Preparation of Wheat-Straw-Fiber-Based Degradable Mulch Film for Sustained Release of Carbendazim and Its Application for Soybean Root Rot Control

Abstract

In order to sustain control over soybean root rot, wheat-straw-fiber-based mulch film (WFM) coated with carbendazim (C) and chitosan (CS) mixture (C-CS-WFM) were prepared through bar coating technology. The Box–Behnken design method was employed to investigate the effects of chitosan concentration, wet film thickness, and carbendazim loading on the dry tensile strength (DTS), wet tensile strength (WTS), and air permeance (AP) of C-CS-WFM. Eventually, the optimization process parameters were determined as follows: a chitosan concentration of 1.83–2.39%, a wet film thickness of 18–24 μm, and a carbendazim loading of 0.05–0.12 g/m². These parameters achieved the desired physical properties of C-CS-WFM, i.e., the DTS is not less than 3.5 kN/m, the WTS is not less than 0.8 kN/m, and the AP does not exceed 2.1 μm/(Pa·s). The results showed that after the introduction of the C-CS coating, the DTS and WTS of C-CS-WFM were enhanced by 11.4% and 14.9%, respectively. In contrast, the AP was reduced by 15.6%. FT-IR analysis indicated that carbendazim was embedded in the C-CS composite material without any chemical interaction. Through SEM and sustained-release kinetic analysis, it was found that the sustained-release mechanism of C-CS-WFM conformed to the Ritger–Peppas kinetic model, and its release mechanism was the physical diffusion and matrix erosion. The results of the in vitro antifungal test and pot experiment demonstrated that C-CS-WFM could effectively inhibit the growth of Fusarium solani and promote the growth of plants. This study provided new ideas for the comprehensive prevention and control of soybean root rot.

1. Introduction

Soybean root rot is a pervasive and detrimental soil-borne disease in global soybean cultivation, primarily caused by a range of pathogenic fungi, including Phytophthora sojae, Fusarium species, and Rhizoctonia solani [1]. These fungi can persist in the soil as mycorrhizae, mycelium, or in the form of thick-walled spores or oospores, which are dormant for extended periods, leading to widespread distribution, significant crop damage, and challenging control measures [2,3,4]. Current control strategies encompass agricultural management, chemical treatments, biological control, and the selection of resistant varieties [5]. Chemical agents, despite their rapid action and ease of use, often suffer from quick degradation and short-lived effectiveness [6,7,8], necessitating the development of new, cost-effective, and enduring application methods.

Mulching is a widely adopted agricultural practice that enhances soil temperature and moisture retention [9,10,11], thereby promoting crop growth and mitigating soil-borne diseases to some extent [12]. However, the non-biodegradable nature of traditional plastic mulch films results in environment contamination and microplastic pollution, posing risks to ecosystems and the food chain [13,14]. Consequently, the development of biodegradable mulch films infused with fungicides presents a promising solution for both disease management and environment sustainability. Biodegradable mulch films are typically made from plant fiber or synthetic biodegradable materials such as polylactic acid (PLA) [15,16] chitosan [17], starch [18,19] and polyhydroxyalkanoates (PHAs) [20]. These films provide an eco-friendly alternative to conventional plastic mulch and can be combined with fungicides for enhanced agricultural efficiency. Fungicide incorporation methods include direct blending, where fungicides are mixed with film-forming materials during fabrication, ensuring uniform distribution but requiring careful control of processing conditions to maintain fungicide activity [21]. Encapsulation involves embedding fungicides in nanoparticles or microcapsules, improving stability and enabling controlled release [22,23]. Another approach, layered structures, uses multilayer films with a dedicated fungicide-releasing layer to precisely control release rates and improve disease management [24]. Biodegradable films integrated with fungicides create an effective controlled release system, reducing application frequency and minimizing environmental contamination. Release mechanisms depend on factors such as temperature, soil moisture, and pH, with degradation synchronized to the crop’s growth cycle, ensuring targeted and efficient disease control [24].

Although researchers have been developing biocidal biodegradable films based on biopolymers such as carboxymethyl chitosan/poly(vinyl alcohol) [25], polyhydroxybutyrate [26], chitosan/hydroxypropyl methylcellulose [27], and poly(vinyl alcohol)/starch [19], However, these materials often face limitations due to high production costs and complex manufacturing processes. In contrast, straw-fiber-based films, derived from agricultural waste, offer a cost-effective and readily available alternative [28,29,30]. Their unique porous structure and biodegradability make them a promising material for agricultural applications, although their mechanical and barrier properties require enhancement to meet practical needs [31].

This study introduces a novel strategy for modifying straw-fiber-based mulch film by coating them with chitosan, a natural polymer known for its film-forming properties and biocompatibility. The coating process incorporates the broad-spectrum fungicide carbendazim, enhancing the mechanical and barrier properties of the mulch film while also improving the stability and longevity of the fungicide [32,33,34,35]. The films were prepared using a classical bar coating method, offering a balance between biodegradability and soil-borne disease control. The study systematically investigated the effects of chitosan concentration, wet film thickness, and carbendazim loading on the dry tensile strength (DTS), wet tensile strength (WTS), and air permeance (AP) of the mulch film. The microstructure of the films was analyzed using scanning electron microscopy (SEM), and the sustained-release mechanism of carbendazim was explored to assess the durability of the film’s efficacy. Additionally, in vitro antifungal property tests and pot experiments were conducted using Fusarium solani (F. solani) as a representative pathogenic fungus to evaluate the film’s performance.

2. Materials and Methods

2.1. Materials

Chitosan (CS) (molecular weight 200 kDa; deacetylation degree 95%) and carbendazim (≥99.5%) were purchased from Shanghai Yuanye Biotechnology Co. (Shanghai, China). All other chemical reagents were analytically pure.

2.2. Preparation of WFM

Wheat straw of Dongnong Winter Wheat 2 was used as raw material to prepare wheat-straw-fiber-based mulch film (WFM) according to a previous work [28]. In detail, wheat straw was processed into semi-finished products using a kneading and cutting machine (kneading and cutting machine feed capacity of 1000 kg/h, spindle speed of 1800 r/min). The semi-finished product was soaked and washed to remove impurities and increase the moisture content of the raw material. Then, the semi-finished products were used to produce wheat straw fiber by extrusion blasting with D200 fiber making machine. Then, the wheat straw fiber and the hardwood unbleached kraft pulp cardboard torn into pieces were separately soaked in water for more than 4 h. Then, according to the standard of GB/T24325-2009 “Pulp Valley (valley) pulper method” [36], the prepared wheat straw fibers and kraft pulp were put into the valley pulper for 30 min of defibering process, and then pulped to the required pulping degree (55 ± 5° SR for wheat straw pulp, and 45 ± 2° SR for kraft pulp) for spare. After that, according to the specific needs of the experiment, the wheat straw pulp and kraft pulp were mixed according to the ratio of 65:35 (both measured in terms of absolute dry pulp), and 1.0% wet strength agent and 1.4% neutral sizing agent were added (both measured in terms of absolute dry pulp). Finally, the WFM was prepared by adjusting the parameters of the paper machine for film pressing and drying.

2.3. Preparation of C-CS-WFM

The C-CS-WFM sample preparation procedure is shown in Figure 1. According to the requirements of the test protocol shown in

Table 1. Factor-level code.

Factors	Levels Used, Actual (Coded Factor)
Independent Variables	Low (−1)	Medium (0)	High (+1)
X1 = Chitosan concentration (%)	1.5	2.0	2.5
X2 = Wet film thickness (μm)	15	20	25
X3= Carbendazim loading (g/m2)	0.05	0.10	0.15

, different concentrations of chitosan solution and carbendazim solution were first prepared by dissolving CS and carbendazim in aqueous acetic acid solution (2%, v/v), respectively, and the solutions were magnetically stirred at 500 rpm for 1 h at room temperature (25 ± 1 °C temperature and 50 ± 2% relative humidity) to ensure that the solids were completely dissolved, and the solutions were left to stand for 1 h to remove air bubbles. Then, the chitosan solution and carbendazim solution were mixed to prepare C-CS mixtures with different concentrations and drug loadings. Finally, the above mixture was coated on a 20 cm × 35 cm WFM with an OSP coating bar with 15 μm, 20 μm and 25 μm wet film thicknesses, respectively, using a laboratory coating machine (MS-RL 320, Ruilin machinery technology co., Ltd., Xianyou, Fujian, China) to obtain the C-CS-WFM samples. All samples were dried at room temperature for 24 h before testing.

2.4. Experimental Design and Statistical Analysis

In this study, a three-factor, three-level Box–Behnken design (BBD) was used to optimize the mechanical and barrier properties of C-CS-WFM. Chitosan concentration (X₁, %), wet film thickness (X₂, μm), and carbendazim loading (X₃, g/m²) were used as independent variables, and dry tensile strength (Y₁, kN/m), wet tensile strength (Y₂, kN/m), and air permeance (Y₃, μm/(Pa·s)) were used as dependent variables, and the coding of the test factor levels is shown in Table 1.

Design Expert 6.0 (Stat-Ease Inc., Minneapolis, MN, USA) was used to design the corresponding BBD tests and statistically analyze the test results.

2.5. Performance Measurement and Characterization

2.5.1. Measurement of Mechanical Properties

According to Equation (1), to calculate the tensile strength S, the unit is expressed in kN/m, with reference to GB/T12914-2018, “Paper and board—Determination of tensile properties—Constant rate of elongation method (20 mm/min)” [37], and GB/T465.2-2008 “Paper and board—Determination of tensile strength after immersion in water” [38]. The film specimen was made into 120 mm × 15 mm, and the dry and wet tensile strength was determined by the Pendulum-type Paper Tensile Strength Measuring Instrument (Jinan Ruidek Instrument Co., Ltd., Jinan, China), and then converted into the dry and wet tensile strength.

$$S=F/\mathrm{W}$$

(1)

where S is the tensile strength (kN/m), F is the average tensile strength (N), and W is the initial width of the sample (mm).

2.5.2. Measurement of Air Permeance

According to Equation (2) to calculate the Schopper permeance (Ps), the unit is expressed in μm/(Pa·s), with reference to GB/T458-2008 “Paper and board-Determination of air permeance” [39] for the determination of the film specimen made of 100 mm × 60 mm, through the HK-TQD01 Schopper permeance tester (Jinan Drake Instrument Co., Ltd., Jinan, China) to determine the air passing through the specimen within 15 s. The volume of air passing through the specimen in 15 s was measured by the HK-TQD01 Schopper-type air permeance tester (Jinan Derrick Instrument Co., Ltd., Jinan, China) and then converted into air permeance.

$$P_s=V/\left(\nabla \mathrm{P}·\mathrm{t}\right)$$

(2)

where P_s is the air permeance (μm/(Pa·s)), V is the volume of air passing through the specimen (mL) during the measurement time, $\nabla \mathrm{P}$ is the pressure difference between the two sides of the specimen (kPa), and t is the measurement time (s).

2.5.3. SEM

The C-CS-WFM specimens were made into 5 mm × 2 mm, patched and then sprayed with gold, and the microstructures of C-CS-WFM before and after immersion were observed comparatively by SEM (SU8010 field emission scanning electron microscope, Hitachi, Chiyoda, Japan).

2.5.4. FT-IR

Freeze-dried powders of carbendazim, chitosan, and C-CS were prepared as samples using KBr compression technique and analyzed by FT-IR spectroscopy in the range of 400–4000 cm⁻¹ using Nicolet iS50 FT-IR spectrometer (Thermo Fisher, Waltham, MA, USA). The test results were used to analyze the interactions between the components of the samples through peak variations.

2.6. Drug Release Kinetics

The carbendazim release test was carried out on C-CS-WFM and wheat-straw-fiber-based mulch film directly coated with the same amount of aqueous carbendazim-loaded solution (C-WFM), in which the C-WFM and the optimized C-CS-WFM were immersed in 120 mL of distilled water at room temperature. At fixed intervals, 10 mL of leachate (containing released contents) was removed from it for determination and the same volume of distilled water was added to the original solution to ensure that its total volume remained constant. Measurements were made at 1 h intervals during 0 to 12 h of immersion and at 12 h intervals during 12 h to 240 h of immersion. A UV-visible spectrophotometer (TU-1810, Beijing Purkinje General Instrument co., LTD, Beijing, China) was used to determine the concentration of carbendazim in the leachate corresponding to different time points at 282 nm according to the Chinese standard GB/T5009.188–2003 [40].

The commonly used first-order kinetic model (Equation (3)), Higuchi kinetic model (Equation (4)) and Ritger–Peppas kinetic model (Equation (5)) were fitted to the experimental data by 0rigin2024. The R² values were compared to explore the release mechanism of carbendazim in water. The equations for each model are given below:

$$\mathit{ln}\left(1-M_t/M_{\infty }\right)=-\mathrm{k}t.$$

(3)

$$M_t/M_{\infty }=\mathrm{k}{t}^{{\displaystyle \frac{1}{2}}}.$$

(4)

$$M_t/M_{\infty }={\mathrm{k}t}^n.$$

(5)

where M_t is the cumulative release of the drug at time t (μg), M_∞ is the total amount of carbendazim added (μg), M_t/M_∞ is the percentage of cumulative release of the drug (%), t is the controlled release time (h), k is the rate constant of the drug release process, and n is the parameter, characterizing the release mechanism.

2.7. In Vitro Antifungal Properties

The F. solani used in this study was provided by JiaMuSi Branch of Heilongjiang Academy of Agricultural Science. It was routinely cultured on potato dextrose agar (PDA) plates at 25 °C in the dark and then cooled to 4 °C to be used as a fungal source in the in vitro activities of film and pot experiment.

In vitro antifungal activity of C-CS-WFM was evaluated by the method where the C-CS-WFM disc (diameter of 6 mm, sterilized under ultraviolet light for 20 min) was placed in the center of PDA plates, and 4 mycelium plugs of F. solani were placed symmetrically around it, and then was cultured at 25 °C in the dark. The blank WFM without carbendazim was as control. Three repeats were tested for each group of experiments.

2.8. Pot Experiment

The pot experiment used Heinong 84 soybean seeds, which were sterilized with 10% sodium hypochlorite. Flower cultivate soil which included native soil, substrate, peat soil and perlite, sterilized at 120 °C for 2 h, filled the pots. The soil surface was covered with WFM and C-CS-WFM. A perforator created 5 holes in the mulch. Subsequently, the soybeans were carefully sowed into the holes. A pot without mulch served as the control group. Three repeats were tested, with 5 plants included in each repeat.

After the soybeans sprouted, a certain amount of diluted spore suspension was accurately aspirate using a pipette. The spore suspension was slowly dripped into the soil around the roots of the soybean plants carefully, making every effort to evenly distribute the spore suspension around the root system (10 milliliters of root irrigation per soybean plant). After the operation was completed, the spore suspension was gently covered with a thin layer of soil to prevent it from being exposed to the air and evaporating or being splashed out. The spore suspension was produced as follows: Take the strains cultivated on PDA for 7 days in the 2.8 bacterial inhibition test. Transfer 5 mL of sterile water to the medium plate with a pipette gun, then scrape the plate colonies with a sterilized inoculation spatula in a sterile centrifugal tube and replenish the tube with 10 mL of sterile water, screw the cap on the tube, shake and mix well, and then vibrate well in a shaker to make sure that the spores are uniform and have no obvious stratification. Then, filter it through gauze to make the spore suspension. This was configured to 1 × 10⁸/mL with saline. After 20 d of infection, we started to investigate the incidence of the disease by measuring the root dry weight of soybeans, the stem length of soybean soybeans, the stem fresh weight of soybeans, grading the disease index of root rot with reference to the standards GB/T17980.88-2004 [41] and calculating the disease index and incidence of soybean root rot as shown in Equations (6) and (7).

$$y=\sum \left[\left(n_i\times a_i\right)/\left(n\times a\right)\right]\times 100$$

(6)

$$x=\left(n_j/n\right)\times 100\%$$

(7)

where y is the soybean root rot disease index, $n_i$ is the number of diseased plants at all levels of soybean, $a_i$ is the value of soybean root rot disease level, n is the total number of plants investigated, a is the highest value of soybean root rot disease level, x is the incidence rate of soybean root rot disease (%), and $n_j$ is the number of diseased plants in soybean.

The test results were also statistically analyzed using Design Expert 6.0 (Stat-Ease Inc., Minneapolis, MN, USA).

3. Results

3.1. Optimization of C-CS-WFM Performance

3.1.1. Experimental Results and Regression Model

The BBD experimental design and results are shown in

Table 2. Experimental design and results.

Run	Independent Variables			Dependent Variables
	Chitosan Concentration (%)	Wet Film Thickness (μm)	Carbendazim Loading (g/m2)	Dry Tensile Strength (kN/m)	Wet Tensile Strength (kN/m)	Air Permeance (μm/(Pa·s))
1	1.50	15.00	0.10	3.50	0.78	2.91
2	2.50	15.00	0.10	3.61	0.81	2.39
3	1.50	25.00	0.10	3.51	0.77	2.30
4	2.50	25.00	0.10	3.70	0.78	1.98
5	1.50	20.00	0.05	3.51	0.76	2.99
6	2.50	20.00	0.05	3.67	0.85	2.38
7	1.50	20.00	0.15	3.49	0.79	2.68
8	2.50	20.00	0.15	3.59	0.75	2.38
9	2.00	15.00	0.05	3.47	0.85	2.36
10	2.00	25.00	0.05	3.52	0.81	1.71
11	2.00	15.00	0.15	3.36	0.79	2.23
12	2.00	25.00	0.15	3.49	0.79	1.60
13	2.00	20.00	0.10	3.67	0.80	2.00
14	2.00	20.00	0.10	3.65	0.81	2.02
15	2.00	20.00	0.10	3.66	0.81	2.02
16	2.00	20.00	0.10	3.64	0.82	2.04
17	2.00	20.00	0.10	3.66	0.80	2.05

.

The results of the experiments were analyzed, and the quadratic equation models of Y₁ (DTS), Y₂ (WTS), and Y₃ (AP) were significant (p < 0.0001). F-tests were performed at a confidence level of 0.05, and the non-significant terms were excluded to obtain the regression models of each objective function as shown in Equations (8)–(10).

Y₁ = 1.92737 − 0.02 X₁ + 0.12658 X₂ + 6.17895 X₃ − 0.00358947 X₂² − 41.89474 X₃² + 0.008 X₁ X₂ + 0.08 X₂ X_3.

(8)

Y₂ = 0.29917 + 0.51472 X₁ − 0.006 X₂ + 1.425 X₃ − 0.090556 X₁² − 1.3 X₁ X₃ + 0.04 X₂ X_3.

(9)

Y₃ = 11.6995 − 9.0955 X₁ + 0.1197 X₂ −13.715 X₃ + 1.987 X₁² − 0.00543 X₂² + 30.7 X₃² + 0.02 X₁ X₂ + 3.1 X₁ X_3.

(10)

where X₁ is the chitosan concentration (%), X₂ is the wet film thickness (μm), and X₃ is the carbendazim loading (g/m²).

Analysis of variance (ANOVA) was used to ensure the accuracy of the model, and the results are shown in

Table 3. ANOVA of regression model.

Source		Sum of Square	df	Mean Square	F Value	p Value
Dry tensile strength	Model	0.15	9	0.02	66.10	<0.0001
	Residual	1.72 × 10−3	7	2.4 × 10−4
	Lack of Fit	1.20 × 10−3	3	4.00 × 10−4	3.08	0.1530
	Pure Error	5.20 × 10−4	4	1.30 × 10−4
	Cor Total	0.15	16
Wet tensile strength	Model	0.01	9	1.28 × 10−3	29.43	<0.0001
	Residual	3.05 × 10−4	7	4.36 × 10−5
	Lack of Fit	2.50 × 10−5	3	8.33 × 10−6	0.12	0.9442
	Pure Error	2.80 × 10−4	4	7.00 × 10−5
	Cor Total	0.01	16
Air permeance	Model	2.25	9	0.25	145.60	<0.0001
	Residual	0.012	7	1.71 × 10−3
	Lack of Fit	9.67 × 10−3	3	3.23 × 10−3	5.56	0.0654
	Pure Error	2.32 × 10−3	4	5.80 × 10−4
	Cor Total	2.26	16

. As can be seen from Table 3, the p value of the regression term for each indicator was <0.05, indicating that the regression equation was highly significant; the p value of the fitted term was >0.05, meaning that the model was significant.

3.1.2. Influence of Test Factors on DTS

Figure 2a illustrates the influence of wet film thickness and chitosan concentration on the DTS of the film. At a carbendazim loading of 0.1 g/m², DTS is directly proportional to chitosan concentration, with higher concentrations leading to greater DTS. The -NH₂ groups in chitosan form hydrogen bonds with the phenolic -OH groups in pulp fibers effectively enhancing the dry tensile strength. The higher the chitosan concentration, the more hydrogen bonds are formed, resulting in greater DTS [42]. The impact of wet film thickness on DTS is such that when the thickness is less than 20 μm, the DTS increases with the increase in wet film thickness. However, when the wet film thickness exceeds 20 μm, the DTS decreases as the thickness increases further. An increase in wet film thickness can increase the amount of chitosan applied to the film surface, thereby increasing the interaction between chitosan and fibers. However, an excessively thick wet film may lead to an uneven film layer after drying, causing the DTS to decrease after reaching a certain level [43]. The influence of chitosan concentration on DTS is greater than that of wet film thickness, with the maximum value occurring at a chitosan concentration of 2.5% and a wet film thickness of 20 μm.

Figure 2b shows the effect of wet film thickness and carbendazim loading on DTS. At a chitosan concentration of 2%, the influence of wet film thickness on DTS first increases and then decreases. The effect of carbendazim loading on DTS also follows an increasing then decreasing trend. When the carbendazim loading is below 0.1 g/m², there is a positive correlation between fungicide loading and DTS, with the DTS increasing as the carbendazim loading increases. When the fungicide loading is above 0.1 g/m², there is a negative correlation, with the DTS decreasing as the carbendazim loading increases. Below a loading of 0.1 g/m², the mixture of fungicide and chitosan can fill the gaps between fibers, slightly enhancing the DTS of the film. Above a loading of 0.1 g/m², an excess of carbendazim can lead to stress concentration, which in turn reduces the DTS of C-CS-WFM [44]. The influence of loading on DTS is greater than that of wet film thickness, with the maximum value occurring at a loading of 0.1 g/m² and a wet film thickness of 20 μm.

3.1.3. Influence of Test Factors on WTS

Figure 3a displays the impact of chitosan concentration and carbendazim loading on the WTS of the film. At a constant wet film thickness of 20 μm, it is observed that WTS is directly proportional to chitosan concentration, with higher concentrations resulting in greater WTS [32]. Conversely, WTS is inversely proportional to the amount of carbendazim loading, with higher loadings leading to lower WTS. Chitosan, when applied to the film surface, forms a dense hydrophobic layer that enhances the water resistance of the straw fiber film. Additionally, the chitosan molecules, containing polar groups such as amino and hydroxyl, interact with the cellulose molecules in the straw fiber film through hydrogen bonding. This molecular interaction strengthens the binding between fibers, thereby improving the WTS of the straw fiber film. The effect of fungicide on WTS is similar to its effect on DTS. The influence of chitosan concentration on wet tensile strength is greater than that of carbendazim loading. The maximum WTS is achieved at a chitosan concentration of 2.5% and a carbendazim loading of 0.05 g/m².

Figure 3b illustrates the effects of carbendazim loading and wet film thickness on the WTS of the film. At a chitosan concentration of 2%, it is evident that WTS is negatively correlated with both carbendazim loading and wet film thickness: higher carbendazim loading and thicker wet film both result in lower WTS. The influence of wet film thickness on WTS is similar to its impact on DTS. The effect of carbendazim loading on WTS is more significant than that of wet film thickness. The maximum WTS is achieved when the wet film thickness is 10 μm and the carbendazim loading is 0.05 g/m².

3.1.4. Influence of Test Factors on AP

Figure 4a shows the impact of wet film thickness and chitosan concentration on the AP of the film. At a constant carbendazim loading of 0.1 g/m², AP decreases as the wet film thickness increases, indicating a negative correlation. Regarding chitosan concentration, AP is negatively correlated when the concentration is below 2%, meaning AP decreases with increasing chitosan concentration. Beyond 2% chitosan concentration, AP increases with concentration, showing a positive correlation. Below 2% chitosan, the increased concentration fills the gaps between wheat straw fibers, forming a dense layer that reduces AP [45,46]. Above 2%, the higher concentration leads to a less fluid composite film, causing uneven coating and increasing AP. The influence of chitosan concentration on AP is greater than that of wet film thickness, with the minimum AP occurring at a chitosan concentration of 2.0% and a wet film thickness of 25 μm.

Figure 4b examines the effects of chitosan concentration and carbendazim loading on AP. At a wet film thickness of 20 μm, AP is negatively correlated with chitosan concentration below 2%, decreasing as chitosan concentration increases. Above 2% chitosan concentration, AP increases with concentration. Carbendazim loading also negatively correlates with AP. In C-CS-WFM, as carbendazim is added to the chitosan matrix as filler, its particles can block the pores in the chitosan film, which increases the gas diffusion path length [47]. Consequently, an increase in carbendazim loading amount raises the particle density, causing a gradual reduction in AP. The impact of chitosan concentration on AP is significantly greater than that of carbendazim loading. The minimum AP is observed at a chitosan concentration of 2% and a carbendazim loading of 1.5 g/m².

3.1.5. Optimization and Model Verification

According to the requirements for the use of C-CS-WFM, the process parameters are optimized to ensure that the DTS is not less than 3.5 kN/m, the WTS is not less than 0.8 kN/m, and the AP does not exceed 2.1 μm/(Pa·s). The optimization results show that the best combination of process parameters is as follows: a chitosan concentration of 1.83–2.39%, a wet film thickness of 18–24 μm, and a carbendazim loading of 0.05–0.12 g/m². The optimization analysis results are shown in Figure 5.

To validate the regression models’ accuracy, we prepared C-CS-WFM samples using an optimized set of parameters: chitosan concentration of 2%, wet film thickness of 20 μm, and carbendazim loading of 0.1 g/m². The resulting DTS, WTS, and AP of the C-CS-WFM samples were tested and are detailed in

Table 4. Mechanical properties and air permeance of different film samples.

Item		DTS (kN/m)	WTS (kN/m)	AP (μm/(Pa·s))
WFM		3.27 ± 0.13	0.70 ± 0.04	2.40 ± 0.32
C-CS-WFM	predicted value	3.66	0.81	2.04
	measured value	3.64 ± 0.01	0.80 ± 0.01	1.99 ± 0.02

. The data indicate that the measured values for DTS, WTS, and AP closely matched the regression model estimates. Additionally, the same properties were tested for WFM samples and are also presented in Table 4. The results demonstrate that the optimized C-CS-WFM outperformed WFM, with a 11.4% increase in DTS, a 14.9% increase in WTS, and a 15.6% decrease in AP.

SEM analysis was conducted to examine the microstructures of WFM and the optimal C-CS-WFM, with results depicted in Figure 6. Figure 6a shows that the wheat straw fibers exhibit a obvious fibrillation and under the influence of additives, the fibers are tightly bound, creating a network structure with numerous pores. The morphology of the optimal C-CS-WFM is presented in Figure 6b, where the chitosan/carbendazim mixture not only fills the interfiber pores but also coats the fiber surfaces, resulting in a smoother surface for C-CS-WFM compared to WFM.

3.2. FT-IR Spectroscopy

FT-IR spectra of chitosan, carbendazim, and the carbendazim/chitosan composite coating are shown in Figure 7. Chitosan, due to the abundance of O-H groups in its molecular structure, exhibits a broad absorption band from 3600 to 3200 cm⁻¹, which overlaps with the weak N-H stretching vibration peaks of the primary amine group. Distinct stretching vibration absorption peaks are observed at 2932 cm⁻¹ and 2875 cm⁻¹, corresponding to the C-H stretching vibrations in chitosan. Additionally, a bending vibration absorption peak is observed at 1597 cm⁻¹, which is attributed to the -NH₂ group [48].

In contrast, the carbendazim molecule, which lacks hydroxyl groups, shows a relatively narrower N-H absorption band in the 3200–3400 cm⁻¹ range compared to chitosan. A moderately strong and sharp peak at 3321 cm⁻¹ is attributed to the N-H stretching vibration of the secondary amine group in carbendazim. The peaks at 1475 cm⁻¹ and 1589 cm⁻¹ are skeletal vibration peaks of the benzene ring, where the 1475 cm⁻¹ peak is absent in chitosan. The 1589 cm⁻¹ peak is similar to the bending vibration absorption peak of chitosan at 1597 cm⁻¹, further highlighting the differences between the two materials. Furthermore, the C=O stretching vibration peak at 1712 cm⁻¹ and peak at 733 cm⁻¹ corresponding to the absorption of ortho-disubstituted benzene are distinctive features of carbendazim that clearly differentiates it from chitosan [49].

The spectrum of the carbendazim/chitosan composite coating is essentially the result of the overlapping peaks from both carbendazim and chitosan, with no new characteristic peaks appearing. Moreover, the characteristic peaks of each component do not show significant shifts. For example, the broad absorption band in the 3600 to 3200 cm⁻¹ range, which results from the overlap of O–H and primary amine N–H stretching vibrations, is observed, similar to chitosan. Additionally, the N–H stretching vibration peak of the secondary amine group in carbendazim remains at 3321 cm⁻¹. These observations indicate that carbendazim has been successfully incorporated into chitosan without any chemical interaction, reinforcing that the two components retain their individual characteristics in the composite coating.

3.3. Drug Release Kinetics

The in vitro release profiles of carbendazim from C-WFM and C-CS-WFM are depicted in Figure 8. For C-WFM, the initial burst release within the first 0–3 h accounted for a 72% cumulative release rate, primarily due to carbendazim loaded on the surface of C-WFM, which quickly dissolved upon immersion in water. In the subsequent 3–24 h, only an additional 6% was released, attributed to carbendazim residing in the fiber pores of the film, with its diffusion slowed by the fibers. Within 72 h, C-WFM achieved a total of 78% release; the unreleased portion may be adsorbed by the micropores on the wheat straw fiber surface, as the fibrous medium itself has strong adsorption capacity within its micropores, which is not easily desorbed, and complete release might require the degradation of the wheat straw fibers [50]. In contrast to C-WFM, the release profile of C-CS-WFM showed a significant change. During the initial burst release phase within 3 h, the cumulative release was only 26%, a 64% decrease compared to the absence of chitosan. Over the next 3–48 h, carbendazim release followed an approximate linear relationship with time, indicating that the addition of chitosan effectively controlled the release of carbendazim. The carbendazim released during the burst phase was adsorbed and retained near the C-CS surface. Between 3 and 48 h, a constant release rate of 1.1 mg/(m²·h) was observed, which reflected the sustained release associated with the erosion of chitosan. C-CS-WFM achieved an 84% cumulative release within 72 h, surpassing C-WFM’s 78%, suggesting that the surface coating method on the film can enhance the effective utilization rate of the fungicide.

To explore the carbendazim sustained-release mechanism of C-CS-WFM, we applied the first-order kinetic model, Higuchi kinetic model, and Ritger–Peppas kinetic model to fit the carbendazim release data, with the results summarized in

Table 5. Different models to fit the data.

Model	R2	Modify R2
The first-order kinetic model	0.9119	0.9056
Higuchi kinetic model	0.9734	0.9735
Ritger–Peppas kinetic model	0.9831	0.9819

. The determination coefficients (R²) for these models were 0.9119, 0.9734, and 0.9831, respectively, with the Ritger–Peppas kinetic model showing the highest R² value. In this model, the exponent n characterizes the mechanism of drug release. The n value of 0.46 obtained in this study, which is greater than 0.45, indicates that the carbendazim sustained release is controlled by a combination of carbendazim diffusion and carbendazim/chitosan coating erosion [51].

To further confirm the sustained-release mechanism of carbendazim in C-CS-WFM, the microstructure of C-CS-WFM after 10 days of immersion in 120 mL of distilled water at room temperature was observed. As shown in Figure 9, the surface of the film exhibited noticeable grooves and gaps compared to the primary C-CS-WFM. This may be attributed to the C-CS film undergoing erosion in the aqueous medium, disrupting the continuous film structure on the surface of C-CS-WFM. The erosion of the C-CS film increased its contact area with water, facilitating the release of carbendazim. Additionally, the rate at which carbendazim dissolves from the film is related to the number of hydrophilic groups in the film. Chitosan, which contains a significant amount of hydrophilic groups (amino and hydroxyl groups), allows for faster water penetration. Under the drive of carbendazim concentration gradient, the embedded carbendazim molecules dissolve and are released into the water more quickly, which also contributes to the sustained release of carbendazim. Therefore, SEM analysis of C-CS-WFM also demonstrates that the release of carbendazim from C-CS-WFM is the result of the combined effects of erosion of the C-CS film and physical diffusion.

3.4. Antifungal Properties and Pot Experiment

As shown in Figure 10, the blank WFM had no antifungal activity against F. solani, while the inhibition zones were formed around the C-CS-WFM. Therefore, the C-CS-WFM had antifungal activity because of the release of carbendazim.

The potential application of C-CS-WFM was further evaluated by a pot experiment. Figure 11a presents the morphology of soybean plants and their roots after 20 days of fungal treatment for C-CS-WFM, WFM, and CK (no mulch film) treatments. The C-CS-WFM treatment group showed significantly better plant morphology, root length, and number of root hairs compared to the other two groups, with WFM outperforming CK. CK exhibited clear symptoms of root rot, with curved, weak plant growth, brown spots on the roots, and fewer root hairs. As shown in Figure 11b–d, both C-CS-WFM and WFM treatments significantly increased the stem length, stem fresh weight, and root dry weight of soybeans, primarily because the film coverage provided better growing conditions that promoted root growth. Notably, the root dry weight of soybeans treated with C-CS-WFM was significantly higher than that of the WFM group, indicating that the growth of seedlings after fungal infection was significantly impeded. The C-CS-WFM effectively suppressed the occurrence of root rot through the sustained release of carbendazim, thereby promoting the growth of soybean plants. Figure 11e,f shows the root rot incidence of soybeans treated with three different methods over 20 days in the presence of F. solani. The CK group had the highest disease incidence at 100%, while WFM and C-CS-WFM groups had rates of 89% and 39%, respectively. The disease indices were 64 for CK, 59 for WFM, and 38 for C-CS-WFM, indicating that the antimicrobial film significantly reduced the probability of soybean plants getting sick through the sustained release of carbendazim.

4. Discussion

Due to the unique fiber composition and porous structure of plant fiber mulches (also known as paper mulches), their design must consider both the performance requirements during application and the functionality after being laid on the ground [52]. Thus, designing straw-fiber-based mulches is a complex, integrated decision-making process.

4.1. Physical Properties of C-CS-WFM

The core physical properties of straw-fiber-based mulches—DTS, WTS, and AP—directly affect their suitability and functional performance in agricultural applications. DTS reflects the mulch’s resistance to tearing under dry conditions and is a critical metric for evaluating mechanical performance, ensuring smooth application and durability in the field. WTS, which describes the mulch’s tensile resistance in wet conditions, is crucial for maintaining integrity and functionality under rain or irrigation. This property is a key consideration in determining whether biodegradable mulches can replace plastic mulches [53]. AP, representing the ability of the mulch to allow gas exchange, is closely related to its fiber structure and porosity. It influences soil–air interactions, affecting root respiration and soil moisture evaporation [54]. The application of chitosan coating enhances fiber bonding, forms hydrogen bonds, and improves the fiber network structure, thereby increasing both DTS and WTS while reducing AP. As a natural biodegradable material, chitosan does not compromise the eco-friendly nature of straw-fiber-based mulches [46].

4.2. Drug Release Performance of C-CS-WFM

Functionalizing mulches is an effective way to reduce agricultural costs, such as by incorporating fungicides onto the mulch surface. Compared to conventional fungicide delivery methods, chitosan coating technology effectively controls fungicide release rates while improving utilization efficiency. C-CS-WFM reduced the initial burst release to 26% within the first 3 h, a 64% decrease compared to the 72% observed for C-WFM. C-CS-WFM maintained a sustained-release rate of 1.1 mg/(m²·h) over 3–48 h. In this study, the cumulative release of C-CS-WFM reached 84% by 72 h, surpassing the 78% of C-WFM, indicating improved fungicide utilization efficiency.

4.3. Cost and Environmental Impact

Compared to traditional polyethylene (PE) mulches, the cost of biodegradable mulches, including those made from PLA, polybutylene adipate terephthalate (PBAT), or plant fibers, remains a critical barrier to widespread adoption. For example, Marí et al. reported that increasing subsidy rates by up to 50.1% of the market price could make biodegradable films and paper mulches economically viable alternatives to PE [55]. Thus, reducing costs and enhancing functionality are essential for advancing biodegradable mulch development.

Raw material costs constitute the primary component of mulch production expenses. In this study, wheat straw was used as the main raw material, costing approximately 50~100 USD/ton in China (market price) or 185 USD/ton in Canada [56], significantly lower than the cost of PLA or PBAT (approximately 2000 USD/ton) [57,58]. Moreover, utilizing agricultural waste offers both economic and environmental benefits.

As a natural biodegradable material, the addition of chitosan does not compromise the environmental performance of straw-fiber-based mulches. Applying a chitosan–fungicide mixture to the mulch surface not only reduces drift losses from spraying and leaching losses from soil drenching but also ensures sustained fungicide release. This prolongs fungicide effectiveness, reduces application frequency, and minimizes overall usage, mitigating the adverse environmental impact of chemical fungicides.

5. Conclusions

Chitosan and carbendazim, as a layer, were applied to WFM via a bar coating method to create C-CS-WFM. This novel film not only excels in mechanical and barrier properties but also exhibits drug-controlled release characteristics.

A three-factor Box–Behnken design was employed to systematically investigate the effects of three key factors—chitosan concentration, wet film thickness, and carbendazim loading—on the performance of the WFM film. The optimization process identified the ideal parameter ranges as follows: a chitosan concentration of 1.83–2.39%, a wet film thickness of 18–24 μm, and a carbendazim loading of 0.05–0.12 g/m². Under these optimized conditions, the resulting C-CS-WFM film exhibited desirable performance metrics, including a DTS of not less than 3.5 kN/m, a WTS of at least 0.8 kN/, and an AP not exceeding 2.1 μm/(Pa·s). This demonstrates the film’s robustness and suitability for practical applications.

A chitosan/carbendazim coating layer was added on top of the optimized WTS using the bar coating method. FT-IR analysis confirmed carbendazim and chitosan did not undergo any chemical interactions in the chitosan/carbendazim coating. The in vitro release profiles revealed that the incorporation of chitosan into C-CS-WFM significantly improved the controlled release and utilization efficiency of carbendazim compared to C-WFM. C-CS-WFM reduced the initial burst release to 26% within the first 3 h, a 64% decrease compared to C-WFM’s 72%. Furthermore, C-CS-WFM maintained a sustained-release rate of 1.1 mg/(m²·h) over 3–48 h, driven by chitosan erosion, while C-WFM exhibited a much slower additional release of only 6% during the same period. By 72 h, C-CS-WFM achieved an 84% cumulative release, surpassing C-WFM’s 78%, indicating an improvement in fungicide utilization efficiency. The carbendazim release data fit the Ritger–Peppas kinetic model better than the first-order and Higuchi kinetic models. An exponent n value of 0.46 from the model suggests that the sustained release is governed by a combination of carbendazim diffusion and chitosan/carbendazim coating erosion. This finding was corroborated by SEM analysis, which showed that after 10 days of immersion in 120 mL of distilled water at room temperature, the C-CS film underwent erosion, disrupting the continuous film structure on the surface of the C-CS-WFM.

The antifungal test showed that C-CS-WFM effectively suppressed F. solani infections, as indicated by clear inhibition zones, unlike blank WFM, which lacked antifungal activity. This confirmed its antifungal properties and ability to promote soybean growth through carbendazim release. In pot experiments, C-CS-WFM significantly improved soybean growth compared to WFM and CK, with healthier plants, longer roots, and denser root hairs. C-CS-WFM achieved higher stem length, stem fresh weight, and root dry weight, demonstrating enhanced fungal resistance and better growth conditions.

In summary, this study highlights the successful integration of chitosan and a broad-spectrum fungicide to develop a high-performance film, offering an eco-friendly and sustainable approach to effectively control soybean root rot while promoting green agricultural practices.