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Article

Forming and Degradation Mechanism of Bowl Seedling Tray Based on Straw Lignin Conversion

by
Liqiang Qi
1,2,3,
Bo Zhang
1,2,3,
Yongcai Ma
1,2,3 and
Wei Zhang
1,2,3,*
1
College of Engineering, Heilongjiang Bayi Agricultural University, Daqing 163319, China
2
Heilongjiang Province Conservation Tillage Engineering Technology Research Center, Daqing 163319, China
3
Key Laboratory of Soybean Mechanization Production, Ministry of Agriculture and Rural Affairs, Daqing 163319, China
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(2), 453; https://doi.org/10.3390/agronomy13020453
Submission received: 26 December 2022 / Revised: 26 January 2023 / Accepted: 31 January 2023 / Published: 2 February 2023
(This article belongs to the Section Agricultural Biosystem and Biological Engineering)
Browse Figures
Versions Notes

Abstract

:
In response to the problems of low straw utilization efficiency and poor returning effect in Northeast China, this paper takes rice straw containing cow dung as the experimental material, and according to the characteristics of lignin glass transformation of the material, proposes a new method to prepare biomass seedling trays. The seedling trays prepared by this method can meet the needs of corn seedling cultivation and transplantation. To study the molding mechanism, scanning electron microscopy and a universal testing machine were used to compare the changes in the internal structure and mechanical properties of the regularly- and hot-compressed seedling trays before and after seedling raising. The results show that the material with water content of 23% has the best hot-pressing effect. The forming mechanism is: that the strength of the molded seedling tray resulted from the mechanical setting force of the multilayered stem fibers with a mosaic structure within the seedling tray. The adhesion and wrapping by lignin prevented water penetration from damaging the multilayered stem fibers and slightly improved their strength. The seedling tray made of straw and manure was completely degraded over 40 days, and the straw degradation rate was improved. This method can increase the overall quality and benefits of straw, providing a foundational reference for high-quality and high-efficiency straw utilization.

1. Introduction

Northeast China is the most important grain-producing region of China. Because of its special geographical location and environmental factors, Northeast China has the most concentrated straw resources in China. The straw output in Northeast China accounts for 20% of the country’s total output, with a theoretical output exceeding 200 million tons [1,2]. In recent years, to protect the resources of black land (the soil color is black) and accelerate the recycling of straw, some straw crops have been properly applied in the fields with fertilizer and feed; however, there are many straw crops which still need to be processed. As the government has issued policies to prohibit the burning of straw, the unprocessed heaps of straw cause many safety hazards [3,4]. For example: Firstly, centralized stacking is a large fire hazard. Secondly, it breeds diseases and pests. Thirdly, it is easy for harmful small animals to hide. In addition, straw stacking on the roadside affects travel and is a potential safety hazard.
As conservation tillage technology has been promoted, straw has begun to be returned to the field through deep burial and surface coverage by means of no-tillage and minimal-tillage sowing and straw residue management, which have effectively reduced the water and soil loss and wind erosion on the black land, increased the organic content in the soil, and improved soil fertility [5,6,7,8]. Therefore, straw utilization and return-to-field have greatly improved, guaranteeing the overall quality of straw utilization, and the overall benefit from utilizing straw has increased. However, the straw returned to field in conservation tillage only accounts for 30–40% of the straw in Northeast China, so there is still much unprocessed straw residue [9,10,11,12]. Regarding the proper utilization of straw crops, scholars in China and abroad have conducted many studies on the effective utilization and proper allocation of straw [13,14,15], and their findings have helped promote the development of proper straw utilization technology [16,17,18,19,20].
Heilongjiang Province, located in a high-latitude area, has a short frost-free season and so the growing period of corn is short. Corn produced by traditional growing methods there have poor quality and low yield. To prolong the growing period of corn, seedling raising and transplantation have been widely applied in recent years, as they can increase the corn growing period by 15–20 days, effectively improving the quality and yield. Current corn seedling trays that are mostly made of plastic have a short service cycle and are not environmentally friendly [21]. Due to the complicated transplant process when using a plastic seedling tray, mechanized and large-scale production is difficult to achieve, and so such trays have not been easily promoted or widely applied. In contrast, seedling trays made of degradable materials can be directly transplanted to the field with the seedlings, which not only simplifies the transplanting process but also causes zero environmental pollution [22,23].
Straw contains much lignin and cellulose. Degradable seedling trays made of straw and other ingredients can simplify the supporting mechanical structure used for transplanting, to popularize the application of seedling raising and transplanting technology and offer a solution for the functional utilization of straw [24,25,26]. With the continuous improvement of agricultural science and technology, scholars have made many breakthroughs in the fields of straw fertilizer [27,28,29], straw feed [30,31], straw energy [32,33], and straw base material [34,35,36].
Therefore, based on the current utilization of straw in Northeast China, this paper aimed to improve the low efficiency of proper straw utilization and the poor effect of straw return-to-field. It used the straw residues from conservation tillage as the experimental materials and proposed a new method to prepare seedling trays, which improved the degradation rate of straw and increased the ease and quantity of straw return-to-field. Under the condition of meeting the requirements of maize seedling cultivation and transplanting, the method is used to prepare the seedling tray. When there are three maize leaves, then field transplanting is carried out with the maize, and then the straw is used reasonably. By using scanning electron microscopy and a universal testing machine, the internal structure and mechanical properties of the seedling tray formed by compression at normal temperature and heating compression before and after the seedling cycle were compared, and the forming and degradation mechanism was studied. This paper develops a new approach that improves proper straw utilization and explores the long-term mechanism for comprehensive straw utilization in order to promote the overall popularization of conservation tillage technology.

2. Materials and Methods

In this study, materials with different water content were first tested for shaping, and then the shaped seedling tray was tested for seedling cultivation after being pressed, and then the seedling tray with different water content and different heating treatment was tested for destruction. Finally, the shaping and degradation mechanism of the seedling tray was analyzed with a scanning electron microscope.

2.1. Materials

Rice straw residues and other ingredients (cattle manure) were taken from the Beingmate industrialized dairy cattle breeding base in Anda city, Heilongjiang Province, China. The composition of the material dry matter is shown in Table 1 [37,38]. This study was carried out in Heilongjiang Province in northeast China, which has a cold temperate and temperate continental monsoon climate. Heilongjiang soil is classed as black soil, which is the most fertile soil in the world.

2.2. Test Procedure

2.2.1. Preprocessing of Materials

Four straw samples were placed in a dark, ventilated environment to reduce their moisture content until they had reached 17%, 20%, 23%, and 26%, respectively, and four mixtures of cattle manure with straw of different moisture content (17%, 20%, 23%, and 26%) were prepared. The dried samples were put into a mixer (Bingcheng BH-12.5, Harbin, China) for 10 min, shaking at 20 r/min, until the samples became loose. The looseness was measured at 0.4 t/m3 (Figure 1).

2.2.2. Molding Test

Regular compression molding: A total of 300 g of the processed sample were put into the compression mold for the corn seedling tray (Figure 2) and were then compressed under a press at 20 MPa. After compression for 20 s, the mold was taken out, and the seedling tray was dried at room temperature for 48 h. The size of the seedling tray mold was 276.5 mm × 42 mm × 35 mm (L × W × H). The specific structure and overall size of the seedling tray are shown in Figure 3.
Hot compression molding: The induction coils of an electromagnetic induction heater were wrapped on the outer surface of the mold material frame, the current of the electromagnetic induction heater was adjusted, and the mold was heated to and kept at 230 °C. A total of 300 g of the processed material (with the same moisture content as used in the regular compression molding) were put into the corn seedling tray compression mold, and then compressed under a press at 20 kN. After compressing at a constant temperature for 20 s and then naturally cooling to <50 °C, the mold was taken out.

2.2.3. Seedling Raising Test

Seedling raising test: At 2–3 days before sowing, the corn seeds (Demeiya 3) were soaked in warm water at 28–30 °C for 8–12 h, then taken out for drying. The seeds were kept at 20–25 °C to accelerate germination. During this process, seeds were turned over every 2–3 h until radicles appeared. Two centimeters of soil was placed in the holes of the molded regularly and hot-compressed seedling trays, a germinated seed was placed in each hole, a layer of topsoil was added to the edge of the tray, and the seedling tray was watered until the soil was completely soaked. The temperature, humidity, and raising time were set as 25 ± 2 °C, 45–65%, and 15 days, respectively.

2.2.4. Breaking Strength Measurement

The crushing strength of the shaped seedling trays with different water content and different treatment methods was measured. The test was repeated three times for each type of seedling tray, and the average value of the three damage results was taken. The molded regularly and hot-compression seedling trays were placed on the test platform of an electronic universal testing machine that followed the GB/T 16491–2008 standard. Under unconstrained support, through three-point bending (Figure 4), the seedling tray was damaged at a constant loading rate of 10 mm/min. The stop was set as 6 mm at the closed-loop control, and the duration was set to 10 s.
During the whole process, the load and deflection of samples were measured to determine the bending strength, bending elastic modulus, and the relationship between bending stress and strain. The span (L) was adjusted to 210 mm, and the position of the upper indenter was also adjusted, with an accuracy of 0.5 mm. The upper indenter was in the middle of the support and was parallel to the axis of the cylindrical surface of the support. The test was based on standard GB/T1449-2005. The sample was placed symmetrically in the middle of the support beam so that the indenter could prepress the sample surface. The instrument was checked to make sure the entire system was under normal conditions.
After the load and displacement reached zero, the test started. After arriving at the target position, the test was stopped, and the data were recorded. After the displacement was reset to zero, a new sample was placed to repeat the aforementioned steps. This method was also applied to the tray after seedling raising.

2.2.5. Scanning Electron Microscopy (SEM) of Molded Materials

Surface samples (1 × 1 cm) were taken from the seedling tray (as shown in Figure 5). The peeling side of the sample faced upwards, and the sample was taped to the sample stage with double-sided tape. The sample was placed in the DII-29030SCTR Ion sputtering instrument (JEOL LTD. Tokyo, Japan) for gold sputtering for 2 × 30 s. The gold-coated sample was put in the SEM (JCM-6000, JEOL, LTD. Tokyo, Japan) with the sample stage for observation. The acceleration voltage of the SEM was set as 15 kV.

3. Results and Discussion

3.1. Compression Molding Test Results and Discussion

3.1.1. Regular Compression Molding

Figure 6 shows the regularly compressed seedling trays made of raw material with different moisture content after being dried at room temperature. For the regularly compressed seedling tray with a moisture content of 17%, the top view (17-1) showed that the outer wall and the upper edge of the hole partition wall were loose, and the side view (17-2) showed that the bottom of the seedling tray (8.3 ± 1.2 mm thick) was stacked with material and had multiple fractured layers.
At the beginning of the molding process, the material was compressed so that porosity decreased, and the material accumulated at the bottom of the mold material frame. When the pressure applied to the mold increased, the pressure in the mold also rose, and the material accumulated at the bottom of the mold produced a component force that expanded outwards along the bottom of the mold. When the component force was greater than the flow stress of the material, the material began to flow and fill the gap between the inner wall of the mold material frame and the holes of the seedling tray from the bottom, creating a sidewall for the seedling tray. The difference in moisture content of the material affected the molding integrity of the seedling tray and the fluidity of the material in the mold [39].
The flow stress of the material increased as the moisture content decreased. After the material with a moisture content of 17% was compressed and molded at the bottom of the seedling tray, it was difficult to fill the sidewall of the material frame once the material had accumulated at the bottom. As a result, the sidewall of the seedling tray could not be created. The thick bottom of the seedling tray experienced density stratification due to the attenuation of pressure conduction, making multiple layers fracture.
The top and side views (20-1, 20-2) of the regularly compressed seedling tray made from material with a moisture content of 20% show that the sidewall of the seedling tray was intact, the thickness of the tray bottom was 6.4 ± 0.3 mm, and there was an unmolded section at the upper edge with a depth of 5.2 ± 2.1 mm. Under pressure, the material filled from the bottom to the sidewall of the material frame, and the material that flowed upward continuously accumulated to generate pressure to shape the top of the seedling tray after contacting the lower surface of the male mold. The increase in material flow distance led to more contact area between the material and the inner wall of the material frame, which in turn increased the flow resistance. The material with a water content of 20% had poor fluidity, so the upper material on the sidewall could not flow to contact the lower surface of the male mold during compression molding.
The material with a water content of 23% and 26% was compressed and completely molded at room temperature. The thickness of the bottom of the seedling tray was 4.6 ± 0.2 mm and 5.1 ± 0.2 mm, respectively. As shown in Figure 6, 23-1, the upper edge of the seedling tray with a water content of 23% was relatively neat, and the outer surface was flat (23-2).
Figure 6, 26-1 shows that the outer surface of the seedling tray with a moisture content of 26% was uneven, there was overflow material on the upper edge of the seedling tray, and a 0.2 mm gap appeared between the upper surface of the material frame and the lower surface of the male mold. During the compression process, the pressure in the mold cavity increased after the material flowed upward along the inner surface of the frame and contacted the lower surface of the male mold.
Comparing the materials with water contents of 26% and 23%, the fluidity of the former was better, and the required internal and external pressure difference was smaller when passing through the same-sized holes. Therefore, the material with a water content of 26% had overflow material on the upper edge of the seedling tray after regular compression molding. Integrity of the seedling tray is required for seedling raising. In the regular compression test, the seedling tray could be molded for material with a moisture of 23% and 26%. Therefore, the material with a moisture content of 23% and 26% were selected for the hot compression test.

3.1.2. Hot Compression Molding

The seedling trays compressed at high temperature after drying at room temperature are shown in Figure 7. 23-H and 26-H show the seedling trays that used material with a moisture content of 23% and 26%, respectively, with 1 indicating the top view and 2 indicating the side view. In Figure 7, 23-H shows a seedling tray prepared with material with a moisture content of 23%. According to 23-H-1 and 23-H-2, the upper edge of the seedling tray was neat, and the outer wall was smooth. Figure 7 26-H shows the seedling tray made of the material with a moisture content of 26%. Despite the smooth outer wall, material overflowed from the edge of the seedling tray, which was again caused by the good fluidity of the material. The test results lay a foundation for the analysis of dissolved material and soil after degradation.

3.2. Seedling Raising Results

As shown in Figure 8, 23-N-1, 26-N-1, 23-H-1, and 26-H-1 show the seedling trays completely soaked with water after sowing. These trays were made with material containing a moisture content of 23% and 26% through regular compression and hot compression: 23-N and 26-N show the regularly compressed seedling trays made of material with a moisture content of 23% and 26%; 23-H and 26-H show the hot-compressed seedling trays made of material with a moisture content of 23% and 26%; 1 indicates the seedling tray after sowing and watering; and 2 indicates the seedling tray after 15 days of seedling raising.
After 15 days of seedling raising, the soils in 23-N-2 and 26-N-2 had separated from the inner walls of the seedling trays, and the sidewalls had significantly expanded; 23-N-2 shows that the junction was broken, and the whole seedling tray was bent and deformed. The junctions of the holes in 26-N-2 were all broken. The soils in 23-H-2 and 26-H-2 were not separated from the inner walls. The sidewall of 23-H-2 was not obviously expanded, while the sidewall of 26-H-2 was slightly expanded, and the width of the seedling tray had increased. The seedling trays in 23-H-2 and 26-H-2 were not broken.
The changes in sidewall size from before to after seedling raising are listed in Table 2. The expansion rate of the seedling trays followed the order 23-H < 26-H < 23-N < 26-N. The soil for raising seedlings showed almost no expansion after being watered, while the expansion rates of 23-N and 26-N were large. Different expansion rates caused the sidewall to expand after the seedling was watered, so the sidewall separated from the soil to produce gaps. During the expansion process, expansions of the sidewall and the hole partition wall produced stresses in different directions, which caused stress failure at the junction between the hole partition wall and the sidewall of the seedling tray, as shown in Figure 8 23-N-2 and 26-N-2. Therefore, only the seedling trays in Figure 8 23-H-2 and 26-H-2 were selected for strength testing after seedling raising.

3.3. Breaking Strength Measurement of Seedling Trays

Figure 9 shows the bending strength results of regularly and hot-compressed seedling trays made of material with a moisture content of 23% and 26%. Figure 9 shows that for the material with the same moisture content, the seedling tray compressed under high temperature was more resilient than the regularly compressed one, although the strength was almost the same. After raising seedlings in the hot-compressed seedling tray, the bending strength of the seedling tray made of material with a moisture content of 23% was higher than that made of material with a moisture content of 26%.
Table 3 shows the bending strength and elastic deformation of the seedling trays. The bending strength of the regularly compressed seedling trays made of material with a moisture content of 23% before seedling raising was 101.6 N, and that of the regularly compressed seedling trays made of material with a moisture content of 26% before seedling raising was 79.6 N. The bending strengths of the hot-compressed seedling trays made of material with a moisture content of 23% before and after seedling raising were 111.3 N and 41.2 N, respectively, showing a decrease of 63.0%.
The bending strengths of the hot-compressed seedling trays made of material with a moisture content of 26% before and after seedling raising were 82.6 N and 20.1 N, respectively, showing a decrease of 75.7%. For the seedling trays molded with the same method, both the bending strength and elastic deformation of trays made of material with a moisture content of 23% before and after seedling raising were greater than those of trays made of material with a moisture content of 26%.
During the process of hot compression, the lignin in the material softens as the temperature of the mold rises and bonds with the surrounding particles after being separated out under the internal stress, and the integrity of seedling trays increases. The strength of lignin itself is not high after solidification. The mechanical strength of biomass material after compression is mainly produced by the solid bridges between biomass particles and the mechanical setting. Therefore, when the material had the same moisture content, hot-compressed seedling trays had a slightly greater strength than regularly compressed seedling trays.
Lignin is elastic, so the hot-compressed seedling trays showed greater elastic deformation than regularly compressed seeding trays when the material contained the same amount of moisture. In the compression molding test, the material with a moisture content of 26% overflowed from the upper exhaust gap, which made the inner pressure produced by compressing the material with a moisture content of 26% lower than that produced by compressing the material with a moisture content of 23%, leading to a lower material density. Therefore, the seedling trays molded with the material with a moisture content of 23% had a higher bending strength.
When the materials had the same moisture content, the hot- and regularly compressed seedling trays had almost the same bending strength, but the regularly compressed seedling tray was still broken during the seedling raising. We speculate that the strength of both regularly and hot-compressed seedling trays was produced by the solid bridge between particles and the mechanical setting, and the lignin during the hot compression was separated out under internal stress to bond with the surrounding particles and encircle the mechanical mosaic structure between some of the particles. Due to the hydrophobicity of lignin [40], the mechanical mosaic structure wrapped with lignin cannot be damaged by water, so the seedling trays can retain some of their strength after watering.

3.4. SEM Analysis of the Effect of Seedling Tray Degradation on Soil

Figure 10 shows the degradation of the biomass seedling tray and growth of the corn roots after 20 days of transplanting. The biomass seedling tray was partially degraded, and some roots entered the soil through the sidewall, although the root system was still restricted by the tray. After being in the soil for 40 days, the biomass seedling tray became loose and decomposed, and only the seedling tray residue with a diameter of less than 5 mm could be seen. The root system began to expand but was still partially restricted by the seedling tray.
During the harvest after transplanting, the root system was fully expanded, was thick and strong, and looked almost the same as that of seedlings planted in the field. The seedling tray had completely decomposed, which meant that the biomass seedling tray could restrict the growth of the root system in early stages, but that the impact on the overall growth of the root system was small. The seedling tray was degraded after 40 days in the soil and completely disappeared with the growth of the seedlings in the later period.
The degraded samples were scanned with SEM, as shown in Figure 11. The 23-N, 26-N, 23-H, and 26-H represent the regularly compressed seedling trays made of material with a moisture content of 23%, regularly compressed seedling trays made of material with a moisture content of 26%, hot-compressed seedling trays made of material with a moisture content of 23%, and hot-compressed seedling trays made of material with a moisture content of 26%, respectively. The fibrous substance in the pictures is the stem fiber, which is mainly composed of lignin and cellulose. Both 23-N and 26-N are SEM images of regularly compressed seedling trays. The stem fibers in the material were multilayered and mosaic.
After molding and drying, the straw gap in the trays with a moisture content of 23% was smaller than that in the tray with a moisture content of 26%. As shown in 23-H and 26-H, most of the gaps between stem fibers in 23-H were filled and adhered by the lignin separated out from the stem fiber, and most of the multilayered stem fibers with a mosaic structure were wrapped by lignin. For the material with a moisture content of 26%, after compression under a high temperature (26-H), the gaps between multilayered stem fibers were also filled and adhered by the lignin. However, not all of them were filled or adhered, and the multilayered stem fibers with a mosaic structure wrapped by lignin were fewer.
The gaps between the cellulose as shown in the SEM images were caused by the internal stress and moisture of the material. During the compression process, the internal stress still existed due to the uneven volume changes in the microstructure of the material. When the external load disappeared, gaps formed in the molded material. After being separated out from the material, lignin adhered to the fibers nearby to create a new structure, and most of the internal stress was offset by the adhesion stress of lignin after the external load disappeared. Therefore, both the expansion rate and the porosity of the molded hot-compressed seedling tray were smaller than those of the regular compression molding seedling tray. The water in the material also filled the gaps during the compression process but evaporated after drying to produce gaps again. Thus, the greater the amount of water, the larger the gap after drying. Therefore, the stem fiber gaps in 26-N and 26-H were larger than those in 23-N and 23-H.
The mechanical setting force of the material is produced due to the stacking and squeezing. Hence, the larger the material gap, the smaller the mechanical setting force. Therefore, in the universal testing machine test, the bending strength of the seedling tray made of material with a moisture content of 23% was greater than that of the seedling tray made of material with a moisture content of 26%.
The lignin flow in the material has two stages. In the first stage, after reaching the softening temperature, the lignin in the stem fiber flows and precipitates under internal pressure, filling the gaps between adjacent stem fibers. In this process, the higher the filling rate, the better the integrity of the seedling tray and the adhesion effect of lignin, and the more resilient the molded seedling tray. In the second stage, as the temperature increases, the fluidity of lignin increases, and the lignin in the gap overflows, covers and wraps more adjacent stem fibers, and creates a lignin layer outside the multilayered stem fibers with a mosaic structure.
Since water is more fluid than liquid lignin, after the biomass material is compressed to discharge internal air, the water in the material first fills adjacent stem fiber gaps, which affects the adhesion of lignin to stem fibers and the film-forming process. Therefore, high moisture content weakens the adhesion of lignin and the film-forming property while improving the fluidity of the material.
Figure 12 shows the SEM images of the four seedling trays, i.e., regularly compressed trays made of material with a moisture content of 23%, regularly compressed trays made of material with a moisture content of 26%, hot-compressed trays made of material with a moisture content of 23%, and hot-compressed trays made of material with a moisture content of 26%. In 23-N, the multilayered stem fibers with a mosaic structure, which amounted to less than one-sixth of the whole tray, were seriously damaged. In 26-N, the multilayered stem fibers with a mosaic structure were completely damaged, and the stem fibers were scattered and loose.
For the seedling trays made of material with a moisture content of 23%, the multilayered stem fibers with a mosaic structure (23-H) were clear, and only a small part were damaged. In 26-H, the multilayered stem fibers with a mosaic structure were seriously damaged by water, and a large area of stem fibers was fractured.
During the process of seedling raising, the water content of soil is high. After the regular compressed seedling tray contacts water, under the adhesive force and cohesive force of molecules, water slowly flows from inside to outside along the gaps between stem fibers in the tray. Lager gaps increase the infiltration velocity. Due to a small stem fiber gap, the infiltration process in the regularly compressed seedling tray made of material with a moisture content of 23% was slow. Therefore, at the end of the seedling raising process, the damage to the seedling tray was less serious than that in the tray made of material with a moisture content of 26%.
Cellulose expands after absorbing water, so the infiltration of water causes the cellulose in the seedling tray to expand and the mechanical setting force to be destroyed. When the seedling tray expands, stress intersections can be produced at the junction of the sidewall and the hole partition wall to cause fracture failure. For the hot-compressed seedling tray, most of the gaps between stem fibers were adhered by lignin, and lignin was not lost with water after being dried. The lignin filled in the gaps hindered the capillary infiltration of water. The multilayered stem fibers with a mosaic structure inside the hot-compressed seedling tray made of material with a moisture content of 23% were wrapped by a large amount of lignin to separate the cellulose from water so that cellulose did not expand, and the multilayered stem fibers with a mosaic structure were not damaged.
For the hot-compressed seedling tray made of material with a moisture content of 26%, a small area of multilayered stem fibers with a mosaic structure was wrapped. The lignin adhering between adjacent stem fibers could decelerate water infiltration but could not fully separate water from cellulose. After transplanting, the seedling tray was buried deep in soil for a long time, and the water retained in both internal and external soil infiltrated into the gaps that were not fully filled, which caused expansion and fracturing inside the stem fibers and weakened the seedling tray. These results support our speculation about the strength of the seedling tray above, i.e., the structure of the seedling tray can change under the support of soil over time.

3.5. Mechanism Analysis and Discussion

The compression molding mechanism of the biomass materials is summarized below. First, compressing the stem fibers in the material causes multilayered stem fibers with a mosaic structure to form, which produces mechanical setting force as the strength source of the seedling tray. Second, the mold is heated up so that the lignin in the material reaches its softening temperature. Under internal stress, the lignin is separated out and adheres to adjacent stem fibers to create a new structure. As the lignin becomes increasingly liquidized, the fluidity of lignin improves, lignin adheres more to its adjacent stem fibers, and a lignin layer forms outside the multilayered stem fibers with a mosaic structure. Third, when the material temperature is lower than the glass transition temperature of lignin, the seedling tray is molded. This study found that the seedling trays prepared with the proposed method are water resistant and can still meet the strength requirements for transplantation even after a cycle of corn seedling raising. Traditional straw return-to-field is restricted by both internal causes (its own structure) and external causes (environment). The results of this study are similar to those obtained in the literature [41,42,43]. However, the proposed method, i.e., compressing straw into seedling trays, is not restricted by the straw structure, but rather it restructures the straw to improve the transformation efficiency of the seedling tray and accelerate straw degradation. While realizing short-term seedling raising goals, the prepared tray can also change the straw structure to facilitate its degradation.
As one of the main corn producing areas in China, Heilongjiang Province has a short frost-free period, so the traditional direct seeding method often suffers from low temperature freezing damage in early spring at the seedling stage. The pot cultivation and transplanting technique of maize can effectively prevent cold damage from low temperature, avoid spring drought, and mature early to prevent autumn frost. Previous studies of our research group showed that pot cultivation transplanting technology could shorten the growth period of corn field and improve the quality and yield of corn. Therefore, using rice straw as the main raw material, and making full use of the involvement of lignin and cellulose to construct a non-adhesive added biomass seedling tray, can meet the needs of agricultural production during corn seedling cultivation and transplanting, can be completely degraded after soil, and provide certain nutrients in the process of corn seedling cultivation and field transplanting. This method can provide reference for making pot seedling trays of other crops in agricultural production.

4. Conclusions

(1)
Based on the glass transition properties of the lignin in the material mixed with straw, this study proposed a new method to prepare biomass seedling trays with a mixed material (straw and cattle manure), which can meet the needs of corn seedling raising and transplantation.
(2)
At the beginning of hot compression, the stem fibers in the material are compressed to produce a mechanical setting force. When the lignin in the material reaches the softening temperature, it flows under internal stress to fill and wrap the gap between fibers and the multilayered stem fibers with a mosaic structure. After cooling down, the seedling tray is molded, and the strength comes from the mechanical setting force of the multilayered stem fibers with a mosaic structure. The adhesion and wrapping of lignin can prevent and decrease the damage from water infiltration into the multilayered, mosaic-structured stem fibers but play a very small role in improving strength.
(3)
During the molding process, the water in the seedling tray prevents the lignin from filling the gaps between stem fibers and reduces the lignin’s wrapping and protection of the multilayered, mosaic-structured stem fibers. Therefore, when the material can completely fill the mold, a lower water content yields a better strength and water resistance of the seedling tray.
(4)
The proposed method, i.e., compressing straw into a seedling tray, is not restricted by the straw structure, and restructures the straw to improve the transformation efficiency of the seedling tray and accelerate degradation. The seedling tray can be mostly degraded after being buried in soil for 40 days and can be completely degraded after the seedling fully grows.

Author Contributions

Writing—Original Draft Preparation, L.Q.; Writing—Review and Editing, W.Z.; Formal Analysis, Y.M.; Software, B.Z.; Data Curation, L.Q.; Supervision, W.Z.; Investigation, L.Q. and B.Z.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by China Agriculture Research System of MOF and MARA (CARS-04-PS30), Natural Science Foundation of Heilongjiang Province (LH2019E073) and Technical Innovation Team of Cultivated Land Protection in North China (TDJH201808).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 13 00453 g001 550
Figure 1. Experimental site and materials (Loose state of mixed straw after treatment).
Figure 1. Experimental site and materials (Loose state of mixed straw after treatment).
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Figure 2. Compression mold for seedling trays.
Figure 2. Compression mold for seedling trays.
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Figure 3. Specific structure and overall size of seedling trays.
Figure 3. Specific structure and overall size of seedling trays.
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Figure 4. Failure analysis of three-point bending of a seedling tray.
Figure 4. Failure analysis of three-point bending of a seedling tray.
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Figure 5. Effect of transplanting in the field.
Figure 5. Effect of transplanting in the field.
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Figure 6. Regular compressed seedling trays. Note: The 17, 20, 23 and 26 represent regularly compressed seedling trays with a moisture content of 17%, 20%, 23%, and 26%, respectively. The “-1” after the numbers represent the top view under this water content, while “-2” represents the side view under this water content.
Figure 6. Regular compressed seedling trays. Note: The 17, 20, 23 and 26 represent regularly compressed seedling trays with a moisture content of 17%, 20%, 23%, and 26%, respectively. The “-1” after the numbers represent the top view under this water content, while “-2” represents the side view under this water content.
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Figure 7. Seedling tray compressed at a high temperature. Note: The 23 and 26 represent water content of 23% and 26%, H means the seedling trays were compressed at high temperature. The “-1” and “-2” represent top view and side view, respectively.
Figure 7. Seedling tray compressed at a high temperature. Note: The 23 and 26 represent water content of 23% and 26%, H means the seedling trays were compressed at high temperature. The “-1” and “-2” represent top view and side view, respectively.
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Figure 8. Results of seedling raising in regularly and hot-compressed seedling trays made of material with different moisture contents.
Figure 8. Results of seedling raising in regularly and hot-compressed seedling trays made of material with different moisture contents.
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Figure 9. Bending strength results of regularly and hot-compressed seedling trays made of material with moisture contents of 23% and 26%.
Figure 9. Bending strength results of regularly and hot-compressed seedling trays made of material with moisture contents of 23% and 26%.
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Figure 10. Transplanting effect of seedling tray.
Figure 10. Transplanting effect of seedling tray.
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Figure 11. SEM image of seedling tray. Note: 23-N and 26-N show the regularly compressed seedling trays made of material with a moisture content of 23% and 26%; 23-H and 26-H show the hot-compressed seedling trays made of material with a moisture content of 23% and 26%.
Figure 11. SEM image of seedling tray. Note: 23-N and 26-N show the regularly compressed seedling trays made of material with a moisture content of 23% and 26%; 23-H and 26-H show the hot-compressed seedling trays made of material with a moisture content of 23% and 26%.
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Figure 12. SEM images of seedling trays after seedling raising.
Figure 12. SEM images of seedling trays after seedling raising.
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Table
Table 1. Dry matter composition of materials.
Table 1. Dry matter composition of materials.
IndicatorContent
Lignin24.6%
Cellulose22.0%
Hemicellulose12.5%
Table
Table 2. Changes in sidewall size from before to after seedling raising.
Table 2. Changes in sidewall size from before to after seedling raising.
23-N26-N23-H26-H
Wall thickness before seedling raising4.3 ± 0.2 mm4.9 ±0.3 mm3.9± 0.1 mm4.2 ± 0.1 mm
Wall thickness after 15 d of seedling raising6.8 ± 0.3 mm9.1 ± 0.4 mm4.5 ± 0.2 mm5.4 ± 0.2 mm
Expansion rate57.4% ± 0.387.5% ± 3.316.7% ± 2.228.8% ± 4.2
Note: 23-N was a regularly compressed seedling tray made of material with a moisture content of 23%, and 26-N was made of material with a moisture content of 26%; 23-H was a hot-compressed seedling tray made of material with a moisture content of 23%, and 26-H was made of material with a moisture content of 26%.
Table
Table 3. Bending strength and elastic deformation of seedling trays.
Table 3. Bending strength and elastic deformation of seedling trays.
Moisture Content of MaterialsMolding MethodFailure LoadElastic Deformation
Before Seedling RaisingAfter Seedling RaisingBefore Seedling RaisingAfter Seedling Raising
23%Regular compression
Hot compression		101.6 N41.2 N2.7 mm2.7 mm
111.3 N3.5 mm
26%Regular compression
Hot compression		79.6 N20.1 N2.2 mm2.3 mm
82.6 N2.8 mm
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MDPI and ACS Style

Qi, L.; Zhang, B.; Ma, Y.; Zhang, W. Forming and Degradation Mechanism of Bowl Seedling Tray Based on Straw Lignin Conversion. Agronomy 2023, 13, 453. https://doi.org/10.3390/agronomy13020453

AMA Style

Qi L, Zhang B, Ma Y, Zhang W. Forming and Degradation Mechanism of Bowl Seedling Tray Based on Straw Lignin Conversion. Agronomy. 2023; 13(2):453. https://doi.org/10.3390/agronomy13020453

Chicago/Turabian Style

Qi, Liqiang, Bo Zhang, Yongcai Ma, and Wei Zhang. 2023. "Forming and Degradation Mechanism of Bowl Seedling Tray Based on Straw Lignin Conversion" Agronomy 13, no. 2: 453. https://doi.org/10.3390/agronomy13020453

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