improving the physical, chemical, and biological characteristics of highly deteriorated soil for sustainable crop production [11,12]. Similarly, the addition of compost to soil influences plant growth positively even in a stressed environment. It contains essential nutrients, beneficial microbes, and humic substances that improve the soil’s physicochemical properties [11]. The compost is being applied to agricultural soil for hundreds of years to improve the fertility status of the soil. Under salt stress, plants are unable to uptake balanced amounts of nutrients and water from the soil, but the addition of compost improves the organic matter of the soil and provides more surface area for microbes to chelate the salted ions [13]. Moreover, the presence of more functional groups and active sites in compost attracts the salted ions and makes them unavailable for plant intake [14].

Various studies have been conducted to mitigate the toxic effects of salt stress by using compost and biochar. Moreover, biochar needs substantial time to provide sustainable nutrition, but it effectively reduces the salt concentration by absorbing the salts due to the presence of active functional sites [11]. It is unclear whether the compost showed instant effects on plant growth as it provides more readily available nutrients to plants. However, studies investigating the impacts of the combined use of compost and biochar on tomato plants under induced salinity are not well reported in the literature.

Based on the findings of the previous studies, this study aimed to evaluate the efficacy of biochar and compost addition on the growth, yield, and fruit quality of tomato grown in saline soil.

2. Materials and Methods

2.1. Chemicals

Analytical grade chemicals that were used in this study were obtained from the local scientific stores of Sigma-Aldrich (St. Louis, MO, USA), Merck (Rahway, NJ, USA), Fischer (Ried im Innkreis, Austria), and Riedel-de-Haen (Seelze, Germany).

2.2. Experimental Setup

A pot experiment was conducted in the wire-house of the Institute of Soil and Environmental Sciences (ISES), University of Agriculture, Faisalabad (UAF). For this experiment, soil was collected from the research area of ISES, UAF. A 2 mm sieve was used to sieve the air-dried soil. For soil spiking, the measured amount of soil was taken in a bucket and electrical conductivity (EC) at 6 dS m⁻¹ [15], which was developed by applying NaCl salt. The salt concentration was determined using Equations (1) and (2) [16]. After the spiking, the soil was kept for homogenization for one month [17].

$${EC}_C={EC}_r-{EC}_s$$

(1)

$$\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}\left(\frac{\mathrm{g}}{\mathrm{k}\mathrm{g}}\right)=TSS\times SP\times {EC}_c\times \frac{100}{1000}$$

(2)

where, EC_c, Calculated EC; EC_r, Required EC; EC_s, EC of soil; NaCl, Sodium chloride; TSS, Total soluble salts; SP, Saturation percentage.

Thereafter, approximately 7 kg of soil (non-saline and saline) was added in respective pots (with dimensions L × W (25 cm × 20 cm)) according to the treatment plan. Eight different treatments each with three replicates (for details see Table S1) were laid out according to a completely randomized design (CRD). A recommended dose of each nutrient, i.e., N (120 kg ha⁻¹), P (40 kg ha⁻¹), and K (70 kg ha⁻¹), was also applied in all pots irrespective of any treatment [18]. Before starting the experiment’s physicochemical parameters of soil, the biochar and compost were measured by following standard protocols [19] and are shown in Table S2. During the entire period of the experiment, metrological data were monitored and are shown in Figure S1.

2.3. Physiological, Agronomic, and Biochemical Attributes

After 35 days of tomato seedling transplantation, the SPAD value was taken by a SPAD meter (SPAD-502, Konica Minolta, Europe, Nieuwegein, The Netherlands). The SPAD value for each leaf was determined by averaging four readings from both sides of the leaf’s midrib. Similarly, a photosynthetic yield analyzer, MINI-PAM-II (WALZ Mess-und Regeltechnik, Germany), was used to measure the photosynthetic active radiation (PAR), electron transport rate (ETR), fluorescence yield (ft), and effective PSII quantum yield (YII) 35 days after seedling transplanting. The membrane stability index (MSI) and relative water contents (RWC) of leaves were determined by following the protocols described by Sairam et al. [20]. Chlorophyll a, b, and total chlorophyll were measured with a method described by Arnon [21] using a UV-visible spectrophotometer (VIS-1100, Biotechnology Medical Services K. Canada, Route Transcanadienne Pointe-Claire, Oka, QC, Canada). Three leaves from the fifth youngest node of the plant were randomly selected to be used as samples for each extract.

The plant’s agronomic attributes were noted at the time of harvesting. Plant height was measured with a measuring tape by stretching the plants along with leaves. After carefully uprooting, the roots were gently washed with tap water and then immediately dried with the help of a paper towel. The length of the primary root was measured by measuring tape and the average value is presented in results. The fresh weight of plants was measured with a weighing balance. After weighing fresh samples, plants were kept in a shady place for air drying. After this, samples were kept in a paper bag for oven drying at 65 °C until constant weight was achieved [19].

Throughout the plant growth, flowers were counted from each plant; the number of fruits per plant was counted periodically; and the average of fruits produced by a plant was recorded. Nitrogen (N), phosphorus (P), potassium (K), and sodium (Na⁺) were measured by following standard procedures. Total N was assessed with the Kjeldahl digestion method as described by Davidson et al. [22] using the Kjeldahl apparatus (DF-4S Mitamura Riken Kogyo Inc., Tokyo, Japan). For the estimation of P, K, and Na⁺, wet digestion method was used as explained by Estefan et al. [19]. Furthermore, P in digested samples was measured through a UV-visible spectrophotometer at 430 nm wavelength. Na⁺ and K⁺ were determined with a flame photometer (FP7, Jenway, London, UK) as illustrated by Chapman and Pratt [23]. Shoot (above-ground plant parts) and root samples were used for digestion and further analysis.

2.4. Quality Parameters

Total soluble solids (TSS) were measured by using a handheld digital refractometer. Titratable acidity (TA) and taste index (TI) were determined using Equations (3) and (4), respectively.

$$TA=\frac{V\times N\times 100\times 0.0064}{m}$$

(3)

where N is the normality of NaOH, 0.0064 is the conversion factor for citric acid, V is the volume of NaOH used (mL), and m is the mass of the tomato sample used (g).

$$TI=TA+\left(\frac{TSS}{20\times TA}\right)$$

(4)

2.5. Statistical Analysis

The data obtained were analyzed by applying analysis of variance (ANOVA) on two factors’ factorial design under CRD in Statistix 8.1 software. The mean comparison among multiple treatments were checked by applying Tukey’s Honest Significance Difference test and the level of significance is p ˂ 0.05, and the letters were used for mean separation [24].

3. Results

3.1. Impact of Compost and Biochar on Physiology and Growth of Tomato Plants under Salinity Stress

The present study revealed the significant impacts of compost and biochar on the physiological characteristics of tomato (

Table 1. Effects of compost and biochar addition on physiological traits of tomato plants grown in saline and non-saline soils.

Treatments	SPAD Value	Ft (µmol m−2 s−1)	PAR (µmol m−2 s−1)	YII (µmol m−2 s−1)	ETR (µmol m−2 s−1)	MSI (%)	RWC (%)	Chl. a (mg g−1 F.Wt.)	Chl. b (mg g−1 F.Wt.)	Total Chl (mg g−1 F.Wt.)
Non-saline soil
Control	47.9 ± 2.7 ab	211 ± 18.4 bc	412 ± 28.0 ab	0.60 ± 0.1 abc	82.4 ± 4.9 bc	83.1 ± 2.4 abc	73.1 ± 1.6 bc	1.73 ± 0.1 c	1.07 ± 0.1 bc	2.80 ± 0.1 b
BC	49.9 ± 2.4 ab	263 ± 38.1 ab	432 ± 29.7 ab	0.67 ± 0.1 ab	93.5 ± 3.1 ab	89.5 ± 3.0 ab	80.2 ± 2.0 ab	1.96 ± 0.1 ab	1.17 ± 0.1 ab	3.13 ± 0.2 ab
C	51.3 ± 2.9 ab	295 ± 28.0 a	466 ± 34.3 a	0.67 ± 0.1 ab	99.9 ± 4.9 a	90.5 ± 2.6 a	82.5 ± 3.1 a	2.11 ± 0.1 a	1.24 ± 0.1 ab	3.35 ± 0.1 a
BC + C	50.2 ± 1.3 c	284 ± 15.7 ab	469 ± 17.1 a	0.69 ± 0.1 a	102.9± 3.9 a	91.4 ± 2.5 a	82.8 ± 3.3 a	2.15 ± 0.1 a	1.28 ± 0.1 a	3.43 ± 0.1 a
Saline soil
S	34.2 ± 2.8 d	162 ± 14.5 cd	270 ± 22.2 d	0.51 ± 0.1 cd	61.5 ± 9.2 d	64.2 ± 3.7 d	64.2 ± 3.9 c	1.42 ± 0.1 d	0.92 ± 0.1 c	2.34 ± 0.2 cd
S + C	45.9 ± 4.0 ab	271 ± 42.6 ab	372 ± 47.6 bc	0.55 ± 0.1 bc	74.5 ± 4.1 cd	80.5 ± 2.0 bc	77.8 ± 2.9 ab	1.76 ± 0.1 bc	1.22 ± 0.1 ab	2.98 ± 0.1 b
S + BC	42.4 ± 3.1 bc	223 ± 21.5 abc	309 ± 38.9 cd	0.56± 0.1 bc	72.8 ± 4.4 cd	77.1 ± 5.7 c	75.4 ± 4.4 ab	1.67 ± 0.1 c	1.18 ± 0.1 ab	2.85 ± 0.1 b
S + BC + C	46.4 ± 3.0 ab	251 ± 22.4 ab	348 ± 23.1 bcd	0.57 ± 0.1 abc	81.7 ± 3.4 bc	82.9 ± 3.4 abc	78.9 ± 3.3 ab	1.84 ± 0.1 bc	1.26 ± 0.1 ab	3.10 ± 0.1 ab

Physiological attributes of tomato plants were taken after 45 days of transplantation. Values represent means ± standard deviations (n = 3). Different small letters in column indicate significant differences (at p ≤ 0.05) among treatments. S, Salinity; BC, Biochar; C, Compost; SPAD, Soil plant analysis and development; Ft, Fluorescence yield; PAR, Photosynthetically active radiation; YII, Quantum yield; ETR, Electron transport rate; MSI, Membrane stability index; RWC, Relative water content; Chl. a, Chlorophyll a; Chl. b, Chlorophyll b; T. Chl., Total chlorophyll.

). A significant reduction in SPAD value, Ft, PAR, YII, ETR, MSI, RWC, Chl. a, Chl. b, and T. Chl. of the tomato plants was observed in the presence of excessive salts (i.e., 6 dS m⁻¹) in soil, and the observed decrease was 28.4%, 23.5%, 34.4%, 16.2%, 25.4%, 22.6%, 12.1%, 18.3%, 13.7%, and 16.5%, respectively, compared to the non-saline control (Table 1). Moreover, the combined application of biochar and compost further improved this effect, and the SPAD value, Ft, PAR, YII, ETR, MSI, RWC, Chl. a, Chl. b, and T. Chl. were enhanced by 26.2%, 35.6%, 22.3%, 11.3%, 24.7%, 22.7%, 18.6%, 22.9%, 26.9%, and 24.65%, respectively, compared to un-amended saline control. Overall, regardless of salinity, individual or combined application of biochar and compost significantly improved the physiological performance of tomato, both in saline as well as non-saline soils, indicating the positive effect of biochar and compost in reducing the toxic effects of salinity on tomato plants (Table 1).

In agronomic attributes of tomato plants, a decrease of 12–44% was recorded in salt-added treatment (

Table 2. Effects of compost and biochar addition on agronomic traits of tomato grown in saline and non-saline soils.

	F/P	SL (cm)	SFW (g)	SDW (g)	RL (cm)	RFW (g)	RDW (g)	Fr/P
	Non-saline soil
Control	45.3 ± 8.0 abcd	62.3 ± 3.5 b	37.2 ± 5.2 bc	7.97 ± 0.6 b	17.3 ± 1.5 bcd	10.9 ± 0.9 bc	2.40 ± 0.2 abc	7.67 ± 1.5 abcd
BC	49.7 ± 6.5 abc	67.3 ± 1.5 ab	40.4 ± 2.5 ab	8.80 ± 0.3 ab	19.3 ± 2.1 abc	12.2 ± 1.2 ab	2.48 ± 0.3 abc	8.67 ± 2.1 abc
C	52.3 ± 4.1 ab	71.6 ± 3.1 a	42.8 ± 3.1 a	9.43 ± 0.2 a	21.3 ± 3.1 ab	13.1 ± 0.7 a	2.81 ± 0.2 ab	9.67 ± 1.2 ab
BC + C	59.3 ±5.7 a	74.67 ± 2.1 a	46.1 ± 5.4 a	9.70 ± 0.2 a	23.3 ± 1.5 a	13.7 ± 0.6 a	2.97 ± 0.3 a	10.3 ± 1.5 a
	Saline soil
S	25.3 ± 6.0 e	41.7 ± 4.2 e	23.5 ± 2.7 e	4.73 ± 0.4 e	12.3 ± 0.6 d	7.27 ± 0.7 d	1.59 ±0.2 d	4.33 ± 1.5 d
S + C	33.7 ± 7.0 cde	52.0 ± 2.6 cd	29.9 ± 5.1 de	6.13 ± 0.4 cd	16.3 ± 0.6 bcd	9.30 ± 0.7 cd	2.00 ± 0.2 cd	6.00 ± 1.0 bcd
S + BC	30.0 ± 4.6 de	49.7 ± 4.2 de	27.8 ± 3.9 de	5.47 ± 0.2 de	15.0 ± 2.6 cd	8.97 ± 0.4 cd	1.89 ± 0.2 cd	5.33 ± 1.5 cd
S + BC + C	35.0 ± 6.6 bcde	59.0 ± 3.0 bc	32.7 ± 2.9 cd	6.47 ± 0.3 c	18.7 ± 2.1 abc	9.70 ± 0.8 c	2.23 ± 0.2 bcd	6.33 ± 0.6 abcd

Agronomic attributes were measured after harvesting. Values represent means ± standard deviations (n = 3). Different small letters in the column indicate significant differences (at p ≤ 0.05) among treatments. S, Salinity; BC, Biochar; C, Compost; F/P, No. of flowers per plant; SL, Shoot length; SFW, SDW, Shoot dry weight; Shoot fresh weight; RL, Root length; RFW, RDW, Root dry weight; Root fresh weight; Fr/P, No. of fruits per plant.

). Individual application of biochar enhanced 8–26% while compost addition improved 12–30% of agronomic characteristics over the respective control. Moreover, co-application of biochar and compost caused moderate to high increase in the number of flowers (27.6%), the shoot length (29.4%), the shoot fresh weight (26.2%), the shoot dry weight (26.8%), the root length (33.9%), the root fresh weight (25.1%), the root dry weight (28.7%), and the number of fruits per plant (31.5%) when compared with the saline control. Overall, the combined use of biochar and compost significantly improved the tomato agronomic traits in both saline as well as non-saline treatments, showing the positive impacts of biochar and compost in relieving the deleterious effects of salinity in tomato plants.

3.2. Impact of Compost and Biochar on Nutrient Uptake of Tomato Plants under Salinity Stress

In this study, the results showed significantly lower N and P contents of the roots and shoots in the saline soil (Figure 1). A decrease of 30.9%, 44.3%, and 40.9% in roots and 35.8%, 45.9%, and 44.2% in shoots were observed in N, P, and K concentrations, respectively, compared with the control. Nevertheless, the sole application of biochar and compost enhanced nutrient concentration in the root and shoot of tomato by 11–22% and 13–29%, respectively. Furthermore, conflate use of biochar and compost enhanced the NPK content in the root by 23–37% and in the shoot by 27–46% under salinity over their respective non-saline control (Figure 1).

Similarly, the concentration of Na⁺ was significantly increased in the shoot and root of the plant in saline soil as compared to the non-saline control, and the increase was about 70.4% and 74.5%, respectively. The individual application of biochar and compost significantly decreased the Na⁺ in shoots (by 28.1% and 37.2%, respectively) and roots (by 31.5% and 42.3%, respectively) as compared to the saline control (Figure 2). Moreover, the combined application of biochar and compost further reduced the concentration of Na⁺ in shoots and roots by 40.2% and 47.6%, respectively, as compared to their respective saline control (Figure 2), suggesting the synergistic effects of biochar and compost for minimizing the Na⁺ accumulation in tomato crop.

The results of this study showed that in saline soil the Na⁺/K⁺ ratio was significantly greater in shoots (83.7%) and roots (85.1%) when compared to the non-saline and un-amended controls (Figure 2). However, the combined use of biochar and compost reduced the Na⁺/K⁺ ratio in shoots and roots by 57.9% and 60.7%, respectively, over respective saline control (Figure 2).

3.3. Impact of Compost and Biochar on Fruit Quality Traits of Tomato under Salinity Stress

Tomato fruit quality was also observed in response to the application of biochar, compost, and salinity. The TSS, TA, and TI levels in tomato fruit were enhanced by applying biochar and compost in saline soil (Figure 3). The highest values of TSS, TA, and TI were observed in the treatment in which biochar and compost were added together in saline pots, and the lowest values were found in the non-saline and un-amended controls. Regardless of whether the treatments were saline or non-saline, the fruit quality attributes were increased in individual or combined application of biochar and compost, indicating the important role of these amendments in improving the fruit quality of tomato under salinity stress.

4. Discussion

Results from this study showed a decrease in physiology and growth attributes of tomato under salinity; however, the application of biochar and compost reduced the phytotoxic impacts of salinity and improved the plant growth by strengthening the physiology and nutrient uptake. Our results are consistent with the research outcomes of Kumar et al. [25] who also reported the reduced plant weight and reduced number of branches due to increased concentration of salts in the growing medium. The possible reason behind the stunted growth is due to the decreased absorption and accumulation of essential nutrients in the plant body [26]. Higher salinity in plants alters cell division, decreasing the cell size by decreasing the water potential in vacuoles and ultimately reducing the dry weight of plants [27]. Reduced weight might be due to various other factors including reduced photosynthetic activity and decreased turgor pressure [28]. Furthermore, our results showed a decreased membrane stability index (MSI) and a decreased relative water content (RWC) when plants were grown under salt stress because of the reduced water intake in plants [29]. The results of this study support the findings of Mozafariyan et al. [30] who also reported a significant reduction in RWC and MSI when the plants were grown in saline conditions.

However, the addition of biochar in salinized soil improved several physicochemical and physiological processes in soil and plants. This observation might be due to an improvement in soil conditions, such as increased macronutrients and water availability for plants [31,32]. The present study showed the negative effects of salts on growth metrics, indicating the possibility of the occurrence of osmotic and ion toxicity in plants [33]. The ion toxicity could be reduced by the addition of biochar. Our results are in line with the findings of He et al. [34] and Rasool et al. [35] who also described higher plant yields and reduced salt (ionic) contents in plant parts from the addition of biochar. Biochar has the ability to absorb and adsorb different cations and anions due to the presence of the active site and functional groups, thus reducing overall ion toxicity [36,37].

Similarly, the addition of compost in saline soil minimized the toxicity of salts to plants directly by decreasing the translocation of harmful salts or indirectly by increasing the other nutrients and water uptake [38]. In stress, plants produce ethylene and hydrogen peroxide that hinder the plant’s physiological processes. However, the addition of compost boosts plant physiology by reducing oxidative damage through the production of peroxide, superoxide dismutase, and catalase that breakdown the ethylene and convert toxic hydrogen peroxide to water and oxygen molecules [39]. Moreover, improved RWC and MSI might be due to the scavenging of ROS and improved water uptake through the addition of compost (as it copes with different abiotic stresses) [38,40].

In the current study, salinity decreased the nutrient uptake in plants, while the addition of compost and biochar played an important role in alleviating the phytotoxic effects of salinity on plants and significantly improved the nutrient status of plants. The findings of this study are in line with the results of Arif et al. [41], who also observed reduced nutrient uptake in plants under salinity stress. Our results demonstrated that salt stress caused a significant reduction in water potential and disturbed ionic balance, and this was probably due to higher amounts of Na⁺ in plant shoots. In fact, it has been reported that sodium ions can compete with other nutritional ions, notably K⁺, and cause K⁺ shortage [42]. So, the plant needs to maintain high potassium levels to offset the excess salt. Sodium and potassium compete for plant absorption under salt stress due to their structural resemblance [43]. This study showed that the salinity in tomato led to a higher Na⁺/K⁺ ratio in root and shoot, and these results are consistent with those of Ur Rahman et al. [44], who reported the highest decline in shoot K⁺ at higher salt concentrations. In fact, Na⁺ in the root causes Na⁺ toxicity, which impairs cell permeability and cell elongation/division [45]. Salt stress causes ionic and osmotic stress in plants, and ionic stressors may be seen at several levels. In tomato, K⁺ in roots was lower than in shoots, indicating that K⁺ in leaves is critical for metabolic activity [46] and this might be a result of the low Na⁺/K⁺ ratio in the cytosol which is important for proper cell metabolism [47].

The compost is known to be beneficial for plants as it enhances the physical, chemical, and biological properties of soil and increases their ability to absorb and translocate nutrients from the soil to the plant [14]. Similar to the findings of our study, Simiele et al. [10] demonstrated enhanced plant growth and productivity with the addition of compost. It is found that the composted soil could improve soil’s physical and chemical properties and increase agricultural production and sustainability [48,49]. Moreover, plant photosynthetic performance may be improved by the synergistic effects of OM, macro- and micronutrients, and biotic and abiotic agents in compost, as demonstrated by Otaiku et al. [50], resulting in an increased biomass output. An appreciable salt tolerance in tomato plants from the compost addition was observed, which lowered Na⁺ levels in plants grown in saline soil [51]. Studies showed that cell osmoregulation, turgor, stomatal function, enzyme activation, protein synthesis, oxidant metabolism, and photosynthesis are impacted by the Na⁺/K⁺ ratio in salt-affected soils [52,53].

The results of this study depicted that the application of biochar and compost in saline soil enhanced the quality parameters of tomato fruit. In response to salinity, plants regulate different metabolic processes that may reduce or relocate the organic acids to various plant parts. i.e., from leaves to fruits [9]. Likewise, the application of biochar and compost raised values of TSS, TA, and TI. Biochar and compost enhanced root elongation and biomass production to provide more surface area for more nutrient and water uptake even in a stressed environment. Similar results were reported by Cao et al. [13] reporting the use of compost for improving the fruit quality of tomato. Similarly, our results also support the findings of Hameeda et al. [54] who reported that biochar application could raise the TSS, TA, and TI values of tomato under salinity.

To our knowledge, this is the first study reporting the integrated impact of biochar and compost on the growth and fruit quality of tomato grown under salinity stress. Findings from this study enhance our knowledge of plant and organic amendment interactions in saline soil and provide new information on the combined usage of biochar and compost in improving crop yield under salinity stress and open new areas for studying the significance of the combined use of biochar and compost in salt-contaminated soils for sustainable pomology and agriculture.

5. Conclusions

In this study, salinity showed deleterious impacts on the growth and physiology of tomato plants: however, salinity caused significantly greater fruit quality traits of tomato including TSS, TA, and TI, probably due to the translocation of beneficial salts from the system or leaves to fruits. In contrast, biochar and compost application enhanced the physiological, agronomic, and biochemical attributes of tomato under salinity. Moreover, the combined use of biochar and compost caused a further increase in all crop attributes than any other treatments. The results of this study demonstrated that the combination of biochar and compost was more efficient than any single amendment. Thus, the integrated use of biochar and compost could be beneficial in alleviating the deleterious impacts of salinity on horticultural plants and improving tomato productivity in salt-affected soil. Based on our findings, further studies are suggested to explore the possible mechanisms involved in improving plant physiology, growth, and productivity under salinity stress.