Proportion and Performance Optimization of Biomass Seedling Trays Based on Response Surface Analysis

Abstract

Nursery trays are essential agricultural tools in rice production. Plastic nursery trays pose problems such as resource waste and environmental pollution. Biomass seedling trays are an effective way to achieve sustainable agricultural development. Previous research has been conducted on biomass seedling tray molding equipment and molding process, but the impact of raw material ratio on seedling tray molding quality and seedling growth is still unclear, and the ratio combination still needs further optimization. In this study, we used slurry concentration, pulp content, adhesive content, and the ratio of straw to cow manure as variables. We selected the bowl hole molding rate and the strong seedling index as evaluation indicators, and carried out biomass seedling tray forming experiments and seedling cultivation experiments. The response surface analysis method was used to optimize the raw material ratio of biomass seedling trays from the perspectives of forming effect and seedling quality. The results show that when the slurry concentration is 30%, the pulp content is 20%, the adhesive content is 530 g, and the mass ratio of straw to cow manure is 2:1; the bowl hole molding rate is 91.03%, and the strong seedling index is 0.22, indicating good molding effect and seedling growth. The verification test results indicate that the theoretical analysis results are accurate, and the model fitting is good. These research results provide a theoretical basis for the preparation of biomass seedling trays and technical support for achieving green development in agriculture.

1. Introduction

Suitable temperature, humidity, nutrient conditions, and planting density are necessary conditions for crop growth [1,2]. The seedling tray is a key agricultural tool that provides a good growth environment and reasonably adjusts spatial distribution for crop seedlings during the agricultural production process. Biomass seedling trays made from straw have the advantages of good ventilation, strong water retention, and easy decomposition. In addition, the organic matter produced by the decomposition of these trays improves soil fertility and structure, providing a good growth environment for crop seedlings and promoting root growth [3,4,5]. Furthermore, these biomass seedling trays can be transplanted to the field together with the seedlings, eliminating the process of separating the seedlings and trays, simplifying the transplanting process, and avoiding the inconvenience of the recovery and storage of conventional plastic seedling trays. At the same time, the production of biomass seedling trays reduces the cost of straw disposal, alleviates the pollution caused by the burning of waste straw, and realizes the high-value utilization of straw. Therefore, developing biomass seedling trays can not only improve the resource utilization of agricultural waste but also reduce environmental pollution caused by plastic seedling trays, which is crucial for promoting the green and sustainable development of agricultural production [6].

At present, research on biomass seedling trays has achieved initial results. Tian et al. [7] mixed straw and cow manure with starch adhesive and designed different seedling container models according to different raw material ratios. When the ratio of straw to cow manure was 1:15, the molding rate of the container reached 98%, and the water content of the container was about 15%. Qianjin Zhu et al. developed a biodegradable vegetable seedling tray suitable for mechanical transplanting, mainly made of straw. Through experiments, the optimized raw material ratio was determined to be a beating degree of rice straw fiber of 50 ± 1 SR, a quantity of 87.5 ± 4 g·m⁻², a rice fiber content of 70%, a neutral sizing agent mass fraction of 1 ± 0.25%, and a wet strength agent mass fraction of 1.5 ± 0.1%. At this point, the strength, air permeability, and degradation period of the vegetable seedling trays all reached a relatively ideal state [8]. Thonyaporn et al. [9] prepared a biodegradable seedling pot using powder ground from the peel fibers of oil palm as the main raw material. It was found that fiber powder with the smallest particle size (33 μm) was obtained at a rotation speed of 200 rpm after milling for 90 min. A TPS composite with 50 wt % of MPC showed the highest tensile strength, thermal stability, and water resistance.

Li et al. [10,11,12] used straw powder as the main raw material and modified starch adhesive instead of thermosetting adhesive to prepare rice seedling trays. Compared with the traditional preparation process, the rice seedling trays reduced the impact of preparation conditions such as forming pressure, temperature, and pressing time on the forming properties, and the bowl molding rate and expansion rate were improved by 0.09% and 0.05%, respectively. Zhang et al. [13] prepared rice seedling trays by adding self-formulated adhesive to rice straw and studied the porosity and expansion rate of rice seedling trays by using the optimized process. They found that the water retention and mechanical strength of the trays met the actual needs of rice seedling nurseries, as well as that the trays formed by mixing clay with straw were more suitable for the needs of industrial production. Ma et al. [14] prepared seedling containers suitable for corn transplanting using corn straw as the main raw material, which realized the transplanting of corn and the efficient use of straw. From the perspective of the green application of straw, Chen et al. [15] Liu et al. [16] Xie et al. [17] Zhang et al. [18] flexibly applied the concept of pot cultivation, mixed straw powder, micro-fertilizer nutrient medium soil, water-soluble biological latex, and other materials in a certain proportion, and prepared biomass seedling trays using roller pressing technology. The production equipment for biomass seedling trays has been studied, and a mechanized cultivation technology system for biomass seedling trays has been established.

Research shows that existing biomass seedling tray molding technology mainly adopts mold pressing preparation processes. Although this method achieves good molding effects, it also has problems such as high energy consumption and cumbersome steps. Pneumatic forming is a method of molding products using positive or negative pressure generated by air force. The action of gas replaces part of the pneumatic molding mold to achieve shaping, so the structure of the molding equipment is simple, no heating and pressure is required, and the energy consumption is low. This method can be used for the preparation of complex-shaped parts [19]. Based on the pneumatic forming technology, Li et al. [20] have innovatively developed a biomass seedling tray forming machine using rice straw as the main raw material, and have designed and optimized the tray structure and forming process. However, further in-depth research is still needed on the ratio of raw materials.

Therefore, this study aimed to prepare biomass seedling trays using rice straw, cow manure, paper pulp, and a starch-based adhesive as the main raw materials. We also conducted a biomass seedling tray forming experiment and seedling cultivation experiment to explore the impact of slurry concentration, pulp content, adhesive content, and the ratio of rice straw to cow manure on the bowl hole molding rate and strong seedling index. The primary and secondary order of each raw material was determined through response surface methodology, and a regression model was established to optimize the mixing ratio of raw materials. The research results aim to provide feasibility for the market-oriented production and large-scale promotion of biomass seedling trays, as well as provide a certain theoretical basis for the subsequent development of biomass seedling trays.

2. Materials and Methods

2.1. Experimental Materials and Equipment

The biomass raw materials used in the experiment were rice straw and dried cow manure collected by the South Subtropical Crops Research Institute of the Chinese Academy of Tropical Agriculture Science, which were crushed into 5–10 mesh particles using an MFKP666X45 pulverizer and stored for later use. The physical and chemical parameters of the biomass raw materials are shown in

Table 1. Physical and chemical parameters of straw and cow manure.

Materials	Total Nitrogen (g·kg−1)	Total Phosphorus (g·kg−1)	Total Potassium (g·kg−1)	PH	EC (us·cm−1)
Straw	8.7	1.21	16.71	8.23	1344
Cow manure	18.03	5.02	5.49	8.05	5952

. Rice straw is rich in cellulose, which can be used to ensure the structural strength and stability of seedling trays, providing a stable growth space for seedling growth while realizing the resource utilization of straw and reducing the treatment burden. Cow manure contains organic matter and nutrients such as nitrogen, phosphorus, and potassium, so using cow manure to prepare biomass seedling trays can provide organic nutrients for seedlings and promote the healthy growth of crops.

Pulp is made from collected wastepaper that has been shredded, stirred, and deinked. The starch-based adhesive was purchased from Jiali Feng Co., Ltd. (Shenzhen, China), with its main ingredients being edible glutinous rice starch, potassium sorbate, edible ethanol, and edible white vinegar. The adhesive can degrade into CO₂ and H₂O within 5–7 days in a natural environment, causing no environmental pollution and no toxicity to rice. Pulp and starch-based adhesives are used to increase the water retention and mechanical strength of seedling trays, firmly bonding rice straw and cow manure together, and gathering soil to provide growth space for seedlings.

The tested rice variety was Fuerdongnong 426, with a thousand-seed weight of 25 g, an average yield of 517.5 kg per mu, and a growth period of about 139 days. The seedling soil was the soil in the 15–20 cm depth range of the experimental field, with an average pH value of 5.54, an average organic matter content of 4.77%, a nitrogen content of 0.218%, a phosphorus content of 0.159%, and a potassium content of 4.09%. The soil conditions were suitable for rice growth and met the requirements of the experiment.

The self-designed TL-YPCX-01-type biomass seedling tray pneumatic forming machine (Independent research) uses pneumatic forming technology to prepare biomass seedling trays [21], as shown in Figure 1. The self-designed YHMW900-100-type microwave hot air coupling multi-functional dryer (Independent research) uses microwave hot air coupling drying technology to dry seedling trays, which has the advantages of high drying rate, uniform heating, and significant sterilization effect [22], as shown in Figure 2.

The remaining equipment used in the experiment included the MFKP666X45 pulverizer (Jinan Yuezhen Machinery Co., Ltd., Jinan, China), WLDH-5 horizontal screw conveyor material mixer (Chengdu Yuanli Machinery Co., Ltd., Chenzhou, China), rice intelligent seedling platform, BSM5203 electronic balance (range: 0–520 g; accuracy: 0.001 g), vernier calipers, stopwatch, cutting tools, etc.

2.2. Experimental Design

2.2.1. Experimental Method

(1)

Biomass seedling tray forming experiment

Considering the raw material ratio requirements for seedling tray production, a test was conducted using factors such as slurry concentration x₁, pulp content x₂, adhesive content x₃, and straw-to-cow manure mass ratio x₄. An orthogonal experiment was conducted using a central composite design (CCD) plan with four factors and five levels. The coding table for the experimental factor levels is shown in

Table 2. Factor level coding.

Level	Slurry Concentration x1 (%)	Pulp Content x2 (%)	Adhesive Content x3 (g)	Straw-to-Cow Manure Mass Ratio x4
+2	40	30	700	3
+1	35	25	600	2.5
0	30	20	500	2
−1	25	15	400	1.5
−2	20	10	300	1

. The raw materials were mixed according to the mixing ratio in the experimental scheme and added to the feed tank of the seedling tray forming machine. The vacuum pressure of the forming machine was set to −0.075 MPa, the holding time was 15 s, the adsorption time was 5 s, and the working program was started to prepare the biomass seedling tray. The moisture content of the biomass seedling trays was high immediately after production, which cannot meet the structural strength requirements for seedling cultivation. Therefore, drying treatment was required. The shaped biomass seedling trays were placed in the microwave hot air coupling multi-functional dryer, the microwave pulse ratio was set to 1.526, the hot air temperature to 61 °C, the hot air speed to 1 m·s⁻¹ s, and the working time to 15 min [23]. Then, the prepared biomass seedling trays were left to rest for 24 h, following which the finished product of the biomass seedling trays was obtained, as shown in Figure 3. The experiment consisted of a total of 30 groups, with each group repeated 50 times to ensure the number of experimental samples, resulting in a total production of 1500 biomass seedling trays.

(2)

Biomass seedling tray seedling cultivation experiment

From the prepared biomass seedling trays, 10 seedling trays were randomly selected from each group (a total of 300 seedling trays) for the seedling experiment. After soaking the rice seeds for 2 days, we allowed them to sprout at 35 °C for 1 day until the seeds showed white buds [24]. A rice precision seeder was used for combined soil covering and sowing operations, with a bottom soil thickness of 10–14 mm, a sowing amount of 120 g, a topsoil thickness of 3 mm, and watering the biomass seedling trays until they were saturated with water. The seedling cultivation experiment was conducted in a seedling greenhouse, and the temperature and humidity were adjusted according to the conventional seedling method during the seedling process [25]. After 30 days, seedlings with stolons can be cultivated.

After the seedling cultivation phase was completed, transplanting was carried out using a transplanter. The biomass seedling trays were torn into individual seedling pots and transplanted to the field along with the seedlings under the action of the seedling needle, as shown in Figure 4. This transplantation method avoids damage to the seedling roots, shortens the seedling recovery period, and promotes the formation of tillers. Regeneration is an essential and complex process in forest dynamics [26,27,28], and this rule also applies to rice. The new branches and leaves produced by tillering can increase the photosynthetic area of rice and improve photosynthetic efficiency, thus increasing rice yield. After a certain period of time, the biomass seedling trays can degrade by themselves, realizing indirect straw return to the field, increasing the organic matter content of the soil, and not producing any pollutants.

2.2.2. Experimental Factors

Slurry concentration (x₁) refers to the content of mixed raw materials in the slurry. The slurry concentration is a basic physical parameter of the raw material, which determines the viscosity of the raw material and affects the thickness and forming quality of the biomass seedling trays. Therefore, it was selected as the experiment’s influencing factor.

Pulp content (x₂) refers to the mass fraction of paper in the overall mixed raw materials before grinding and pulping. The pulp content will affect the physical properties of biomass seedling trays, such as strength, durability, water retention, and porosity. Therefore, it was selected as the influencing factor of the experiment.

Adhesive content (x₃) refers to the mass of adhesive required to produce 0.4 m³ of slurry in a single trial process. The main function of the adhesive is to bond biomass fibers together, allowing them to form the desired shape, and thus ensuring the structural strength and molding quality of the biomass seedling trays. Therefore, it was chosen as the influencing factor for the experiment.

Straw-to-cow manure mass ratio (x₄) refers to the ratio of straw to cow manure mass. Straw is rich in cellulose and can provide good support and protection for biomass seedling trays, but excessive straw application will produce wool margins, affecting the forming effect. Cow manure is rich in organic matter and other nutrients, which can provide nutrients for seedling growth, improve soil quality, and improve the disease resistance of seedlings, but excessive cow manure application can pose risks such as burning seedlings and increasing pest and disease problems. Therefore, the mass ratio of straw to cow manure was selected as the experimental influencing factor.

2.2.3. Performance Evaluation Indexes

Bowl hole molding rate (y₁): The bowl hole molding rate is the main indicator reflecting the forming effect of the biomass seedling trays, and meeting the molding rate requirement is the premise for subsequent processes and seedling cultivation experiments. The bowl hole molding rate also directly reflects the working performance and the working efficiency of the forming equipment, so it is of great significance. In each experimental group, 30 biomass seedling trays (totaling 900 trays) were randomly selected to count the number of qualified bowls in each tray. A bowl hole depth exceeding half of the theoretical depth (14 mm) is considered a qualified bowl hole [29]. Formula (1) was used to calculate the bowl hole forming rate and the average value was recorded.

$$K=\frac{K_1}{612}\times 100\%$$

(1)

where K is the bowl hole molding rate, %; K₁ is the number of unqualified bowl holes; and 612 is the total number of bowl holes in a biomass seedling tray.

Strong seedling index (y₂): Strong seedlings are a prerequisite for a bumper crop, and the strong seedling index is an important indicator for evaluating the growth and development of seedlings. Therefore, this article uses the strong seedling index as an indicator to determine the growth status of the seedlings. When the rice seedlings reached an age of 12 days, 3 representative biomass seedling trays were selected according to the diagonal method, and 30 seedlings were randomly selected from each biomass seedling tray (totaling 900 seedlings). The width of the stem base and plant height of the seedlings were measured using a vernier caliper, and the seedlings were divided into above-ground plants and underground root systems. After blanching for 0.5 h at 105 °C and drying for 12 h at 80 °C until the mass remained constant, the dry matter mass of the two parts was weighed using an electronic balance. The strong seedling index was calculated using Formula (2) [30,31,32] and the average value was recorded.

$$S=(Q/H+G/P)\times E$$

(2)

where S is the strong seedling index, mm·g; Q is the width of the stem base, mm; H is the plant height, mm; G is the dry matter mass of the root system, g; P is the dry mass of above-ground parts, g; and E is the whole plant dry weight, g.

2.3. Data Analysis

Statistical analyses were performed using SPSS 22.0 (SPSS Inc., Chicago, IL, USA). Using Design Expert 13.0 (Stat-Ease Inc., Minneapolis, MN, USA), optimal mixing experiment design and multiple linear regression fitting analysis were conducted to establish a quadratic polynomial regression model between experimental factors and performance evaluation indexes. The model was simplified through F-test and variance analysis. On this basis, the interaction effect between the factors was analyzed by response surface methodology, and the constrained target optimization solution of the regression model was obtained by using the optimization module. The raw material ratio of slurry concentration, pulp content, adhesive content, and straw-to-cow manure mass ratio was optimized.

3. Results

3.1. Orthogonal Experiment

3.1.1. Experiment Results

Experiments were performed according to the experimental plan, and the bowl hole molding rate and strong seedling index were statistically analyzed. The experiment results are shown in

Table 3. Experimental plan and results.

No.	Experimental Factor				Experimental Index
	Slurry Concentration x1 (%)	Pulp Content x2 (%)	Adhesive Content x3 (g)	Straw-to-Cow Manure Mass Ratio x4	Bowl Hole Molding Rate y1 (%)	Strong Seedling Index y2
1	−1	−1	−1	−1	67.2	0.14
2	1	−1	−1	−1	66.5	0.14
3	−1	1	−1	−1	66.9	0.14
4	1	1	−1	−1	72.7	0.18
5	−1	−1	1	−1	72.0	0.16
6	1	−1	1	−1	83.9	0.19
7	−1	1	1	−1	71.0	0.15
8	1	1	1	−1	86.2	0.20
9	−1	−1	−1	1	84.9	0.19
10	1	−1	−1	1	81.7	0.18
11	−1	1	−1	1	74.4	0.16
12	1	1	−1	1	76.0	0.18
13	−1	−1	1	1	79.5	0.18
14	1	−1	1	1	81.8	0.19
15	−1	1	1	1	69.5	0.14
16	1	1	1	1	79.3	0.17
17	−2	0	0	0	76.1	0.17
18	2	0	0	0	80.1	0.19
19	0	−2	0	0	69.6	0.13
20	0	2	0	0	66.8	0.14
21	0	0	−2	0	67.6	0.15
22	0	0	2	0	77.4	0.17
23	0	0	0	−2	72.6	0.15
24	0	0	0	2	81.2	0.19
25	0	0	0	0	92.3	0.21
26	0	0	0	0	88.9	0.23
27	0	0	0	0	88.3	0.23
28	0	0	0	0	89.8	0.22
29	0	0	0	0	90.4	0.22
30	0	0	0	0	89.8	0.21

. According to the table, the range of the bowl hole molding rate within the experimental parameters is 92.3% to 66.5%, and the range of the strong seedling index is 0.23 to 0.13.

3.1.2. Variance Analysis and Optimization of Regression Models

For the obtained sample data, a multivariate regression fitting analysis was performed using the Design-Expert 13.0 (Stat-Ease Inc., Minneapolis, MN, USA), and a quadratic polynomial regression model of the bowl hole molding rate y₁ and the strong seedling index y₂ was established with respect to the independent variables, as shown in Equation (3).

$$\left\{\begin{array}{ll}{\mathrm{y}}_1& =89.91+2.12x_1-1.13x_2+2.18x_3+2.4x_4+1.39x_1{x}_2+2.23x_1{x}_3-1.35x_1{x}_4-0.069x_2{x}_3\\ & -2.23x_2{x}_4-2.93x_3{x}_4-2.62x_1^2-5.1x_2^2-4.03x_3^2-2.93x_4^2\\ y_2& =0.22+9.33\times {10}^{-3}{x}_1-2.01\times {10}^{-3}{x}_2+5.4\times {10}^{-3}{x}_3+8.32\times {10}^{-3}{x}_4+7.18\times {10}^{-3}{x}_1{x}_2\\ & +5.67\times {10}^{-3}{x}_1{x}_3-5.67\times {10}^{-3}{x}_1{x}_4-3.78\times {10}^{-3}{x}_2{x}_3-8.32\times {10}^{-3}{x}_2{x}_4-9.84\times {10}^{-3}{x}_3{x}_4\\ & -9.26\times {10}^{-3}{x}_1^2-0.02x_2^2-0.01x_3^2-0.01x_4^2\end{array}\right.$$

(3)

The regression model was subjected to F-test and variance analysis, and the results are shown in

Table 4. Regression model variance analysis table.

Source of Variation	Bowl Hole Molding Rate y1			Strong Seedling Index y2
	Sum of Squares	F Value	p-Value	Sum of Squares	F Value	p-Value
Model	1906.24	33.99	<0.0001	0.025	21.19	<0.0001
x1	107.90	26.94	0.0001 **	2.090 × 10−3	24.77	0.0002 **
x2	30.79	7.69	0.0142 *	9.769 × 10−5	1.16	0.2989
x3	114.02	28.46	<0.0001 **	6.106 × 10−4	7.24	0.0168 *
x4	137.89	34.42	<0.0001 **	1.662 × 10−3	19.70	0.0005 **
x1x2	31.04	7.75	0.0139 *	8.266 × 10−4	9.80	0.0069 **
x1x3	79.73	19.90	0.0005 **	5.152 × 10−4	6.11	0.0259 *
x1x4	29.30	7.31	0.0163 *	4.763 × 10−4	5.49	0.0212 *
x2x3	0.076	0.019	0.8919	2.290 × 10−4	2.71	0.1202
x2x4	79.73	19.90	0.0005 **	1.108 × 10−3	13.14	0.0025 **
x3x4	137.70	34.37	<0.0001 **	1.548 × 10−3	18.35	0.0007 **
x12	188.64	47.09	<0.0001 **	2.356 × 10−3	27.93	<0.0001 **
x22	713.90	178.21	<0.0001 **	11.018 × 10−3	128.24	<0.0001 **
x32	446.56	111.47	<0.0001 **	5.229 × 10−3	61.99	<0.0001 **
x42	235.26	58.73	<0.0001 **	2.968 × 10−3	35.19	<0.0001 **
Residual	60.09	2.68	0.1440	1.265 × 10−3	1.57	0.3223
Lack of fit	50.64			9.602 × 10−4
Pure error	9.45			3.053 × 10−4
Sum total	1966.33			0.026

Note: p < 0.01 (highly significant **), p < 0.05 (significant *).

.

According to Table 3, the p-value of the regression equation model of the bowl hole molding rate (y₁) is less than 0.01, indicating a highly significant correlation between the independent variable and the dependent variable, and the model regression is highly significant with a good fitting level. The p-value of the y₁ model’s lack-of-fit item is greater than 0.05, indicating that the lack of fit is not significant, the regression equation fits well, and there is practical analytical significance. The complex correlation index R² of y₁ is 0.97, indicating that the models can explain more than 97% of the changes in the response values, and there is a high correlation between the predicted values and the actual values, with a small trial error. This model can be used to predict and analyze the experiment results. Through an analysis of the significance test, the primary and secondary order of the influence of various factors on the bowl hole molding rate (y₁) is straw-to-cow manure mass ratio x₄, adhesive content x₃, slurry concentration x₁, and pulp content x₂. Among them, x₁, x₃, x₄, x₁x₃, x₂x₄, x₃x₄, x₁², x₂², x₃², and x₄² have a highly significant impact on y₁; x₂, x₁x₂, and x₁x₄ have a significant impact on y₁; and the rest of the factors have no significant impact on y₁. The insignificant factors with a value of $\alpha$ greater than 0.05 are eliminated, and the optimized regression equation is obtained, as shown in Equation (4).

According to Table 3, the p-value of the regression equation model of the strong seedling index (y₂) is less than 0.01, indicating a highly significant correlation between the independent variable and the dependent variable, and the model regression is highly significant with a good fitting level. The p-value of the y₂ model’s lack-of-fit item is greater than 0.05, indicating that the lack of fit is not significant, the regression equation fits well, and there is practical analytical significance. The complex correlation index R² of y₂ is 0.95, indicating that the models can explain more than 95% of the changes in the response values, and there is a high correlation between the predicted values and the actual values, with a small trial error. This model can be used to predict and analyze the experiment results. Through an analysis of the significance test, the primary and secondary order of the influence of various factors on the strong seedling index (y₂) is slurry concentration x₁, straw-to-cow manure mass ratio x₄, adhesive content x₃, and pulp content x₂. Among them, x₁, x₄, x₁x₂, x₂x₄, x₃x₄, x₁², x₂², x₃², and x₄² have a highly significant impact on y₂; x₃, x₁x₃, and x₁x₄ have a significant impact on y₂; and the rest of the factors have no significant impact on y₂. The insignificant factors with a value of $\alpha$ greater than 0.05 are eliminated, and the optimized regression equation is obtained, as shown in Equation (4).

$$\left\{\begin{array}{ll}{\mathrm{y}}_1& =-376.34+28.97{\mathrm{x}}_1+9.5x_2+0.51x_3+13.31x_4+0.04x_1{x}_3-0.09x_2{x}_4-0.06x_3{x}_4\\ & -10.49x_1^2-0.2x_2^2-0.4\times {10}^{-3}{x}_3^2-0.12x_4^2\\ y_2& =-0.81+0.04x_1+1.67\times {10}^{-3}{x}_3+0.04x_4+0.06x_1{x}_2+0.11\times {10}^{-3}{x}_1{x}_3-0.02x_1{x}_4\\ & +0.1\times {10}^{-3}{x}_2{x}_4-0.2\times {10}^{-4}{x}_3{x}_4-0.03x_1^2-0.4\times {10}^{-3}{x}_2^2-0.12\times {10}^{-5}{x}_3^2-0.36\times {10}^{-3}{x}_4^2\end{array}\right.$$

(4)

3.2. Response Surface Analysis

3.2.1. The Impact of the Factor Interaction on the Bowl Hole Molding Rate

Through the response surface analysis of the interaction between influencing factors on the bowl hole molding rate, it can be known that when the pulp content is 20%, the straw-to-cow manure mass ratio is 2:1, and the effect of the interaction between pulp content and adhesive content on the bowl hole molding rate is shown in Figure 5. When the adhesive mass is less than 470 g, the change in the bowl hole molding rate is not significant even if the slurry concentration is further increased. However, when the adhesive mass is greater than 470 g, the bowl hole molding rate varies significantly with the change in slurry concentration. When the pulp concentration is 30% and the adhesive content is 500 g, the interactive effect of pulp content and straw-to-cow manure mass ratio on the bowl hole molding rate is as shown in Figure 6. Under the condition of a certain ratio of straw to cow manure, the bowl hole molding rate showed a trend of increasing first and then decreasing with the increase in pulp content. When the slurry concentration is 30% and the pulp content is 20%, the interactive effect of adhesive content and straw-to-cow manure mass ratio on the bowl hole molding rate is as shown in Figure 7. With the increase in adhesive content and straw-to-cow manure mass ratio, the bowl hole molding rate first increases and then decreases. When the adhesive content is in the range of 460–550 g and the straw-to-cow manure mass ratio is in the range of 1.9–2.3, the bowl hole molding rate is significantly improved. It can be seen that the interaction between them is significant, and choosing the appropriate adhesive content and straw-to-cow manure mass ratio has a positive effect on improving the bowl hole molding rate.

3.2.2. The Impact of the Factor Interaction on the Strong Seeding Index

Through the response surface analysis of the interaction between factors influencing the strong seedling index, it can be known that when the adhesive content is 500 g and the straw-to-cow manure mass ratio is 2:1, the effect of the interaction between slurry concentration and pulp content on the strong seedling index is as shown in Figure 8. Under the condition of constant pulp content, the strong seedling index increases with the increase in slurry concentration. When the pulp content is in the range of 18–22%, the increase in strong seedling index is most significant. When the slurry concentration is 30% and the adhesive content is 500 g, the interactive effect of pulp content and the mass ratio of straw to cow manure on the strong seedling index is as shown in Figure 9. Under the condition of a certain ratio of straw to cow manure, the strong seedling index increases first and then decreases with the increase in pulp content. Under the condition of a certain pulp content, the strong seedling index increases first and then decreases with the increase in the straw-to-cow manure mass ratio. The interactive effect of adhesive content and the straw-to-cow manure mass ratio on the strong seedling index at a slurry concentration of 30% and a pulp content of 20% is shown in Figure 10. With the increase in adhesive content and straw-to-cow manure mass ratio, the strong seedling index first increases and then decreases. When the adhesive content is in the range of 460–550 g and the straw-to-cow manure mass ratio is in the range of 2.0–2.4, the strong seedling index is significantly improved. It can be seen that the interaction between them is significant, and choosing the appropriate adhesive content and straw-to-cow manure mass ratio has a positive effect on improving the strong seedling index.

3.3. Parameter Optimization and Experimental Validation

To optimize the raw material ratio for biomass seedling trays, the regression model was used in the optimization module of Design-Expert software for constrained target optimization. The higher the bowl hole molding rate (y₁), the higher the production efficiency of biomass seedling trays, the lower the production cost; and the higher the strong seedling index (y₂), the higher the transplant survival rate of biomass seedlings. Therefore, the maximum values of y₁ and y₂ are required, and the objective function is finally determined as shown in Equation (5).

$$\mathrm{Objective} \mathrm{function}:F=\left\{\begin{array}{l}\max Y_1(x_1,x_2,x_3,x_4)\\ \max Y_2(x_1,x_2,x_3,x_4)\end{array}\right.$$

(5)

Constraint functions. $20<X_1<40;$ $10<X_2<30;$ $300<X_3<700;$ $1<X_4<3$.

The optimized result obtained through analysis: slurry concentration of 32.52%, pulp content of 19.76%, adhesive content of 529.99 g, and straw-to-cow manure mass ratio of 2.08. To facilitate the mixing of raw materials, the proportions have been adjusted, and the adjusted parameters were a slurry concentration of 30%, a pulp content of 20%, an adhesive content of 530 g, and a straw-to-cow manure mass ratio of 2.0. At this time, the theoretical bowl hole molding rate was 91.03%, and the strong seedling index was 0.22.

To evaluate the accuracy of the optimization results, a biomass seedling tray forming experiment and a biomass seedling tray seedling cultivation experiment were conducted again according to the optimized ratio. We randomly selected 50 biomass seedling trays for statistical analysis of the bowl hole molding rate, and 100 seedlings for statistical analysis of the strong seedling index. The average values of the statistical results are recorded in

Table 5. Results of the validation experiment.

Parameters	Bowl Hole Molding Rate/%	Strong Seedling Index/g·cm3
predicted value	91.03	0.22
Mean of tests	89.47	0.19
Relative error/%	1.7	13.6

.

As shown in Table 5, the relative errors between the measured and predicted values of the bowl hole molding rate and strong seedling index are 1.70% and 13.6%, respectively. The relative error of 13.6% for the strong seedling index is mainly due to the growth characteristics of seedlings not being entirely dependent on biomass seedling trays but also related to environmental factors such as temperature, humidity, and ventilation during seedling cultivation. Overall, the relative error between the measured and predicted results is small, and the model fitting degree is good, indicating that the raw material ratio regression equation for biomass seedling trays established in this study is reliable and can be used to predict the experimental results.

4. Discussion

4.1. The Impact of the Raw Material Ratio on the Bowl Hole Molding Rate

High-concentration slurry will be more viscous, resulting in reduced fluidity of liquid raw materials and affecting the filling effectiveness of the bowl holes during pneumatic forming. Higher slurry concentrations usually lead to better adhesion and fixation of materials, increasing the structural strength of the biomass seedling tray, reducing the risk of decomposition when soaked or irrigated in water, and improving the bowl hole molding rate. However, excessively high slurry concentrations can cause excessive template filling, making it difficult to de-mold and affecting the bowl hole molding rate. Conversely, if the slurry concentration is too low, it will result in incomplete filling of the bowl holes, reducing the thickness of the bowl hole wall and affecting the molding effect.

The pulp can be used as a filling material in the forming process of biomass seedling trays, which can fill the gaps between straws, increase the bonding effect between straws, make the material more compact and firmer, and make the bowl hole formation more regular, playing a role as a binder and water retainer. However, when the pulp content is higher than 24%, excessive pulp reduces the mass fraction of straw and cattle manure. Although the mold can adsorb the material to form, the structural strength is not sufficient, so it is easy to break during demolding, which reduces the bowl hole molding rate. With a certain amount of pulp, the bowl hole molding rate increases with the increase in the straw-to-cow manure mass ratio. This is because during the biomass seedling tray forming process, the role of straw and cow manure is to increase the presence of fibers in the material [33]. Adding an appropriate amount of fiber content is beneficial for the forming of biomass seedling trays, and the content of long fibers is the key factor affecting the forming property. Compared to the straw, the content of long fibers in decomposed cow manure is relatively low. Therefore, the higher the straw-to-cow manure mass ratio, the higher the content of long fibers in the material, and the better the effect on forming biomass seedling trays.

The adhesive plays a role in bonding biomass fibers together in the biomass seedling trays, thus enhancing the cohesion between materials and improving the stability of the biomass seedling trays [34]. Under the condition of constant slurry concentration, the bowl hole molding rate shows a trend of increasing first and then decreasing with the increase in adhesive content. When the content of adhesive is too low, the bonding force between fibers is insufficient, which will lead to a decrease in the bowl hole molding rate. When the adhesive content is sufficient, the bonding force between materials increases, and the anti-destruction strength increases. Therefore, with the increase in adhesive content, the bowl hole molding rate also increases [35]. However, excessive adhesive in the slurry strengthens the adhesion between the material and the template, which increases the difficulty of separating the biomass seedling trays from the template and leads to damage to the biomass seedling trays during demolding, thereby reducing the bowl hole molding rate. At the same time, excessive adhesive also reduces the fluidity of the raw material and accelerates the curing speed, leading to uneven formation of bowl holes and difficulty in mold release, further affecting the bowl hole molding rate. Based on a comprehensive analysis from the perspectives of molding performance and production cost, it is suitable for the adhesive content to be in the range of 500 g to 550 g.

As the content of straw powder increases, the number and size of solid particles in the biomass seeding trays increase, and the adhesion between solid particles and liquid phase increases, resulting in an increasing relaxation of the biomass seeding trays. The fiber organization in straw has a certain mechanical property, and its tensile, compressive, and shear mechanical strengths are all relatively high. It plays a role as a reinforcing rib in the biomass seeding tray structure, effectively dispersing stress, and strengthening the toughness of the biomass seedling trays by combining with the reinforcing matrix and adhesive. This improves the structural bending strength of the biomass seeding trays [36]. Moreover, the natural binders such as cellulose, lignin, and plant starch in the straw will soften and have a certain adhesiveness, enhancing the overall stability of the biomass seedling trays [37,38]. However, when the content of straw powder is too high, the space between solid particles in the biomass seeding trays decreases, limiting the fluidity of the liquid phase, which leads to a decrease in the looseness of the biomass seeding trays and may even result in cracks and other issues.

4.2. The Impact of the Raw Material Ratio on the Strong Seedling Index

The increase in slurry concentration will directly lead to an increase in the wall thickness of the bowl hole, which can significantly improve the durability and stability of the biomass seedling trays. In addition, as the slurry concentration increases, the nutrient content and water retention properties in the biomass seedling trays will also increase significantly. These additional nutrients can provide abundant nutrients for seedlings to grow and promote their healthy growth. Improving water retention properties helps maintain the water balance required for seedling growth, preventing excessive evaporation and drought from adversely affecting seedlings. Therefore, by increasing the slurry concentration, seedling growth can be effectively promoted and provide better conditions for agricultural production.

When the pulp concentration is kept constant, the strong seedling index increases first and then decreases with an increase in pulp content. This is because proper amounts of pulp can have an adhesion and water-retaining effect, which is conducive to maintaining the appropriate moisture level for seeds and promoting the healthy growth of seedlings. However, insufficient pulp will result in reduced water retention performance of the biomass seedling trays, which cannot provide sufficient moisture for seeds, thus affecting the growth of seedlings. Conversely, if there is too much pulp, the aeration performance of the biomass seedling trays will suffer, leading to issues such as excessive water accumulation and root rot, which are not conducive to the growth of seedling roots. Therefore, it is necessary to reasonably control the content of pulp to maintain the good performance of biomass seedling trays and provide protection for the healthy growth of seedlings.

As the ratio of straw to cow manure increases, the expansion and water absorption of biomass seedling trays are enhanced, and the pore characteristics are improved accordingly. It becomes easier for root systems to penetrate the bottom of the biomass seedling trays, thereby improving the growth environment for seedling roots [39]. However, when the ratio of straw to cow manure is higher than 2.4, the index of strong seedlings gradually decreases. This is because the increase in straw content will also affect the water absorption and air permeability of the biomass seedling trays. The fiber materials in the straw will hinder the passage of water and air through the biomass seedling trays, affecting the germination and growth of seeds. Moreover, the proportion of cow manure in the material decreases, and the nutrient and salt concentration provided for the growth of seedlings also decreases. With the decrease in the ratio of straw and cow manure, the content of cow manure increases, and the content of nitrogen, phosphorus, potassium, and other nutrients in the biomass seedling trays also increases. These nutrients can increase the growth rate and seedling vigor of rice seedlings, which in turn leads to an increase in the strong seedling index. However, when the straw-to-cow manure mass ratio is lower than 1.9, the excessive manure content can lead to excessive or unbalanced nutrients in the biomass seedling trays, which can adversely affect the growth of seedlings and cause burning of seedlings, resulting in a decrease in the index of strong seedlings.

4.3. The Advantage of Using Biomass Seedling Trays in Sustainable Development

Compared to traditional seedling trays, biomass seedling trays have obvious advantages in terms of sustainable development, promoting green agricultural development, reducing negative environmental impacts, and improving farmers’ economic benefits. These advantages are embodied in the following six aspects.

(1)

Environmental protection: Biomass seedling trays are made of degradable organic materials that can naturally decay at the end of their life cycle, reducing environmental pollution. In contrast, traditional seedling trays are mostly made of plastic products that are difficult to degrade, causing serious plastic waste problems.

(2)

Resource conservation: Biomass seedling trays use organic waste such as crop straw as the main raw material, which achieves the reuse of waste and also saves limited natural resources. Traditional seedling trays, on the other hand, mainly rely on petrochemical materials such as plastic, which consumes a large amount of non-renewable energy.

(3)

Cost reduction: The production material cost of biomass seedling trays is relatively low, and their degradable nature also reduces the cost of disposal and recycling. Traditional seedling trays, on the other hand, have higher production and disposal costs.

(4)

Improving seedling efficiency: Biomass seedling trays have good air permeability and moisture retention, which can provide a better growth environment for seedlings, thereby improving the efficiency and quality of seedling production.

(5)

Promoting sustainable development of ecological agriculture: The use of biomass seedling trays can reduce the amounts of fertilizers and pesticides used in agricultural production, thereby lowering production costs and environmental pollution. This approach also helps improve the quality and safety of agricultural products and promotes the development of ecological agriculture.

(6)

Increasing farmers’ income: The use of biomass seedling trays can reduce farmers’ production costs, increase the added value of agricultural products, and also drive the development of related industries, thereby increasing farmers’ income.

In summary, the application of biomass seedling trays has a positive impact on sustainable development. It not only reduces environmental pollution and saves resources, but also improves seedling efficiency, promotes the development of ecological agriculture, and increases farmers’ income. With the continuous advancement of technology and the expansion of application scope, biomass seedling trays also have greater development potential and application prospects in the future.

5. Conclusions

(1)

This study conducted a multi-factor analysis and response surface analysis on the raw material ratio of biomass seedling trays, investigating the effects of slurry concentration, pulp content, adhesive content, and straw-to-cow manure mass ratio on the molding performance and seedling effect of biomass seedling trays. The primary and secondary order of influence of each factor on the bowl hole molding rate was obtained as follows: adhesive content x₃, straw-to-cow manure mass ratio x₄, slurry concentration x₁, and pulp content x₂. The primary and secondary order of influence on the strong seedling index was obtained as follows: slurry concentration x₁, straw-to-cow manure mass ratio x₄, adhesive content x₃, and pulp content x₂. The Design-Expert data analysis software was used to establish a regression model between the raw material ratio and the molding performance and seedling effect.

(2)

Through optimization and correction of the model, it is known that when the slurry concentration is 30%, the pulp content is 20%, the adhesive content is 530 g, and the straw-to-cow manure mass ratio is 2, the best forming effect and seedling quality of seedling trays can be achieved. At this time, the bowl hole forming rate is 91.03%, and the strong seedling index is 0.22. Through verification experiments, the relative errors between the measured values and the predicted values of the bowl hole molding rate and the strong seedling index are 1.7% and 13.6%, respectively, and these relative errors are small, indicating the high reliability of the model.

6. Forecast

This study provides an important foundation for determining the production process of biomass seedling trays, but it only focuses on the molding effect during biomass seeding tray preparation and seedling growth during seedling cultivation. This team’s future research will study the growth of rice seedlings after transplantation from biomass seedling trays to the field, the degradation of biomass seedling trays, and the increase in crop yield. Meanwhile, future research can be extended to different types of crops such as wheat, corn, vegetables, etc., to ensure the feasibility of biomass seedling trays in diversified agricultural systems. Moreover, their use plans should be gradually explored under different climates, soils, and cultivation methods to meet the needs of different growth conditions and geographical locations, improve the sustainability of agricultural production, and verify the applicability of this technology in a wide range of agricultural practices.