Sustainable Practices for Arid Climates: Evaluating Combined Mulches with Biostimulant in Combating Soil Salinity and Cowpea Cultivation

Abstract

Salinity and water security are significant challenges in arid climates, necessitating effective practices to enhance crop productivity in these stressful environments. To address this, a study was conducted during the summer seasons of 2022 and 2023 using a randomized, completely block setup with three replications. The research assessed the effects of different mulch materials, unmulched (bare soil), white plastic, rice straw, and sawdust, combined with biostimulant foliar applications (control, bulk chitosan at 250 mg/L, and two concentrations of chitosan nanoparticles at 125 mg/L and 62.5 mg/L) on physiochemical and biological properties of salt-affected soil, as well as on the growth and yield of cowpeas. The findings of this study indicate that different mulch materials exert distinct effects based on their type. For instance, white plastic mulch with chitosan nanoparticles at a concentration of 62.5 mg/L markedly decreased soil salinity (by 10.80% and 14.64%) and ESP (by 6.93% and 6.80%). In contrast, white plastic mulch paired with a control foliar application significantly increased the soil moisture content (by 23.93% and 27.63%) compared to un-mulched soil. The combination of organic mulches and biostimulant foliar treatments significantly enhanced soil health by increasing the pH, organic carbon, nutrient content, and beneficial bacteria while reducing the bulk density and suppressing harmful fungi. Biostimulant foliar treatments have a modest affected soil property. Additionally, this study highlights that integrating specific mulching materials with biostimulant foliar treatments can significantly improve cowpea’s vegetative growth, yield, and nutrient content. This suggests that combining mulches and biostimulants may provide a sustainable solution for enhancing cowpea production in saline environments.

1. Introduction

Agricultural soils worldwide are facing alarming depletion due to the pervasive impacts of global climate change [1]. Salt-affected soils are primarily found in arid and semi-arid regions, including both saline and sodic soils [2]. The accumulation of soluble salts and sodium ions in soils can arise from various sources. In these regions, fluctuating temperatures, rising sea levels, and unpredictable rainfall are increasingly causing droughts, which can lead to secondary salinity and sodicity [3]. Concurrently, anthropogenic activities exacerbate these challenges, leading to widespread soil salinization and sodification—a critical global issue [4,5]. Omar et al. [6] estimated that salt-affected soils impact around 1 billion hectares of land worldwide, with approximately 200 million hectares located in Africa. In Egypt, salt-affected soils are a significant concern, particularly in the northern–central part of the Nile Delta and along its eastern and western margins [7]. Approximately 56% of cultivated lands in the northern Delta are affected by soil salinity [8].

Soil salinization and sodification disrupt nutrient availability and promote clay dispersion, swelling, and soil pore plugging, which collectively reduce infiltration, hydraulic conductivity, water retention, the drainage capacity, organic matter content, and microbial activity [9]. Furthermore, under these conditions, root extension is inhibited which results in low infiltration rates, potentially causing waterlogging or the formation of perched water tables that suffocate roots [10]. Additionally, poor air movement into the subsoil reduces the oxygen availability for growing plants [11] and, also, can lead to various biochemical and fertility issues, including deficiencies in essential nutrients [12]. Furthermore, these conditions may induce specific ionic toxicities from sodium (Na⁺), chloride (Cl⁻), borate (H₃BO₄), and bicarbonate (HCO₃⁻), as well as osmotic stress affecting both microorganisms and plant cells; significantly hindering food production and agricultural progress [13,14].

Cowpea (Vigna unguiculata) is a leguminous crop belonging to the Fabaceae family [15]. The total area used for the cultivation of the cowpea plant in Egypt is estimated at 1853 ha, with a mean production of 7180 tons of dry seeds [16]. Cowpea is highly sensitive to salt stress, which negatively affects its morphological, physiological, and yield parameters [17]. Salt stress also disrupts the uptake and transport of essential nutrients, leading to deficiencies in phosphorus (P), potassium (K), and calcium (Ca), which are crucial for various metabolic processes necessary for plant development [18]. Therefore, effective management strategies are crucial for cultivating cowpea sustainably in salt-affected soils [19].

Mulches are materials applied to the soil surface to improve the soil quality and promote plant growth. They enhance the agricultural efficiency by conserving resources, reducing waste, and increasing productivity [20]. Mulches can be divided into three primary categories: organic (from plant or animal sources), synthetic (such as plastic films, nonwoven fabrics, and paper films), and living mulches, which are cover crops like clover and grass [21,22].

In dry land agriculture, plastic mulching offers several advantages: It helps retain soil moisture, reduces soil compaction and erosion, moderates soil temperatures, and improves soil organic matter and nutrient levels [23,24]. Additionally, it plays a crucial role in managing salt-affected soils by inhibiting salt accumulation and regulating its distribution, showing a strong correlation between cumulative evaporation and soil salinity [25]. However, the widespread use of plastic films raises environmental concerns due to their persistence in the environment and contribution to pollution [26,27].

An eco-friendly alternative is organic mulches, which are both cost-effective and environmentally sustainable [17]. Research has shown that straw mulching effectively reduced evaporation, regulated temperatures, and enhanced microbial activities, which are significant advantages for agricultural practices [28,29,30]. Similarly, sawdust mulch can lower the soil pH while increasing the moisture retention and nutrient content [31]. It also improved the soil organic matter and porosity, making it less dense and more favorable for plant growth [32,33].

Applying foliar fertilizers is an effective strategy for increasing element levels in crops and enhancing crop yields [34]. Leaves absorb nutrients efficiently through their stomatal openings and the epidermal layer [35], making this method a promising strategy for increasing plant tolerance to salt stress [36].

The use of biostimulants in sustainable agricultural practices is increasingly recognized as a promising, safe, and eco-friendly alternative for enhancing crop production [37]. Derived from natural sources, these biostimulants are particularly valued for their ecological benefits and biodegradability, which help minimize their environmental impact. Among them, chitosan, a cationic polysaccharide derived from chitin, is made up of repeating units of N-acetyl glucosamine and D-glucosamine [38]. Its structure includes three functional groups that allow for chemical modifications and influence its solubility [39]. Chitosan is more soluble in acidic environments than chitin [40] and is valued for its nontoxic, biocompatible properties, making it promising for sustainable agriculture [41].

Nanoagriculture utilizes nanomaterials, such as nanofertilizers, which are used to improve the soil quality and plant health [42]. Chitosan nanoparticles (ChNPs) are particularly effective due to their lower molecular weight, better bioavailability, longer half-life, and higher surface area-to-volume ratio [43,44]. ChNPs have been found to be more effective than bulk chitosan in alleviating salt stress due to their superior interaction with plant tissues and lower toxicity [45].

Applying chitosan or ChNPs can enhance various plant traits, such as chlorophyll levels, height, and yield, especially under stress conditions [46]. They can improve seed germination, promote growth, and enhance stress tolerance by increasing the antioxidant enzyme activity and reducing oxidative damage [47]. Research has shown that ChNPs can mitigate the negative effects of salinity on cowpea plant growth and improve physiological responses by activating stress signaling pathways [48]. Additionally, when combined with metallic nanoparticles, ChNPs exhibit altered structural and functional properties that further improve stress resilience [49].

There is limited research specifically examining mulching combined with chitosan (or its nanoparticles’) effectiveness in salt-affected soils and cowpea plants. So, this study aims to investigate how combined mulches with biostimulant foliar treatments (chitosan or its nanoparticles) influence the characteristics of salt-affected soils as well as the cowpeas growth and yield.

2. Materials and Methods

2.1. Site

Two field experiments were conducted from March to July in 2022 and 2023 growing seasons at agricultural research farm located in Sakha research station, Kafer El-Sheikh Governorate, Egypt.

The average of climatic data from the automated meteorological station for both seasons indicated maximum and minimum temperatures of 31.37 and 16.71 °C, respectively. Relative humidity averaged 56.64%, with monthly precipitation at 0.327 mm and wind speeds averaging 3.27 m/s at a height of 2 m.

The soil was classified as heavy clay, consisting of 56.37% clay, 25.64% silt, and 17.99% sand, and it was saline–sodic in nature. Initial soil properties are summarized in

Table 1. Average Initial Soil Properties.

Soil pH 1	EC (dS/m) 2	ESP
8.70	8.04	16.66
Soil organic carbon (g/kg)	Soil moisture content (%)	Soil bulk density (g/cm3)
0.530	29.51	1.36
Available N (g/kg)	Available P (g/kg)	Available K (g/kg)
18.62	8.63	180.49
Bacterial counts (CFU × 107 g−1 dry soil)	Fungal counts (CFU × 104 g−1 dry soil)
3.67	2.46

¹ 1: 2.5 soil water, ² soil paste extract.

.

2.2. Design

Each treatment was assigned to one of 48 plots in a randomized, completely block design, with 3 replications. Each plot measured 6 m² (3 m × 2 m), resulting in a total area of 288 m². The various treatment combinations tested were as follows:

-

T1: un-mulched without foliar application (CH₀);

-

T2: un-mulched with foliar by bulk chitosan at 250 mg/L (CH₁);

-

T3: un-mulched with nano-sized chitosan at 125 mg/L (CH₂);

-

T4: un-mulched with nano-sized chitosan at 62.5 mg/L (CH₃);

-

T5: white plastic mulches (30 μm) with CH₀;

-

T6: white plastic mulches (30 μm) with CH₁;

-

T7: white plastic mulches (30 μm) with CH₂;

-

T8: white plastic mulches (30 μm) with CH₃;

-

T9: rice straw mulches (15 cm) with CH₀;

-

T10: rice straw mulches (15 cm) with CH₁;

-

T11: rice straw mulches (15 cm) with CH₂;

-

T12: rice straw mulches (15 cm) with CH₃;

-

T13: sawdust mulches (15 cm) with CH₀;

-

T14: sawdust mulches (15 cm) with CH₁;

-

T15: sawdust mulches (15 cm) with CH₂;

-

T16: sawdust mulches (15 cm) with CH₃.

2.3. Bulk Chitosan Characterization and Chitosan Nanoparticles’ Synthesis

Chitosan was sourced from shrimp shell, with a deacetylation degree (DD) of 90–95% and a molecular weight of 100 cP. The synthesis of nano-sized chitosan was achieved through milling, as described by Sari et al. [50]. The resulting nanoparticles had an average particle size distribution of 88.67 nm (Figure 1a), confirming their nano-scale dimensions. Scanning electron microscopy (SEM) images indicated that the nanoparticles were spherical in shape (Figure 1b), consistent with the desired morphology. Additionally, the chitosan nanoparticles exhibited an average zeta potential of +47.01 mV, categorizing them as strongly cationic and stable [51].

Chitosan solution for foliar spray was prepared by dissolving various concentrations in 800 mL of distilled water with 1% acetic acid. The mixture was stirred until fully dissolved, and then the final volume was adjusted to 1 L.

2.4. Cultural Practices

In both seasons, rows were established in each plot, measuring 5 m in length, 60 cm in width, and 30 cm in height, with a plant spacing of 25 cm. Cowpea seeds (cv. Kafr El-Sheikh) were sown on 20 March and harvested at full maturity (July) in both seasons.

Mulching materials were applied 10 days after sowing. According to Tavares et al. [18], cowpea plants began to show signs of salinity stress 15 days after sowing, prompting foliar applications of chitosan and nano-chitosan at 15, 30, and 45 days post-sowing. Agricultural practices and irrigation followed the recommended guidelines for cowpea cultivation.

2.5. Parameters Assessed

Soil samples were collected at a depth of 60 cm, both before and after crop harvest, to analyze various physicochemical characteristics and soil microbial communities. The sampling methods, as well as the procedures for chemical analysis (soil pH, EC, and ESP), physical analysis (particle size distribution, soil bulk density, and soil moisture content), and soil fertility (nutrient content and soil organic carbon), followed the protocols outlined by Page et al. [52], Sheldrick and Wang [53], Campbell [54], and Klute [55]. Colony-forming units (bacterial and fungal) were counted as described by Vieira and Nahas [56].

Plant growth measurements included plant height (cm), leaf area (cm²) estimated by Leaf Area Meter LLAM-A10, fresh weight of leaves, branches and roots (g/plant), and chlorophyll content (µg/mL), which was determined by Chlorophyll Content Meter Model CCM-200; these were measured 60 days after sowing the cowpeas. Yield and quality measurements at harvest included: pod length (cm), number of pods/plant, seeds weight (g/plants), and straw and seed yield (kg/ha). Macro nutrients (N, P, and K) of plants post-harvest were analyzed using standard methods of Peterburgski [57], Jackson [58], and Snell and Snell [59].

2.6. Data Processing

All collected data were analyzed using One-Way analysis of variance (ANOVA), with Tukey’s test for comparisons significant differences established at p ≤ 0.05. The statistical software used for analysis was Minitab version 21. Principal Component Analysis and correlation biplot were analyzed using R software (ver. 4.3.1).

3. Results

3.1. Soil Properties

The data in in

Table 2. (a,b). The soil parameters across different combination treatments.

(a)

Parameters

Soil pH

EC (dS/m)

ESP

SMC (%)

BD (g/cm3)

Organic Carbon (g/kg)

Treatments

2022

2023

2022

2023

2022

2023

2022

2023

2022

2023

2022

2023

T1

8.72 a

8.74 a

8.08 a

7.90 a

16.72 a

16.64 a

31.38 e

31.08 e

1.38 a

1.39 a

0.591 i

0.572 g

T2

8.67 ab

8.63 b

7.97 a

7.76 ab

16.64 a

16.48 ab

30.91 ef

30.52 ef

1.37 a

1.37 ab

0.648 fgh

0.657 defg

T3

8.62 abc

8.57 bc

7.95 ab

7.71 b

16.61 a

16.40 abc

30.65 f

30.24 ef

1.36 b

1.36 bc

0.656 efgh

0.671 cdef

T4

8.58 cd

8.50 cd

7.94 ab

7.69 b

16.51 ab

16.36 abc

30.31 f

30.70 f

1.35 bc

1.34 cd

0.665 defgh

0.686 bcde

T5

8.60 bcd

8.57 bc

7.32 e

6.96 gh

15.81 ef

15.83 defgh

38.89 a

39.67 a

1.35 cd

1.34 de

0.620 hi

0.575 fg

T6

8.53 cde

8.45 de

7.22 e

6.82 hi

15.71 f

15.65 fgh

38.67 ab

39.34 a

1.34 cd

1.32 def

0.680 cdef

0.698 bcde

T7

8.48 efg

8.38 ef

7.20 e

6.76 i

15.67 f

15.56 gh

38.36 ab

39.23 a

1.34 de

1.32 efg

0.690 cdef

0.713 abcd

T8

8.45 efg

8.36 ef

7.19 e

6.74 i

15.56 f

15.51 h

38.09 b

38.91 a

1.33 ef

1.32 efg

0.699 cde

0.730 abcd

T9

8.57 cd

8.52 bcd

7.77 bc

7.47 c

16.29 bc

16.09 cde

35.24 d

36.13 c

1.32 fg

1.31 fg

0.630 ghi

0.607 efg

T10

8.46 efg

8.34 fg

7.67 cd

7.31 cde

16.17 cd

15.91 defg

35.37 d

35.71 cd

1.32 fg

1.31 fg

0.696 cde

0.697 bcde

T11

8.45 efg

8.32 fg

7.65 cd

7.23 def

16.12 cd

15.82 defgh

35.21 d

35.36 cd

1.31 gh

1.30 gh

0.707 bcd

0.714 abcd

T12

8.42 g

8.3 fg

7.63 cd

7.21 def

16.01 de

15.78 efgh

34.95 d

35.10 d

1.31 h

1.30 gh

0.718 bc

0.729 abcd

T13

8.53 def

8.46 de

7.66 cd

7.35 cd

16.34 bc

16.17 bcd

36.79 c

37.97 b

1.29 i

1.28 hi

0.674 cdefg

0.660 defg

T14

8.46 efg

8.33 fg

7.57 d

7.20 def

16.23 cd

15.97 def

36.63 c

37.92 b

1.29 i

1.27 ij

0.747 ab

0.759 abc

T15

8.43 fg

8.29 fg

7.54 d

7.15 ef

16.18 cd

15.88 defg

36.44 c

37.68 b

1.28 i

1.26 ij

0.761 a

0.777 ab

T16

8.40 g

8.25 g

7.51 d

7.11 fg

16.08 cd

15.83 defgh

36.20 c

37.35 b

1.27 j

1.25 j

0.771 a

0.792 a

p values

**

**

*

**

*

*

**

**

**

**

**

**

(b)

Parameters

Available N (g/kg)

Available P (g/kg)

Available K (g/kg)

Bacterial Count (CFU × 107 g−1 Dry Soil)

Fungal Count (CFU × 104 g−1 Dry Soil)

Treatments

2022

2023

2022

2023

2022

2023

2022

2023

2022

2023

T1

20.19 m

19.03 f

9.38 de

8.82 ef

169.32 f

163.92 e

3.85 e

3.62 e

4.79 a

4.75 a

T2

21.25 l

19.66 ef

9.35 de

8.59 ef

174.39 f

179.56 d

4.23 ef

4.75 d

4.17 b

4.40 a

T3

22.17 k

19.75 ef

9.37 de

8.22 f

174.66 f

179.69 d

4.36 ef

5.09 d

3.88 bc

3.48 b

T4

22.77 j

20.08 def

9.06 e

7.89 f

177.19 f

180.99 d

4.58 e

5.21 d

3.74 cd

3.41 b

T5

23.65 i

22.20 cde

11.21 abc

11.98 ab

219.23 e

228.37 c

6.07 d

6.43 c

3.64 cd

3.05 bc

T6

24.41 h

22.60 bcd

10.45 bcde

10.32 c

224.51 de

236.52 c

6.23 d

6.40 c

3.43 de

3.02 bc

T7

25.18 g

23.67 abc

9.81 cde

9.65 cde

224.60 de

236.55 c

6.07 d

6.62 c

3.26 ef

2.92 bc

T8

25.37 g

23.17 abc

9.29 de

8.98 def

231.44 cde

236.68 c

6.19 d

6.94 c

3.14 efg

2.83 bcd

T9

27.45 cd

24.49 abc

12.11 a

13.16 a

237.80 bcd

254.47 b

7.60 c

7.83 b

3.11 efg

2.97 bc

T10

27.85 c

25.20 ab

12.52 ab

12.94 a

241.59 abc

260.92 ab

7.77 bc

7.98 b

3.04 fgh

2.62 cd

T11

28.58 b

24.76 abc

11.32 abc

12.86 a

241.87 abc

261.31 ab

7.81 bc

7.80 b

2.95 fghi

2.59 cd

T12

29.18 a

25.47 a

10.75 bcd

12.20 ab

242.74 abc

261.69 ab

7.89 bc

8.34 ab

2.86 fghij

2.55 cd

T13

25.91 f

23.22 abc

11.76 ab

12.53 ab

252.92 ab

266.33 ab

8.07 abc

8.42 ab

2.91 ghij

2.64 cd

T14

26.38 e

23.27 abc

11.71 ab

12.41 ab

254.31 a

269.39 a

8.12 abc

8.28 ab

2.71 hij

2.48 cd

T15

26.50 e

24.87 abc

10.48 bcde

11.59 b

254.50 a

269.71 a

8.36 ab

8.52 ab

2.63 ij

2.44 cd

T16

27.09 d

23.79 abc

8.97 de

10.16 cd

254.55 a

270.90 a

8.64 a

8.92 a

2.54 j

2.22 d

p values

**

*

*

**

**

**

*

**

**

**

EC: soil salinity, ESP: exchangeable sodium percentage, SMC: soil moisture content, and BD: soil bulk density. T1: un-mulched with CH₀, T2: un-mulched with CH₁, T3: un-mulched with CH₂, T4: un-mulched with CH₃, T5: white plastic mulches with CH₀, T6: white plastic mulches with CH₁, T7: white plastic mulches with CH₂, T8: white plastic mulches with CH₃, T9: rice straw mulches with CH₀, T10: rice straw mulches with CH₁, T11: rice straw mulches with CH₂, T12: rice straw mulches with CH₃, T13: sawdust mulches with CH₀, T14: sawdust mulches with CH₁, T15: sawdust mulches with CH₂, T16: sawdust mulches with CH₃. p values: ** p < 0.01 and * p < 0.05. A letter is using for comparisons significant differences between treatments within the same row (Tukey’s test at p ≤ 0.05). The lowest exchangeable sodium percentage (ESP) values, 15.56 and 15.51, were observed with the T8 treatment (white plastic mulches with CH₃) in both seasons. However, no significant differences in ESP values were noted across the combinations of all mulch materials with biostimulant foliar treatments in 2023.

a,b compare the studied soil parameters across different combinations of mulch materials with biostimulant foliar treatments for the 2022 and 2023 seasons.

In both years, the soil parameters in the control treatment (T1) were consistently the lowest, indicating that the control treatment, which lacked mulch and biostimulant foliar applications, resulted in suboptimal soil conditions compared to the other treatments.

The combinations of white plastic with biostimulant foliar treatments (T5 to T8) resulted in a clear reduction in soil salinity and an increase in the soil moisture content, irrespective of the biostimulant applications, in both growing seasons (Table 2a). The highest reduction in salinity was observed with the T8 treatment (white plastic mulches with CH₃), showing a decrease of 10.80% in 2022 and 14.67% in 2023.

Similarly, the white plastic mulches with CH₀ (T5) significantly improved the soil moisture content, increasing it by 23.93% in 2022 and 27.63% in 2023.

The combinations of sawdust with biostimulant foliar treatments (T13 to T16) resulted in a noticeable reduction in the bulk density, though no significant effect was observed from the biostimulant foliar applications (Table 2a).

Both organic mulches with biostimulant foliar treatments (T10, T11, T12, T14, T15, and T16) exhibited a lower soil pH and soil organic carbon values. However, no significant differences in the soil pH were observed across the different mulch and biostimulant combinations with the two concentrations of the chitosan nanoparticles CH₂ or CH₃ in 2022 (Table 2a).

Additionally, both organic mulches with biostimulant foliar treatments improved the availability of soil nutrients. Soil microbial communities showed a significant increase in bacterial growth, accompanied by a marked decrease in fungal growth, although biostimulant foliar applications had no observable effect on microbial dynamics (Table 2b).

In this study, we observed that under different combinations of mulch materials when applying biostimulant foliar treatments (CH₁, CH_2, and CH₃), the soil moisture content values (Table 2a), available phosphorus, and fungal growth (Table 2b) were decreased in the 2022 and 2023 seasons than with CH₀.

Figure 2 shows the relationships between the studied soil parameters. A strong positive connection was observed between the soil pH, EC, ESP, bulk density, and fungal growth. Conversely, these parameters exhibited a negative connection with the organic carbon, available soil nutrient content, soil moisture content, and bacterial communities.

The correlation results also revealed a clear positive relationship between soil organic carbons and all soil parameters studied.

3.2. Plant Growth and Yield

This section examined the effects of various mulching materials combined with biostimulant foliar treatments on the growth parameters, biomass, and micro-elemental concentrations in cowpea seeds (see

Table 3. The growth parameters across different combination treatments.

Parameters	Plant Height (cm)		Chlorophyll Content (µg mL−1)		Leaf Areas (cm2/Plant)		Fresh Weight (g/Plant)
							Leaves		Branches		Roots
Treatments	2022	2023	2022	2023	2022	2023	2022	2023	2022	2023	2022	2023
T1	25.50 f	26.80 e	26.40 f	27.75 e	44.52 f	48.29 e	4.79 j	5.38 h	3.89 e	4.34 b	0.405 f	0.421 g
T2	31.50 e	34.06 cd	30.80 e	31.40 d	53.07 e	54.40 d	5.41 i	5.99 g	4.45 cd	4.60 b	0.484 ef	0.499 f
T3	33.75 cd	34.50 c	34.00 cd	35.05 bc	56.30 c	65.31 b	6.72 g	6.96 de	4.92 bc	5.13 b	0.545 cde	0.571 de
T4	34.75 c	35.75 c	37.28 ab	37.73 ab	61.19 ab	68.19 ab	7.84 cde	7.79 bc	5.88 a	6.11 a	0.592 abc	0.623 bc
T5	32.85 de	34.30 cd	33.87 cd	34.62 c	54.53 ce	60.51 c	6.10 h	6.34 ef	4.93 bc	4.93 b	0.513 de	0.533 ef
T6	38.00 ab	39.50 ab	36.62 b	37.17 b	59.84 b	67.35 ab	7.64 def	7.82 bc	5.85 a	6.22 a	0.597 abc	0.621 abc
T7	38.25 ab	40.00 ab	37.70 ab	38.43 ab	61.98 ab	68.94 ab	8.11 bcd	8.27 abc	6.11 a	6.20 a	0.619 abc	0.645 ab
T8	39.25 a	40.50 a	38.80 a	39.03 a	63.00 a	71.06 a	8.92 a	8.88 a	6.26 a	6.35 a	0.656 a	0.659 a
T9	32.50 de	34.25 cd	30.68 d	31.83 d	54.23833	58.88 c	5.64 hi	6.18 efg	4.52 cd	4.75 b	0.510 de	0.530 ef
T10	37.75 ab	39.25 ab	36.45 b	36.95 b	59.45 b	67.36 a	7.53 ef	7.85 bc	5.83 ab	6.07 a	0.605 abc	0.607 bc
T11	38.00 ab	39.75 ab	36.80 b	37.70 ab	61.31 ab	68.24 ab	8.05 bcd	8.22 abc	5.90 a	6.11 a	0.607 abc	0.635 ab
T12	38.75 ab	40.00 ab	38.38 ab	38.88 a	62.39 ab	70.40 a	8.55 ab	8.73 ab	6.18 a	6.28 a	0.645 a	0.651 ab
T13	31.90 e	32.30 d	30.85 cd	31.40 d	52.89 ce	53.95 d	5.53 i	6.01 fg	4.33 de	4.36 b	0.485 de	0.498 f
T14	37.20 b	38.00 b	36.60 b	36.73 bc	58.73 bc	66.65 b	7.31 f	7.78 bc	5.70 ab	5.98 a	0.564 bcd	0.593 cd
T15	37.75 ab	38.25 b	36.25 b	37.53 ab	59.25 b	67.70 ab	7.92 cde	8.00 abc	5.88 a	6.02 a	0.595 abc	0.626 bc
T16	38.75 ab	39.50 ab	37.90 ab	38.12 ab	61.65 ab	69.26 ab	8.24 bc	8.72 ab	6.16 a	6.25 a	0.630 ab	0.642 ab
p values	**	**	**	**	**	**	**	**	**	**	**	**

T1: un-mulched with CH₀, T2: un-mulched with CH₁, T3: un-mulched with CH₂, T4: un-mulched with CH₃, T5: white plastic mulches with CH₀, T6: white plastic mulches with CH₁, T7: white plastic mulches with CH₂, T8: white plastic mulches with CH₃, T9: rice straw mulches with CH₀, T10: rice straw mulches with CH₁, T11: rice straw mulches with CH₂, T12: rice straw mulches with CH₃, T13: sawdust mulches with CH₀, T14: sawdust mulches with CH₁, T15: sawdust mulches with CH₂, T16: sawdust mulches with CH₃. p values: ** p < 0.01. A letter is used for comparisons of significant differences between treatments within the same row (Tukey’s test at p ≤ 0.05).

,

Table 4. The yield and its parameters across different combination treatments.

Parameters	Pod Length (cm)		No. of Pods/Plant		Seed Weight (g/Plants)		Seed Yield (kg/ha)		Seed Yield (kg/ha)
Treatments	2022	2023	2022	2023	2022	2023	2022	2023	2022	2023
T1	10.83 g	11.33 g	14.67 f	15.67 e	16.15 f	16.38 h	1613.90 h	1637.32 h	2185.96 e	2250.34 d
T2	11.83 f	12.75 ef	16.17 cdef	18.33 cd	17.66 ef	18.43 gh	1764.89 gh	1797.42 gh	2655.91 cde	2720.84 bcd
T3	12.50 e	13.00 ef	17.00 bcde	18.67 cd	18.07 def	17.98 fgh	1806.44 fgh	1841.83 fg	2907.88 abcd	2887.56 abc
T4	13.83 cd	14.46 cd	18.00 abcd	19.46 bcd	20.33 bc	20.78 bcde	2031.83 bcde	2077.60 bcde	2865.26 abcd	2930.73 abc
T5	13.00 de	13.83 bcd	16.33 cdef	19.67 bcd	20.06 bcd	20.39 bcdef	2005.56 bcdef	2038.30 cdef	2857.88 abcd	2921.19 abc
T6	13.67 cd	14.83 bc	18.00 abcd	20.25 abc	20.59 bc	21.36 abcd	2058.53 abcd	2110.42 abcde	3222.50 ab	3174.68 abc
T7	14.69 bc	15.58 ab	18.67 abcd	21.00 a	22.11 ab	22.50 ab	2143.38 abc	2182.79 abcd	3240.66 ab	3217.90 ab
T8	16.00 a	16.17 a	20.00 a	21.33 a	22.91 a	23.13 a	2256.79 a	2278.61 a	3257.58 a	3269.26 a
T9	12.83 e	13.23 def	16.42 cdef	18.00 d	19.16 cde	19.47 defg	1881.82 defg	1912.38 efg	2697.40 bcde	2768.23 abcd
T10	13.50 cd	14.83 bc	17.67 bcde	19.67 bd	20.40 bc	21.13 abcd	2105.66 bc	2136.76 abcd	3097.98 abcd	3110.28 abc
T11	14.40 c	15.50 ab	18.33 abcd	20.33 ab	21.70 ab	22.02 abc	2168.75 b	2200.57 abc	3162.69 abc	3191.03 abc
T12	15.50 ab	15.67 ab	19.33 ab	21.00 a	22.17 ab	22.44 ab	2216.21 ab	2242.65 ab	3221.76 ab	3235.11 ab
T13	12.50 e	12.63 f	15.17 cdef	18.25 cd	19.03 cde	18.98 efg	1835.48 efg	1864.04 fg	2558.50 de	2695.61 cd
T14	13.33 de	13.83 cd	16.67 cdef	19.17 bcd	19.44 cde	20.01 cdefg	1942.72 cdefg	2000.06 def	3058.42 abcd	3086.98 abc
T15	14.00 c	14.25 cd	18.00 abcd	19.50 bcd	21.13 abc	21.45 abcd	2112.16 abc	2143.78 abcd	3114.25 abc	3151.24 abc
T16	15.00 b	15.50 ab	19.00 ab	21.00 a	21.87 ab	22.18 ab	2185.65 ab	2217.09 abc	3184.58 abc	3180.97 abc
p values	**	**	**	**	**	**	**	**	**	**

T1: un-mulched with CH₀, T2: un-mulched with CH₁, T3: un-mulched with CH₂, T4: un-mulched with CH₃, T5: white plastic mulches with CH₀, T6: white plastic mulches with CH₁, T7: white plastic mulches with CH₂, T8: white plastic mulches with CH₃, T9: rice straw mulches with CH₀, T10: rice straw mulches with CH₁, T11: rice straw mulches with CH₂, T12: rice straw mulches with CH₃, T13: sawdust mulches with CH₀, T14: sawdust mulches with CH₁, T15: sawdust mulches with CH₂, T16: sawdust mulches with CH₃. p values: ** p < 0.01. A letter is used for comparisons of significant differences between treatments within the same row (Tukey’s test at p ≤ 0.05).

and

Table 5. The macro nutrients content in cowpeas seed across different combination treatments.

Parameters	N%		P%		K%		Na%
Treatments	2022	2023	2022	2023	2022	2023	2022	2023
T1	2.09 h	2.14 e	0.187 g	0.192 f	0.723 f	0.726 f	0.227 a	0.211 a
T2	2.17 efg	2.19 cd	0.198 f	0.208 e	0.733 f	0.735 ef	0.175 c	0.162 b
T3	2.18 efg	2.19 cd	0.210 e	0.223 d	0.742 ef	0.744 ef	0.169 c	0.155 bc
T4	2.21 bcde	2.21 c	0.222 d	0.238 bcd	0.751 def	0.753 de	0.167 c	0.155 bc
T5	2.16 fg	2.15 de	0.224 cd	0.234 bcd	0.736 f	0.736 ef	0.189 b	0.171 b
T6	2.20 cdef	2.19 cd	0.223 d	0.227 cd	0.740 f	0.742 ef	0.143 d	0.143 c
T7	2.21 bcde	2.22 bc	0.232 bcd	0.239 bcd	0.754 de	0.751 de	0.142 d	0.130 c
T8	2.25 ab	2.27 ab	0.241 ab	0.251 ab	0.772 c	0.766 cd	0.146 d	0.131 c
T9	2.17 efg	2.15 de	0.236 bc	0.243 abc	0.759 cd	0.756 cde	0.169 c	0.156 bc
T10	2.21 bcde	2.24 abc	0.232 bcd	0.235 bcd	0.748 de	0.752 de	0.136 d	0.130 c
T11	2.23 abcd	2.26 ab	0.241 ab	0.247 ab	0.769 cd	0.773 c	0.136 d	0.134 c
T12	2.26 a	2.28 a	0.250 a	0.259 a	0.790 b	0.794 b	0.137 d	0.133 c
T13	2.15 g	2.15 de	0.224 cd	0.226 cd	0.7475	0.749 de	0.166 c	0.155 bc
T14	2.19 defg	2.24 abc	0.230 bcd	0.235 bcd	0.7685	0.770 cd	0.140 d	0.133 c
T15	2.20 cdef	2.24 abc	0.236 bc	0.244 abc	0.790 b	0.791 b	0.140 d	0.137 c
T16	2.24 abc	2.28 a	0.242 ab	0.253 ab	0.811 a	0.812 a	0.141 d	0.136 c
p values	**	*	**	**	**	*	**	**

T1: un-mulched with CH₀, T2: un-mulched with CH₁, T3: un-mulched with CH₂, T4: un-mulched with CH₃, T5: white plastic mulches with CH₀, T6: white plastic mulches with CH₁, T7: white plastic mulches with CH₂, T8: white plastic mulches with CH₃, T9: rice straw mulches with CH₀, T10: rice straw mulches with CH₁, T11: rice straw mulches with CH₂, T12: rice straw mulches with CH₃, T13: sawdust mulches with CH₀, T14: sawdust mulches with CH₁, T15: sawdust mulches with CH₂, T16: sawdust mulches with CH₃. p values: ** p < 0.01 and * p < 0.05. A letter is used for comparisons of significant differences between treatments within the same row (Tukey’s test at p ≤ 0.05).

) over two growing seasons (2022 and 2023).

3.2.1. Vegetative Growth Parameters

Conversely, the combination of un-mulched soil and the control (tap water), T1, resulted in the poorest performance (Table 3). The best growth outcomes were observed with the combination of white plastic and CH₃ (T8), which resulted in increases of 53.92% and 51.12% for plant height, 47.00% and 40.66% for the chlorophyll content, 41.53% and 47.17% for leaf areas, 86.15% and 65.27% for the fresh weight of leaves, 61.09% and 46.44% for the fresh weight of branches, and 62.06% and 56.72% for the fresh weight of roots relative to the control treatment in both seasons, respectively. However, the statistical analysis (p ≤ 0.05) indicated that the all combination between mulch materials and CH₂ and CH₃ significantly outperformed the control (T1) in promoting plant growth.

3.2.2. Yield and Its Parameters

The combination of mulch materials and biostimulant foliar treatments significantly influenced the yield parameters (Table 4). In particular, the combination of white plastic and CH₃ (T8) achieved the highest values in the pod length (16.17 cm), no. of pods per plant (21.33), no. of seed per plant (23.13 g), seed yield (2278.61 kg h⁻¹), and straw yield (3269.26 kg h⁻¹) in 2023. However, the yield parameters were not significantly affected by any combination of mulches with either foliar bulk chitosan (CH₁) or nano-sized chitosan particles (CH₂ and CH₃) in both seasons.

3.2.3. Macro Nutrients Content in Cowpeas Seed

The data presented in Table 5 indicate that the integration of mulch materials with biostimulant foliar treatments generally resulted in enhanced nitrogen, phosphorus, and potassium concentrations within the seeds, while concurrently inducing a significant reduction in the sodium content compared to the control treatment (T1), which was accompanied by a notable increase in sodium concentrations. Specifically, the combination of rice straw mulch and the CH₃ biostimulant (T12) recorded the higher values for both the nitrogen and phosphorus content in the seeds, along with a substantial increase in the potassium content. In contrast, the use of sawdust mulch combined with the CH₃ treatment (T16) resulted in the highest potassium accumulation. Furthermore, all treatments combining various mulch materials with either CH₂ or CH₃ biostimulants resulted in statistically comparable effects on the nutrient content of cowpea seeds, highlighting a consistent improvement across these combinations.

3.3. Principal Component Analysis

The Principal Component Analysis (PCA) biplot elucidates the differential impacts of the different combination treatments (denoted as T1 through T16) on the examined soil and plant parameters (Figure 3). The first principal component (Dim1) accounts for 72.8% of the total variance, while the second component (Dim2) explains 13.4%. These components represent the axes along which the maximal variance in the dataset is captured. The accompanying percentages quantify the proportion of variability attributable to each respective component. The positioning of the treatment symbols along these axes reflects the extent to which these treatments modulate the corresponding soil and plant traits. Furthermore, the vectors representing the variables are indicative of their relationships with the principal components. The orientation and magnitude of each vector are pivotal in determining the strength and direction of the correlation between the variables and the components. Vectors aligned with an axis are positively correlated with that axis, whereas those oriented oppositely are negatively correlated. The length of each vector serves as a surrogate measure of the strength of this correlation, with longer vectors denoting more pronounced associations.

This biplot furnishes a nuanced visual representation of the interplay between treatments and the array of plant traits. It facilitates the discernment of treatment similarities and differences, as well as the identification of those traits most heavily influenced by the treatments.

It is of particular note that the tripartite set of treatments, encompassing white plastic (T7–9) in conjunction with CH₁, CH₂, and CH₃ biostimulants, manifests a pronounced positive correlation, thereby suggesting a remarkable degree of homogeneity in their respective influences on the plant traits under scrutiny. Conversely, the T1 treatment, situated at more disparate loci on the biplot, insinuates a distinct divergence in its effects on the array of soil and plant parameters examined. Furthermore, the overarching trend is unmistakable: all permutations of the mulch–biostimulant combinations consistently tend to yield the most favorable outcomes concerning the seed yield, as corroborated by their positive orientation along the relevant principal component axes.

4. Discussion

The analysis of the soil and plant parameters across the different treatment combinations (T1–T16), as presented in the preceding tables and figures, reveals a range of significant effects that underscore the complex interactions between mulch materials, biostimulant foliar treatments, and the resultant soil and plant responses. The control treatment (T1), which lacked both mulch and biostimulant foliar applications, consistently displayed the least favorable outcomes in terms of key soil health indicators and plant performance, thereby reinforcing the notion that both organic mulches and biostimulants play crucial roles in enhancing edaphic conditions and promoting plant growth.

The incorporation of white plastic mulch combined with biostimulant foliar treatments (T5–T8) exhibited a pronounced and consistent improvement across soil parameters in both growing seasons, particularly in the form of notable reductions in soil salinity and significant increments in soil moisture retention. The remarkable attenuation of soil salinity, particularly in T8 (white plastic mulch coupled with CH₃), suggests the efficacy of this combination in mitigating the deleterious impacts of soil salinization—an affliction that severely compromises plant growth and impedes nutrient uptake. This reduction is plausibly attributable to the mulch’s capacity to curtail evaporation, thus facilitating the enhanced retention of both moisture and nutrients within the rhizosphere. Concurrently, the T8 treatment exhibited the lowest exchangeable sodium percentage (ESP) values, a crucial marker of soil sodicity, in both years. The substantial decline in ESP underscores a likely amelioration of ion exchange processes within the soil matrix, which could catalyze the availability of essential nutrients while mitigating the risks of nutrient imbalances commonly associated with sodic soils. However, the absence of statistically significant differences in ESP during 2023 suggests that extrinsic factors such as temporal variations in climatic conditions or fluctuations in the biostimulant application methodology may have influenced the treatment outcomes.

The marked improvement in the soil moisture content observed in T5 (white plastic mulch with CH₀) further accentuates the synergistic effects of mulching and biostimulant foliar application in preserving optimal soil water levels. The reported increase in moisture retention highlights the role of white plastic mulches as potent agents for moisture conservation, particularly in arid or semi-arid environments where water availability is a critical limiting factor.

This observation is congruent with findings in the literature by Haque et al. [60] and Khalifa et al. [33], who asserted that white plastic mulches, in particular, outperform organic mulches in mitigating soil salinity and enhancing moisture retention.

The use of sawdust mulch combined with biostimulant foliar treatments (T13–T16) led to a reduction in the bulk density, although no statistically significant effects were observed from the biostimulants themselves. This suggests that the physical properties of the sawdust mulch material, rather than the foliar treatments, were primarily responsible for this outcome. A decrease in the bulk density enhances the soil porosity, thus promoting root penetration and facilitating water infiltration, which are pivotal factors for supporting plant growth. This finding corroborates earlier studies by Khalifa et al. [33], who posited that organic mulches such as sawdust can elevate soil organic matter levels and improve the soil structure. Furthermore, the decomposition of sawdust could play a role in the amelioration of the bulk density by augmenting the soil organic content, as suggested by Paunovic et al. [31]

The observed reductions in both the soil pH and organic carbon content in treatments incorporating organic mulches (T10, T11, T12, T14, T15, and T16) were anticipated, given the known acidifying effect of organic mulches and their influence on microbial activity. However, the lack of significant differences in the soil pH among treatments involving CH₂ and CH₃ biostimulants in 2022 suggests that the pH dynamics were primarily governed by the mulch materials themselves rather than the foliar treatments. This is consistent with the work of Paunovic et al. [31], who highlighted the acidifying nature of sawdust mulch, and Ahmed et al. [48], who noted that chitosan could potentially alter the pH, but under specific conditions. The interplay between mulch-induced pH adjustments and biostimulant applications remains complex and warrants further exploration to delineate their combined effects on soil chemistry [33].

Microbial dynamics provided another intriguing facet of this study, with biostimulant foliar treatments driving a significant increase in bacterial populations, coupled with a decrease in fungal biomass. This shift in the microbial composition may suggest that biostimulants, particularly chitosan, preferentially stimulate bacterial growth while inhibiting fungal proliferation, which is of particular interest given the critical role of bacteria in nutrient cycling and the potential benefits of fungal suppression in controlling plant pathogens. However, the absence of a clear, overarching impact on the total microbial community structure implies that biostimulants may exert their influence more selectively on specific microbial taxa or under particular environmental conditions. These findings align with those of Caboň et al. [61] and Khalifa et al. [33], who observed that mulching can have differential effects on microbial communities, with organic mulches such as sawdust possibly creating less favorable conditions for fungal growth. Additionally, chitosan and its nanoparticles have antimicrobial properties which may be responsible for the observed decrease in fungal populations, as suggested by Divya et al. [62] and Fu et al. [63]

Salinization poses substantial challenges to cowpea production by limiting the growth and yield potential [17,18,64]. The marked improvements in vegetative growth parameters with the mulch materials and CH₂ and CH₃ biostimulants biostimulant treatments across both seasons reflect the synergistic effects of mulching and biostimulant application on plant development.

Significant increases in the plant height, chlorophyll content, leaf area, and fresh weights of leaves, branches, and roots provide strong evidence for the effectiveness of this treatment combination in enhancing plant biomass. The significant increases in vegetative growth and yield parameters, the same as quality seeds, suggest that these treatments create a more conducive environment for plant growth, likely through improved soil conditions, better moisture retention, enhanced nutrient availability, and concurrently inducing a significant reduction in the sodium uptake. These findings corroborate the growing body of the literature supporting the role of mulches in alleviating abiotic stressors such as water deficits and soil compaction, thus facilitating enhanced plant development [65]. The studies conducted by Ahmed et al. [48], Ray et al. [66], and Tartoura et al. [67] have established chitosan and its nanoparticles as a particularly effective agent in mitigating salinity stress, thereby optimizing the yield and productivity of cowpea plants. This efficacy is attributed to its role in enhancing key plant physiological processes, including boosting photosynthetic activity and increasing resistance to stress. As a result, chitosan not only aids in improving plant growth under saline conditions, but also contributes to the production of higher-quality seeds, thus promoting the overall crop performance in saline soils.

The PCA biplot further elucidates the relative effectiveness of the various treatments by mapping the correlations between soil and plant parameters. The positive correlation between treatments involving white plastic mulches (T7–9) and biostimulants (CH₁, CH₂, and CH₃) demonstrates a consistent pattern of enhanced plant performance and soil health, suggesting that these treatments share common underlying mechanisms. The distinct divergence of the control treatment (T1) on the biplot underscores the detrimental effects of the absence of mulch and biostimulant applications on both soil and plant parameters. The PCA also highlights the critical role of soil moisture, available nutrients, and organic carbon in driving plant growth and yield, with the treatments consistently improving these parameters.

5. Conclusions

This study provides compelling evidence that the integration of mulch materials, particularly white plastic, with biostimulants, such as nano-chitosan-based biostimulant foliar treatments, significantly improves soil health, enhances plant growth, and boosts yield in cowpea cultivation. The synergistic effects of mulching and biostimulants help mitigate environmental stressors, including soil salinity and water scarcity, while promoting nutrient availability and the microbial balance. These findings have important implications for sustainable agricultural practices, particularly in areas affected by soil degradation and water scarcity. Future research should continue to explore the long-term effects of these treatments on soil health, microbial dynamics, and crop productivity under varying environmental conditions.