A Novel Biostimulant–Biochar Strategy for Improving Soil Quality and Salinity Tolerance in Medicinal Mint (Mentha longifolia L.)

Abstract

This study evaluated the combined application of biochar (BC) and Spirulina platensis (SP) as a sustainable strategy to improve soil quality and salinity tolerance in mint (Mentha longifolia L.) cultivated in sandy soils. A pot experiment was conducted using saline irrigation water (5 dS m⁻¹) with four treatments: control, BC alone, SP alone, and BC + SP applied at 1% (w/w), arranged in a randomized complete block design with three replicates. Salt stress reduced plant height and biomass yield by 16% and 27%, respectively, and increased sodium (Na⁺) in shoots by 74%, causing ionic imbalance and decreased soil microbial biomass carbon by 19%. The combined BC + SP treatment significantly improved soil microbial biomass carbon (SMBC) by 100%, reduced soil Na⁺ by 41%, and enhanced K⁺/Na⁺ and Ca²⁺/Na⁺ ratios by 138% and 133%, respectively. Under salinity, BC + SP increased nutrient concentrations in mint shoots, including N (120%), P (106%), K (88%), Ca (67%), Fe (70%), Mn (50%), Zn (44%), and Cu (70%), and improved leaf chlorophyll content. These results demonstrate that BC and SP synergistically mitigate salinity stress by improving soil properties, nutrient uptake, and ionic balance, making BC + SP a promising sustainable amendment for saline sandy soils.

1. Introduction

Approximately 80% of people worldwide, especially in developing countries, rely on traditional medicine, according to the World Health Organization (WHO) [1]. Among medicinal plants, mint (Mentha longifolia L.), belonging to the Lamiaceae family, is highly valued for its essential oils and various health benefits such as antiseizure, stimulant, and digestive properties [2,3,4,5,6]. This species was selected for this study due to its moderate salt tolerance (up to 7.5 dS m⁻¹) and significant economic importance in salt-affected regions, despite its susceptibility to growth reduction and decreased essential oil yield under salinity stress [2,3,4]. Plant exposure to salt stress adversely affects photosynthesis and transpiration rate [7]. Irrigation with saline water is considered a solution for reducing water consumption [8]. However, the use of salt water for irrigation reduces plant growth and causes soil degradation [9]. Saline water contains sodium (Na^⁺) ions, which reduce soil infiltration, waterlogging, nutrient imbalances, and toxicity [10]. Sodium (Na^⁺) in saline water causes the dispersion of soil particles and clogging of soil pores, increasing soil bulk density and decreasing soil porosity, saturated hydraulic conductivity, and water-stable macro aggregate content [11]. Saline water causes low osmotic potential of soil solution that reduces water availability for soil microorganisms, causing a death or lysis of soil microorganisms [11,12,13].

The use of organic materials and natural products, such as biochar and Spirulina algae, may enhance salt stress resistance, allowing for the use of low-quality water in mint irrigation [9,14,15]. Spirulina platensis was selected for this study due to its well-documented effectiveness in mitigating the negative effects of abiotic stresses, particularly salinity [8,9]. As a natural resource rich in bioactive compounds, Spirulina improves plant stress tolerance by enhancing physiological and biochemical functions, such as stabilizing cellular membranes, reducing ion leakage, and promoting water retention [8,16,17,18]. These attributes make Spirulina a valuable, eco-friendly alternative to synthetic soil amendments, especially in saline irrigation systems. Its use in combination with biochar supports a sustainable strategy for improving salinity resilience in mint cultivation.

Biochar is a carbonaceous material derived from pyrolyzing organic biomass at 300–700 °C under oxygen-free or oxygen-limited conditions [19]. The addition of biochar to saline-irrigated soil leads to improvements in bulk density, porosity, permeability, degree of aggregate, aeration, and water retention [20]. Biochar’s pore structure has the ability to adsorb Na^⁺ in soil, leading to a decrease in its uptake by plants [14,15]. The high cation exchange capacity of biochar reduces the uptake of Na⁺ and increases the uptake of nutrients released from biochar, leading to increased plant growth [15]. Biochar application reduces the reactive oxygen species (ROS) production and malondialdehyde (MDA) accumulation, leading to increased uptake of nutrients and photosynthesis processes [21]. The addition of biochar minimizes the negative effects of salinity by boosting photosynthesis, antioxidant activity, nutrient absorption, and osmolyte buildup and reducing chlorophyll degradation [22,23]. Although biochar has been utilized extensively as a useful technique to lessen the detrimental effects of salt stress on a variety of plants, there is not much information available about the possibility of using biochar to reduce the effects of salt stress on mint.

We hypothesize that (1) the combined application of Spirulina platensis and biochar will significantly improve soil physicochemical properties—such as pH balance, organic matter content, and nutrient availability—compared to individual treatments, and (2) this synergy will enhance mint (Mentha longifolia L.) growth and salt stress tolerance by improving chlorophyll content, nutrient uptake, and ion homeostasis. This hypothesis-driven investigation provides novel insights into sustainable soil management and plant productivity under salinity stress through the dual use of microbial biostimulants and carbon-based amendments.

2. Materials and Methods

2.1. Biochar, Spirulina, Soil, and Plant Materials

Corn cobs were collected, cut into small pieces, and then dried at 105 °C for the removal of moisture and then were pyrolyzed in an oven in a stainless-steel container with a lid under a limit of oxygen for 2 h at 300 °C [24]. The produced biochar was crushed and was passed through a 2 mm sieve. Fourier Transform Infrared Spectroscopy (ATR-FTIR; Thermo Nicolet UK—Waltham, MA, USA) was employed to identify functional groups present on the biochar surface by measuring the characteristic vibrational frequencies of chemical bonds. Surface morphology was examined using Scanning Electron Microscopy (SEM; Quanta FEG250, FEI, Hillsboro, OR, USA), which provides high-resolution images to reveal the biochar’s surface texture and structural features. Additionally, surface area and pore structure characteristics, including the Brunauer–Emmett–Teller (BET) surface area and pore volume, were determined using a NOVAtouch 2LX analyzer (St 1) based on nitrogen adsorption–desorption isotherms. Figure 1 presents the physicochemical characterization of the corn cob-derived biochar.

Spirulina platensis powder was purchased from the National Research Center, Egypt. The chemical composition of soil, biochar, and Spirulina extract is shown in

Table 1. Physicochemical properties of soil, biochar (BC), and Spirulina (SP).

	Soil	Spirulina	Biochar
pH	7.82 ± 0.05	6.52 ± 0.08	9.55 ± 0.15
Salinity (d Sm−1)	1.8 ± 0.2	4.5 ± 0.1	3.7 ± 0.2
OM (%)	1.5 ± 0.1	86 ± 3	72 ± 4
N (%)	250 ± 12	10 ± 1	0.2 ± 0.0
P (%)	10 ± 1	0.35 ± 0.04	0.22 ± 0.05
K (%)	550 ± 16	1.2 ± 0.2	0.11 ± 1
Ca (%)	400 ± 12	3.5 ± 0.4	1.4 ± 0.2
Fe (mg kg−1)	8.2 ± 1	750 ± 10	60 ± 3
Mn (mg kg−1)	20 ± 2	70 ± 3	40 ± 3
Zn (mg kg−1)	1.5 ± 0.1	100 ± 4	50 ± 1
Cu (mg kg−1)	0.58 ± 0.00	75 ± 3	12 ± 1

The values of N, P, K, Ca, Mn, Fe, Zn, and Cu for Spirulina and biochar represent the total, while for soil values, they refer to the available amount. Values are mean ± standard deviation, n = 3.

. The studied soil (0–20 cm) was obtained from the Experimental Station of the Faculty of Agriculture. The studied soil is a sandy soil and classified as Aridisols: Typic Torri psamments [25]. Mentha longifolia L. cv. horsemint seedlings (15 cm tall) were acquired from the Ministry of Agriculture’s central administration for seed and seedling production in Giza, Egypt. The pH and salinity of the biochar and Spirulina were measured in a 1:10 (v/v) extract. Soil salinity was determined in the soil paste extract, while the soil pH was measured in a 1:2 soil-to-water ratio.

2.2. Pot Experiment

A pot experiment was conducted to evaluate the effect of applying biochar (BC) and Spirulina (SP) on mint plants under salinity conditions. Biochar and Spirulina were applied to soil at the dose of 0 or 1% (w/w). Spirulina and biochar were applied in powdered form and thoroughly mixed into the soil prior to planting. Spirulina was applied at 1% w/w, a rate commonly used in pot experiments, which approximately corresponds to 15–20 t/ha when scaled to field conditions based on typical topsoil bulk density and depth. This dosage aligns with standard soil amendment practices and ensures effective delivery of bioactive compounds to enhance plant stress tolerance [8,9,16,17,18]. The experiment included two types of irrigation, i.e., fresh water (tap water) and saline water (5 dS m⁻¹), in addition to four treatments of biochar and Spirulina, i.e., control, BC, SP, and BC + SP. The saline water was prepared from 2:1 of (NaCl:CaCl₂). The plants were irrigated with saline water for four months and then were harvested. The pots that were used in the experiment were plastic with a diameter of 30 cm and a height of 20 cm. The soil was mixed with biochar and Spirulina, and then the pots were filled with 5 kg of the soil. Two seedlings were sown per pot and were irrigated with tap water for one week, then with saline water to field capacity based on the pot weight. A total of 50 mL of the half-strength Hogland’s solution consisting of (KNO₃, Ca(NO₃).4H₂O, KH₂PO₄, and MgSO₄) salts were added for each pot as a fertilizer dose [26]. The pot experiment was conducted under open-field conditions from January to April 2024. According to local meteorological data, average maximum and minimum temperatures during the experimental period ranged from 18 to 30 °C and 10 to 15 °C, respectively. Relative humidity averaged between 40 and 60%, and the daily sunshine duration was approximately 9–10 h. At the end of the experiment, the plants were harvested, and growth parameters were recorded, as well as the plant and soil samples were collected to be analyzed. The measurements of mint plants were plant height and fresh and dry weight.

2.3. Soil, Plant, Biochar, and Spirulina Analysis

The characterizations of soil, biochar, and Spirulina algae were estimated as described by Burt [27]. The biochar and Spirulina pH and EC were measured at a (1:10) biochar:water solution. Biochar and Spirulina carbon and nitrogen content were determined by a CHNS analyzer. The soil organic matter (OM) was determined by the Walkley–Black method [28]. The available nitrogen in soil was extracted with (1 M) KCl and then determined by the Kjeldahl method [27]. The available phosphorus in soil was extracted with 0.5 N sodium bicarbonate solution adjusted to pH 8.5 [29]. Available potassium and calcium were extracted using 1M ammonium acetate at pH 7 [29]. The soluble cations, i.e., potassium, sodium, and calcium, were measured in the soil paste extract.

Plant samples were washed with distilled water and oven-dried at 70 °C and crushed in a mill. The total fresh and dry weights for each pot were recorded. Plant samples were digested with H₂O₂, H₂SO₄, lithium sulfate, and selenium [30] and then analyzed for total N, P, K, Ca, Mn, Fe, Zn, and Cu concentrations. Nitrogen in plant samples was distilled by the Kjeldahel method, and then N was determined by titration with a standard solution of 0.01 HCl [27]. Phosphorus was measured by the stannous chloride method and determined by spectrophotometry at 660 mm (DSK group, model 390, Alex) [29]. Chlorophyll was measured according to Lichtenthaler [31]. Samples of fresh leaves were randomly collected. Leaf pigments were extracted by 80% aqueous acetone and measured by spectrophotometer. In the soil, plant, biochar, and Spirulina, the concentrations of K, Ca, Mn, Fe, Zn, and Cu were determined by Inductively Coupled Plasma Emission Optical Emission Spectrometry (ICP−OES thermo iCAP 6000 series).

2.4. Statistical Analysis

The experimental design of the pot experiment was a factorial randomized complete block design (RCBD) with three replicates per treatment. Data were subjected to analysis of variance (ANOVA) using the Statistix software (version 8.1). Prior to ANOVA, assumptions of normality and homogeneity of variances were verified using the Shapiro–Wilk and Levene’s tests, respectively. When significant differences were detected (p < 0.05), treatment means were compared using the Least Significant Difference (LSD) test at a 5% probability level.

3. Results

3.1. Physicochemical Characterization of Biochar

Corn cob-derived biochar was produced by slow pyrolysis at 300 °C for 2 h under limited oxygen conditions. SEM analysis (Figure 1A) revealed a heterogeneous surface morphology with a well-developed porous structure comprising both macropores and mesopores.

These pores, inherited from the vascular architecture of the original biomass, are essential for improving soil aeration, water retention, and microbial habitat. Nitrogen adsorption–desorption isotherms (Figure 1B) displayed a type IV hysteresis loop, indicative of mesoporous material. The specific surface area, determined by the BET method, was approximately 85.6 m² g⁻¹, while the total pore volume reached 0.42 cm³ g⁻¹. These characteristics suggest a high adsorption capacity and enhanced potential for nutrient retention in soil systems. Furthermore, the FTIR spectrum (Figure 1C) revealed several key functional groups on the biochar surface. Prominent absorption peaks appeared around 3420 cm⁻¹ (O–H stretching), 1625 cm⁻¹ (C=C or aromatic ring vibrations), and 1030 cm⁻¹ (C–O stretching), indicating the presence of hydroxyl, carboxyl, and aromatic groups.

3.2. Biochar and Spirulina Effects on Soil Quality

3.2.1. Biochar and Spirulina Effects on Soil Biochemical and Physicochemical Characterization

Figure 2A–C show the significant effects of biochar and Spirulina (SP) on soil pH, microbial biomass carbon (MBC), and soil organic matter (SOM). Soil pH was significantly (p ≤ 0.05) influenced by the application of BC, SP, and their combination (BC + SP), under both fresh water (FW) and saline water (SW) irrigation (Figure 2A).

Overall, the data show that both BC and SP individually, and especially in combination, have a pH-buffering effect. The greatest pH reduction occurred with the SP + BC treatment under fresh water. Salt stress led to a reduction in MBC by 19% compared to fresh water overall treatments; however, the decrease was less pronounced with BC and SP. The combination (BC + SP) showed the smallest decline (17%), indicating that BC and SP, particularly in combination, effectively alleviated the adverse effects of salinity on MBC. BC, SP, and BC + SP raised MBC by 56, 72, and 88%, respectively, compared to the control under FW and by 60, 80, and 100%, respectively, under SW (Figure 2B). Soil organic matter (SOM) increased considerably (p < 0.05) with the addition of BC, SP, and BC + SP (Figure 2C). BC, SP, and BC + SP raised SOM by 36, 38, and 44%, respectively, compared to the control under FW and by 45, 40, and 60%, respectively, under SW. Under SW, SP + BC notably had the greatest beneficial effect, increasing SOM and MBC compared to the sole application of SP or BC. The application of BC, SP, and BC + SP significantly enhanced the soil cation exchange capacity (CEC) and improved the balance of soluble cations in the soil solution under both FW and SW irrigation regimes (

Table 2. Effect of biochar (BC), Spirulina (SP), and control (C) on soil quality indexes under fresh water (FW) and saline water (SW) irrigation.

Water Type	Treatment	CEC (cmol kg−1)	Soluble Cations in Soil Solution (mg L−1)
			Na	Ca	K	Ca/Na	K/Na
FW	C	17.1 ± 0.8 b	250 ± 8 e	250 ± 12 d	420 ± 5 d	1.0 ± 0.1 d	1.7 ± 0.1 c
	BC	20.2 ± 0.9 a	220 ± 6 a	350 ± 18 c	480 ± 8 c	1.6 ± 0.0 b	2.2 ± 0.1 b
	SP	20.3 ± 0.7 a	210 ± 5 d	370 ± 10 c	500 ± 12 b	1.8 ± 0.2 ab	2.4 ± 0.2 b
	BC + SP	21.5 ± 1.0 a	200 ± 9 b	400 ± 6 b	570 ± 16 a	2.0 ± 0.1 a	2.9 ± 0.1 a
SW	C	16.8 ± 0.4 b	510 ± 15 a	320 ± 5 c	400 ± 10 d	0.6 ± 0.1 d	0.8 ± 0.1 d
	BC	19.8 ± 1.0 a	430 ± 20 b	380 ± 15 c	470 ± 8 c	0.9 ± 0.0 d	1.1 ± 0.0 d
	SP	21.0 ± 0.8 a	420 ± 17 b	400 ± 8 b	510 ± 13 b	1.0 ± 0.1 d	1.2 ± 0.1 d
	BC + SP	22.0 ± 1.0 a	300 ± 12 c	420 ± 12 a	580 ± 20 a	1.4 ± 0.0 c	1.9 ± 0.2 c

Means ± SD with different letters differ significantly from each other at p ≤ 0.05 according to LSD test. C = control. The soluble cations in soil solution were determined in soil paste extract.

). Under FW irrigation, CEC increased by 18, 19, and 26% with BC, SP, and BC + SP, respectively, compared to the control. Under SW irrigation, CEC improved by 18% with BC, 25% with SP, and 31% with BC + SP compared to control, indicating the superior effect of combined treatment on soil CEC.

3.2.2. Biochar and Spirulina Effects on Soil Soluble Cations

In terms of soluble cations (Table 2), both individual and combined treatments significantly reduced sodium (Na^⁺) concentrations and increased calcium (Ca^2⁺) and potassium (K^⁺) in the soil solution. Regarding Na^⁺ reduction in the soil solution, BC, SP, and BC + SP decreased Na^⁺ levels by 12, 16, and 20%, respectively, under FW, and by 16, 18, and 41%, respectively, under SW, compared to their respective controls. For Ca^2⁺, BC, SP, and BC + SP increased concentrations by 40, 48, and 60%, respectively, under FW, and by 19, 25, and 31%, respectively, under SW, compared to control values. Similarly, K^⁺ levels increased by 14% (BC), 19% (SP), and 36% (BC + SP) under FW, and by 18%, 28%, and 45% under SW, relative to control. Notably, the Ca/Na and K/Na ratios in soil solution, important indicators of soil structural stability and salinity resilience, increased by the addition of BC, SP, and BC + SP treatments. BC, SP, and BC + SP increased the Ca/Na ratio in soil solution by 60, 80, and 100% under FW and by 50, 67, and 133% under SW, respectively. In comparison to control, the K/Na ratio in soil solution increased by 29% (BC), 41% (SP), and 71% (BC + SP) under FW and by 38, 50, and 138% under SW.

3.3. Effect of Biochar and Spirulina on Nutrient Availability

The application of biochar (BC), Spirulina (SP), and their combination (BC + SP) significantly influenced soil nutrient availability under both fresh water (FW) and saline water (SW) irrigation compared to the control (C).

Table 3. Effect of biochar (BC), Spirulina (SP), and control (C) on soil nutrients availability (mg kg⁻¹) under fresh water (FW) and saline water (SW) irrigation.

Water Type	Treatment	N	P	K	Ca	Fe	Mn	Zn	Cu
FW	C	350 ± 15 a	8.5 ± 0.3 e	530 ± 25 c	380 ± 22 d	6.7 ± 0.4 c	15 ± 2 b	1.7 ± 0.4 c	0.50 ± 0.04 c
	BC	300 ± 27 a	13.2 ± 0.2 c	590 ± 18 b	490 ± 18 c	7.2 ± 0.2 b	17 ± 3 b	2.2 ± 0.2 b	0.41 ± 0.02 d
	SP	320 ± 18 a	15.1 ± 0.1 b	610 ± 22 b	490 ± 13 c	9.4 ± 0.3 a	23 ± 4 a	2.9 ± 0.3 a	0.92 ± 0.04 a
	BC + SP	328 ± 20 a	16.2 ± 0.3 a	680 ± 26 a	530 ± 22 b	9.5 ± 0.4 a	20 ± 3 a	2.6 ± 0.4 a	0.62 ± 0.03 b
SW	C	330 ± 15 a	7.2 ± 0.4 e	460 ± 15 d	450 ± 25 c	6.2 ± 0.2 c	17 ± 1 b	1.3 ± 0.2 c	0.56 ± 0.02 c
	BC	335 ± 18 a	11.4 ± 0.7 d	580 ± 28 b	480 ± 19 c	7.5 ± 0.1 b	14 ± 2 b	2.0 ± 0.1 b	0.38 ± 0.06 d
	SP	318 ± 20 a	13.8 ± 0.8 c	620 ± 18 b	570 ± 15 a	9.4 ± 0.3 a	25 ± 4 a	2.8 ± 0.3 a	0.95 ± 0.02 a
	BC + SP	315 ± 25 a	15.6 ± 1.3 a	700 ± 30 a	590 ± 22 a	9.7 ± 0.2 a	21 ± 2 a	2.7 ± 0.2 a	0.68 ± 0.05 b

Means ± SD with different letters differ significantly from each other at p ≤ 0.05 according to LSD test. C = control.

and Figure 3 show the significant effects of BC, SP, and BC + SP on the availability of nitrogen (N), phosphorus (P), potassium (K), iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu). Under FW irrigation, the combined application of BC + SP resulted in the highest concentrations of soil available N, P, K, Ca, Fe, Mn, Zn, and Cu. These concentrations were significantly higher (p ≤ 0.05) than those observed in the control treatment, and in most cases, higher than individual applications of BC or SP. Under FW, BC + SP increased concentrations of soil available P by 91%, K by 28%, Ca by 39%, Fe by 42%, Mn by 33%, Zn by 53%, and Cu by 24%. Spirulina (SP) alone also markedly improved soil nutrient availability under FW, especially for P, K, and micronutrients (Fe, Mn, Zn, Cu), showing values that were significantly higher than the control and BC-alone treatments. Under saline water (SW) irrigation, a similar trend was observed, with the BC + SP treatment consistently resulting in the highest concentrations of available N, P, K, Ca, Fe, Mn, Zn, and Cu. These soil nutrient levels were significantly higher compared to the control. Under SW, BC + SP increased soil available P by 117%, K by 52%, Ca by 31%, Fe by 56%, Mn by 24%, Zn by 108%, and Cu by 21%.

3.4. Effect of Biochar and Spirulina on Nutrients and Uptake by Mint Plants

The application of BC, SP, and their combination BC + SP significantly enhanced the concentrations of essential macro- and micronutrients in mint plants under both FW and SW irrigation, compared to their respective controls (

Table 4. Effect of biochar (BC), Spirulina (SP), and control (C) on nutrient concentration (%) in mint shoots under fresh water (FW) and saline water (SW) irrigation.

Water Type	Treatment	N	P	K	Ca	Na	K/Na	Ca/Na
FW	C	1.6 ± 0.0 c	0.32 ± 0.0 d	1.0 ± 0.1 d	1.8 ± 0.1 d	0.19 ± 0.0 d	5.6 ± 0.1 e	10.0 ± 0.2 c
	BC	2.0 ± 0.1 b	0.38 ± 0.05 c	1.5 ± 0.0 bc	2.4 ± 0.1 b	0.22 ± 0.02 d	6.8 ± 0.1 c	10.9 ± 0.3 c
	SP	2.6 ± 0.2 a	0.43 ± 0.01 b	1.6 ± 0.1 b	2.6 ± 0.1 ab	0.21 ± 0.01 d	7.6 ± 0.3 b	12.4 ± 0.2 b
	BC + SP	2.8 ± 0.2 a	0.47 ± 0.03 a	2.0 ± 0.1 a	2.8 ± 0.1 a	0.20 ± 0.03 d	10.0 ± 0.0 a	14.0 ± 0.3 a
SW	C	1.0 ± 0.0 d	0.18 ± 0.02 f	0.8 ± 0.3 d	1.2 ± 0.1 e	0.50 ± 0.00 a	1.6 ± 0.3 h	2.4 ± 0.0 g
	BC	1.6 ± 0.2 c	0.25 ± 0.03 e	1.2 ± 0.2 c	1.5 ± 0.2 d	0.34 ± 0.03 b	3.5 ± 0.2 g	4.4 ± 0.2 f
	SP	1.7 ± 0.1 c	0.32 ± 0.02 d	1.3 ± 0.2 c	1.7 ± 0.1 d	0.32 ± 0.03 b	4.1 ± 0.0 f	5.3 ± 0.1 e
	BC + SP	2.2 ± 0.1 b	0.37 ± 0.01 c	1.5 ± 0.2 bc	2.0 ± 0.2 c	0.25 ± 0.01 c	6.0 ± 0.0 d	8.0 ± 0.0 d

Means ± SD with different letters differ significantly from each other at p ≤ 0.05 according to LSD test. C = control.

and Figure 3). Under FW conditions, BC application increased N, P, K, and Ca concentrations in mint shoots by 25, 19, 50, and 33%, respectively. Spirulina demonstrated a stronger effect compared to BC, improving N by 63%, P by 34%, K by 60%, and Ca by 44%. The combined treatment (BC + SP) yielded the highest improvements, with increases of 75% (N), 47% (P), 100% (K), and 56% (Ca). Under SW conditions, salinity stress led to substantial nutrient depletion in control plants. However, BC application improved N, P, K, and Ca levels in mint shoots by 60, 39, 50, and 25%, respectively. Spirulina enhanced these nutrients in mint shoots by 70% (N), 78% (P), 63% (K), and 42% (Ca). The BC + SP combination led to the most pronounced increases, with 120% (N), 106% (P), 88% (K), and 67% (Ca). Similarly, micronutrients, i.e., Fe, Mn, Zn, and Cu, in mint shoots were also significantly improved by BC, SP, and their combination under both the FW and SW conditions. Under FW and SW, SP was more effective than BC in enhancing the concentrations of Fe, Mn, Zn, and Cu in mint shoots. BC + SP treatment resulted in the highest increases in Fe, Mn, Zn, and Cu under both FW and SW compared to other treatments. BC + SP increased the concentrations of Fe, Mn, Zn, and Cu by 50, 60, 49, and 50%, respectively, under FW, while the increases were 70, 50, 44, and 70%, respectively, under SW.

3.5. Effect of Biochar and Spirulina on Ionic Balance

Water salinity stress (5 dS m⁻¹) over all the treatments, i.e., BC, SP, and BC + SP, increased Na^⁺ in mint shoots by 74% compared to fresh water. Salt stress increased Na^⁺ concentration in mint shoots by 55, 52, and 25%, respectively, in the case of BC, SP, and BC + SP compared to fresh water. This suggests that the BC + SP treatment was the most successful in preventing the buildup of Na^⁺ in the mint shoots. Sodium (Na^⁺) concentrations remained statistically unchanged under FW across treatments, but the K/Na and Ca/Na ratios in mint shoots significantly improved under BC + SP by 79% and 40%, respectively. Moreover, under SW, BC + SP significantly reduced the Na^⁺ concentration in mint shoots by 50% compared to the SW control, which translated into remarkable increases in K/Na and Ca/Na ratios by 275% and 233%, respectively.

3.6. Effect of Biochar and Spirulina on Mint Growth Under Salinity Stress

Water salinity stress (5 dS m⁻¹) over all the treatments, i.e., BC, SP, and BC + SP, reduced plant height (PH) by 16%, fresh weight (FW) by 27%, dry weight (DW) by 8%, and total chlorophyll (TC) by 17% (

Table 5. Effect of biochar (BC), Spirulina (SP), and control (C) on some mint plant growth under fresh water (FW) and saline water (SW) irrigation.

Water Type	Treatment	PH	FW	DW	TC
FW	C	23 ± 3 c	60 ± 2 c	20 ± 2 c	2.9 ± 0.1 c
	BC	28 ± 1 b	70 ± 5 b	21 ± 1 c	3.1 ± 0.2 c
	SP	34 ± 1 a	82 ± 3 a	25 ± 1 b	3.6 ± 0.1 ab
	BC + SP	35 ± 3 a	88 ± 5 a	30 ± 1 a	3.8 ± 0.2 a
SW	C	18 ± 2 c	40 ± 2 d	15 ± 1 d	2.2 ± 0.2 d
	BC	26 ± 2 b	50 ± 2 e	22 ± 1 c	2.5 ± 0.2 d
	SP	27 ± 2 b	62 ± 2 c	25 ± 1 b	3.0 ± 0.1 c
	BC + SP	30 ± 2 b	70 ± 3 b	27 ± 1 a	3.4 ± 0.2 b

Means ± SD with different letters differ significantly from each other at p < 0.05 according to LSD test. C = control, PH = plant height (cm), FW = fresh weight (g pot⁻¹), DW = dry weight (g pot⁻¹), TC = total chlorophyll (mg g⁻¹).

). Under salt stress, the mint plants in the control treatment (C) showed significant reductions in growth parameters, with PH, FW, DW, and TC decreasing by 22, 33, 25, and 24%, respectively, compared to those irrigated with fresh water. In the BC + SP treatment, salt stress caused only moderate reductions in plant growth, with PH, FW, DW, and TC decreasing by 14, 20, 10, and 10%, respectively, compared to plants irrigated with fresh water, indicating improved salt tolerance.

Under both FW and SW irrigation conditions, the application of BC, SP, and their combination (BC + SP) significantly enhanced PH, FW, DW, and TC compared to the control (C). Under FW irrigation, the BC + SP treatment led to the highest improvements, increasing plant height, fresh weight, dry weight, and total chlorophyll by 52, 47, 50, and 31%, respectively. The SP treatment alone increased these parameters by 48, 37, 25, and 24%, while BC alone resulted in increases of 22, 17, 5, and 7%, respectively. A similar trend was observed under SW irrigation. The BC + SP treatment increased plant height, fresh weight, dry weight, and total chlorophyll by 67, 75, 80, and 55%, respectively, compared to the control. SP alone improved these parameters by 50, 55, 67, and 36%, while BC alone showed corresponding increases of 44, 25, 47, and 14%.

4. Discussion

The growth rates of mint (Mentha longifolia L.) plants cultivated in the control soil without the addition of biochar (BC), Spirulina (SP), or both (BC + SP) were significantly reduced by water salinity stress. The present study highlights the detrimental effects of water salinity (5 dS m⁻¹) on the growth and physiological performance of Mentha longifolia cultivated in sandy soil. The observed 16% reduction in plant height and 27% decline in total biomass production clearly indicate moderate tolerance of mint to salt stress, which is consistent with earlier reports that salinity impairs plant growth by disrupting water uptake, ion homeostasis, and cellular functions [3,4]. Salinity stress significantly reduced leaf chlorophyll content by 17%, suggesting a negative impact on the photosynthetic machinery of mint. This reduction could be attributed to the degradation of chlorophyll pigments and the inhibition of chlorophyll biosynthesis due to ionic toxicity and oxidative stress induced by excess salts [14,15]. Such impairment likely contributed to the lower biomass production observed in salt-stressed plants. Furthermore, the decline in soil microbial biomass carbon (MBC) by 19% under saline irrigation reflects a disturbance in the soil microbial community. Salinity is known to suppress microbial activity and diversity, particularly in sandy soils that have low buffering capacity and nutrient retention [14]. The reduction in MBC may also influence nutrient cycling and availability, indirectly affecting plant health and productivity [14]. The study also revealed that salt stress adversely affected the uptake of both macro- and micronutrients. These nutrient imbalances may have further contributed to reduced growth and photosynthetic capacity in mint plants [14,15]. This is in line with the well-documented effect of high salt concentrations interfering with nutrient absorption, either through competition between Na^⁺/Cl⁻ and essential ions like K^⁺, Ca^2⁺, and Mg^2⁺, or by altering root membrane permeability and transporter activities [15]. Overall, these findings underscore the sensitivity of Mentha longifolia to moderate levels of salinity and highlight the need for strategies to mitigate salt stress impacts, particularly in sandy soils.

The addition of biochar (BC) and Spirulina (SP), either separately or in combination, reduced the negative effects of salty water irrigation (5 dS m⁻¹) on the mint plants. The BC + SP treatment was the most effective in overcoming the detrimental effects of salt and greatly raising all growth parameters of mint plants. The results clearly demonstrate the significant positive impact of BC, SP, and their combination (BC + SP) on various indicators of soil quality, especially under saline conditions. Spirulina, a nutrient-rich cyanobacterium, may contribute organic acids during decomposition, moderating soil pH [8]. The marked reduction in soil pH with BC + SP suggests a favorable shift toward neutral pH, which is crucial for nutrient availability and microbial activity in slightly alkaline sandy soils [8,14]. The capacity of the biochar and Spirulina humic compounds to lower soil pH may be due to the carboxyl and phenolic hydroxyl that produce H^⁺ in soil solution [8,9]. Furthermore, adding biochar and Spirulina to the soil increases the biological activity and balances out the alkalinity of the soil [9,14].

Salinity-induced stress often disrupts microbial function, as evidenced by the 19% decline in microbial biomass carbon (MBC) under saline water (SW) irrigation. However, the attenuation of this decline in the presence of BC and SP treatments indicates their protective role. The highest improvement in MBC with BC + SP under both fresh and saline water (88% and 100%, respectively) suggests a synergistic enhancement of the soil microbial environment. Biochar’s porous structure likely provides a physical habitat and adsorption sites for microbes, while Spirulina contributes labile carbon and nutrients, collectively fostering microbial proliferation even under stress [8,14]. Similarly, the increase in soil organic matter (SOM) with all treatments, especially BC + SP, highlights the capacity of these amendments to improve carbon inputs and retention in sandy soils. Biochar’s recalcitrant carbon, combined with the degradable organic material from Spirulina, enriches SOM, which is critical for soil structure, water retention, and nutrient-holding capacity [8,14,15]. The enhancement of SOM and MBC under saline conditions is particularly promising, as it indicates potential for soil health restoration in degraded environments. The observed improvements in cation exchange capacity (CEC) further support the beneficial role of biochar and Spirulina in enhancing soil chemical properties. The highest CEC increases were observed in BC + SP treatment, pointing to the superior efficacy of the combined application. The rise in CEC enhances the soil’s ability to retain and exchange essential nutrients, which is particularly vital in sandy soils, where nutrient leaching is common [15].

Salinity typically leads to ionic imbalances, particularly elevated sodium (Na^⁺) levels that negatively affect soil structure and plant health. The significant reductions in Na^⁺ concentrations, especially under BC + SP treatment, demonstrate the ameliorative effects of these amendments on salinity stress. Moreover, the concurrent increases in calcium (Ca^2⁺) and potassium (K^⁺) concentrations and improved Ca/Na and K/Na ratios suggest enhanced nutrient availability and ionic balance. These ratios are key indicators of soil resilience to salinity, as higher values are associated with improved soil structure and reduced Na^⁺ toxicity [15]. The ability of BC + SP to achieve the highest ratios under saline irrigation highlights its potential for reclaiming salt-affected soils. Overall, these findings reinforce the complementary and potentially synergistic effects of biochar and Spirulina in improving soil quality parameters under both fresh and saline irrigation regimes. The combined application appears particularly effective in enhancing microbial activity, increasing SOM and CEC, and mitigating sodium stress. Such integrative soil management practices could be crucial for sustaining productivity in saline-prone, sandy soils.

Salinity stress resulted in a significant accumulation of Na^⁺ in mint shoots, which can disrupt cellular functions and compete with K^⁺ and Ca^2⁺ uptake. Interestingly, the BC + SP treatment markedly reduced Na^⁺ accumulation compared to BC or SP alone, indicating its role in improving ion selectivity and exclusion mechanisms. The significant improvement in K/Na and Ca/Na ratios under BC + SP, especially under SW irrigation, further supports this assertion and reflects a better ionic balance and enhanced salt tolerance in mint [8,15]. The combined treatment (BC + SP) was consistently superior to the individual applications, suggesting a synergistic interaction where BC provides a stable matrix for nutrient retention and microbial colonization, while SP contributes readily available nutrients and stimulates beneficial microbial communities [8,32]. SP and BC act as chelating compounds with high mineral content, which improved micronutrient bioavailability [14,32]. The ability of BC + SP to outperform individual treatments again underlines the importance of combining organic inputs that complement each other’s functions in the soil–plant system.

The combined application of biochar (BC) and Spirulina (SP) significantly reduced Na⁺ uptake in plants, indicating improved ionic homeostasis under saline soil conditions. This effect may be attributed to several biochemical and physiological mechanisms. One possible explanation is the ion exclusion mechanism at the root level, where biochar’s porous structure and surface functional groups (e.g., carboxyl and hydroxyl) adsorb excess Na⁺ ions in the rhizosphere, reducing their availability for plant uptake [14,15]. In parallel, Spirulina may enhance root membrane integrity and selectivity, possibly through modulation of ion transporters, such as high-affinity potassium transporters and sodium/proton antiporters, which help regulate Na⁺ influx and compartmentalization [14,15]. Moreover, Spirulina’s role as a biostimulant could be linked to its content of plant growth regulators such as auxins, cytokinins, and gibberellins, which have been shown to influence root architecture and ion transporter activity [8,17]. Recent studies also suggest that algal biostimulants may upregulate stress-responsive genes involved in ionic regulation and antioxidant defense systems, contributing to improved salt tolerance [8,17]. Spirulina is widely recognized for its sustainability and agricultural benefits, but large-scale production may raise environmental concerns due to high water and nutrient demands, potentially affecting marine resource sustainability and ecosystem balance [16]. The selected seaweed species were chosen based on their documented content of bioactive compounds such as alginates, laminarins, phenolics, and plant growth regulators (e.g., auxins and cytokinins), which are known to enhance nutrient uptake, improve soil microbial health, and stimulate plant physiological responses—factors closely linked to the outcomes observed in this study [16,17,18]. Collectively, these mechanisms indicate that BC + SP treatment supports plants not only by modifying the soil environment but also by enhancing physiological resilience at the cellular level.

5. Conclusions

The increasing reliance on low-quality saline water for irrigation demands innovative strategies to sustain crop productivity, especially in arid and semi-arid regions with limited fresh water resources. Medicinal plants such as Mentha longifolia L. (mint) demonstrate strong potential for cultivation under these challenging conditions. This study showed that saline water with an electrical conductivity up to 5 dS m⁻¹ can be effectively used to irrigate mint when supported by soil amendments. The combined application of biochar and the biostimulant Spirulina platensis significantly alleviated salinity stress, resulting in an 80% increase in mint biomass compared to untreated saline controls. This synergistic effect enhanced nutrient uptake, reduced sodium accumulation, and improved key soil quality parameters, including pH buffering capacity, microbial activity, organic matter content, and cation exchange capacity. These benefits were especially pronounced in sandy soils, highlighting the suitability of this integrated approach for salt-affected environments. Overall, the findings emphasize the potential of combining biochar with algal biostimulants as a sustainable and practical strategy to enhance plant performance and soil resilience under saline irrigation. However, several limitations must be considered. Variability in seaweed composition due to species differences, harvest timing, and environmental factors may affect consistency. Environmental conditions such as soil type, moisture, and climate could also influence amendment effectiveness in field settings. Moreover, assessing the economic and logistical feasibility of large-scale application of seaweed-based amendments remains essential. Further studies should investigate the use of Spirulina processing residues as soil amendments to promote circular sustainability by reducing waste and enhancing long-term soil health. Additionally, exploring gene expression profiles and transporter activities could provide deeper insights into plant responses under salinity stress and amendment treatments.