The Quality of Seedbed and Seeding Under Four Tillage Modes

Abstract

Crop residue management and soil tillage (CRM and ST) are key steps in agricultural production. The effects of different CRM and ST modes on the quality of seedbed, seeding, and harvest yield are not well determined. In this study, the system of maize (Zea mays L.)–soybean (Glycine max (L.) Merr) rotation under ridge-tillage in the semi-arid regions of Northeast China was chosen as the study conditions. Four modes were investigated: deep tillage and seeding (DT and S), stubble field and no-tillage seeding (SF and NTS), three-axis rotary tillage and seeding (TART and S), and shallow rotary tillage and seeding (SRT and S). Results show that the DT and S mode produced the best quality of seedbed and seeding. Among the conservation tillage modes, the SRT and S mode produced the shortest average length of roots and straw, the best uniformity of their distribution in the seedbed, and the highest soybean yield. Both the SRT and S and SF and NTS modes yielded a higher net profit as their cost-effectiveness. When considering only the quality of seedbed and seeding under conservation tillage as a prerequisite, it can be concluded that the SRT and S mode is both advantageous and sustainable.

1. Introduction

Crop residue management and soil tillage are key steps in agricultural production and have been used to create favorable environments for crop growth since the inception of agriculture [1,2,3]. Currently, major crop residue management approaches include straw removal and straw returning [4]. Straw returning, where crop residues are incorporated into the soil of the field where they grew, is increasingly recognized as a cornerstone of sustainable agriculture due to its numerous benefits [5].

Extensive research demonstrates that straw returning significantly improves soil health. It enhances soil biological activity by increasing the abundance of earthworms and microorganisms, which in turn secrete substances that improve soil structure stability [6,7]. The decomposition process of straw also releases essential nutrients like nitrogen, phosphorus, and potassium back into the soil, directly enhancing its fertility [8,9]. Furthermore, incorporating straw promotes the accumulation of plant-derived carbon, contributing to long-term soil health and even the remediation of contaminated soil [10,11].

Currently, there are three main kinds of CRM and ST modes, which including the deep plowing (corresponding to DT and S mode in this study), mulch (corresponding to SF and NTS mode in this study), and rotary tillage mode (corresponding to TART and S and SRT and S modes in this study) [12,13,14]. Under the deep plowing mode, the moldboard plow is used for burying stubble and straw deeply into the soil, which improves seedbed quality and retains nutrients from residues in the soil. However, a large quantity of soil would be disturbed during this process, which results in the loss of soil moisture [15,16]. Under the mulch mode, the stubble and straw would be cut and spread on the soil surface, which would protect the soil moisture and decrease the operational costs [17]. However, the recovery of the spring soil temperature would be impeded in the high-latitude regions [18]. Additionally, the excessive coverage of the straw on the seedbed surface would make the terrain adaptability of the seeder decrease too [19]. Overall, despite the different approaches, a common challenge persists: the presence of straw on the surface or mixed within the soil often compromises the quality of the seedbed, which can negatively affect seeding accuracy and germination rates [20,21,22].

Scientifically evaluating the trade-off between the long-term benefits of soil improvement from different CRM and ST modes and the short-term impairments to the quality of seeding caused by residue cover or mixing is a topic of great research significance. Although this evaluation requires substantial supporting data, there is a significant lack of research specifically quantifying the impacts of different CRM and ST modes on the subsequent quality of seedbed and seeding. This knowledge gap is particularly acute for the specific agricultural system of maize (Zea mays L.)–soybean (Glycine max (L.) Merr) rotation under ridge-tillage in the semi-arid regions of Northeast China.

To address this, our study systematically evaluates four distinct CRM and ST modes: deep tillage and seeding (DT and S), stubble field and no-tillage seeding (SF and NTS), three-axis rotary tillage and seeding (TART and S), and shallow rotary tillage and seeding (SRT and S). By quantifying their effects on the quality of seedbed, seeding, and harvest yield, this research aims to provide a scientific reference for farmers in semi-arid regions to select optimal CRM and ST modes.

2. Materials and Methods

2.1. Site Description

In May 2023, a field test was conducted in Gannan County, Qiqihar City, Heilongjiang Province, China (47.91° N, 123.46° E). This region is characterized by a mid-temperate continental monsoon climate, with a mean annual temperature of approximately 3.5 °C and a frost-free period of 130 to 140 days. The mean annual precipitation is approximately 450 mm, occurring primarily during the summer months from June to August. In terms of thermal resources, Gannan County is situated in the second accumulated temperature zone, where the annual effective accumulated temperature (≥10 °C) ranges from 2500 to 2700 °C · d.

The predominant soils at the experimental site in Gannan County are Chernozem and Phaeozem, accounting for 54.87% and 18.82% of the area, respectively. The moisture contents of soil in the surface layer (0 mm–50 mm depth) and second layers (50 mm–150 mm depth) were 19.18% and 23.05%, respectively, which were measured through the oven drying method based on the “Method for the determination of soil water content” (GB 7172-87). An oven (DHG-9030, Bluepard Instruments Co., Ltd., Shanghai, China) and an analytical balance with the type of WT20003 (WANT Balance Instrument Co., Ltd., Changzhou, China) were used to dry the soil and weigh its mass. The hardness of soil in the two layers was 0.5 MPa and 0.58 MPa, which was measured by soil-hardness with the type of 41010 (STEPS GmbH, Pocking, Germany).

Two characteristics are included in the planting mode of the experimental site: the maize–soybean rotation and ridge-tillage. The maize was planted in the experimental site under the mode of single-row planting on ridges with a plant spacing of 25 cm and a ridge spacing of 65 cm last year, and a large quantity of straw was produced. In September 2022, the maize straw was broken and spread in the field by a combined harvester. The height of the stubble ranges from 8 cm to 44 cm. The quantity of straw per unit area is 1.1 kg·m⁻². The condition of the field surface before tillage is shown in Figure 1.

2.2. Equipment Description

The rotary plow with the type of JIANFENG 430 (Yucheng YINONG Machinery Manufacturing Co., Ltd., Yucheng, China) (Figure 2a) was used for carrying out the tillage operations of the DT and S mode. As shown in Figure 3a, the core structure of a rotary plow consists of a sharp plowshare (Figure 3a, ①) at the front and a curved moldboard (Figure 3a, ②) behind it. In operation, as a tractor pulls the plow forward, the plowshare first makes a horizontal cut to sever the soil, creating a continuous strip known as a furrow slice. This slice then slides upward along the moldboard’s unique helical surface and is completely inverted, thereby burying surface weeds and crop residue deep into the soil while simultaneously bringing the lower soil layers to the surface.

The no-tillage seeding machine, which was a DEBONT 1205 (Debont Corp, Harbin, China) (Figure 2b), was used for carrying out the operations of the SF and NTS mode. As shown in Figure 3b, the primary structure of a no-tillage seeder comprises a front-mounted residue management unit (Figure 3b, ⑦), a furrow opener (Figure 3b, ④), a seed metering (Figure 3b, ②), and a rear-mounted furrow-closing and press-wheel assembly (Figure 3b, ③). During operation, the residue management unit at the front of the machine first cuts through surface crop straw and stubble, clearing it to the sides. This creates a clean seeding zone within the residue cover. Subsequently, the furrow opener creates a narrow seed furrow in the untilled soil. The seeding system then deposits seeds at the bottom of this furrow. Finally, the furrow-closing and press wheel assembly covers the seeds.

The three-axis rotary tiller with the type of HUAYANG 1GZL 350 Ⅲ (Changtu Huayang Agricultural Machinery Repair and Manufacturing Co., Ltd., Changtu, China) (Figure 2c) was used for carrying out the operations of the TART and S mode. As shown in Figure 3c, the implement is equipped with three sequential blade shafts, each with distinct operational parameters and functions. The front blade shaft (Figure 3c, ①), which has the smallest rotation radius, rotates forward at a relatively high speed to pulverize root stubble. The second blade shaft (Figure 3c, ②) has the largest rotation radius and rotates in reverse at a relatively low speed to mix the pulverized straw with the soil. The third blade shaft (Figure 3c, ③), with a rotation radius intermediate to the other two, rotates forward at a high speed to level the soil surface.

The shallow rotary tiller designed by our team independently (Figure 2d) was used for carrying out the tillage operations of the SRT and S mode. The structure of the shallow rotary tiller is shown in Figure 3d. The machine is composed of three parts, namely the front, middle, and rear. Three sets of the vertical combined blades (Figure 3d, ①) are included in the front part for stubble-breaking, which are aligned with three ridges, respectively. Its working process is shown in Figure 4a. As the shallow rotary tiller advances, the stubble comes into contact with the moving blades and is dragged to the fixed blades along the direction of the knife edge. Then, it was fixed and cut by the moving and fixed blades.

To ensure the fixed blade and the moving blade can achieve stable cutting of the straw, a prerequisite is that they must grip the straw firmly without slippage. A force analysis of this gripping process is illustrated in Figure 4b. According to this analysis, both the fixed blade and the moving blade must satisfy the condition expressed in Equation (1).

$${\alpha }_1+{\alpha }_2<\lambda <f_1+f_2$$

(1)

where α₁, α₂ are the inclination angles of the cutting edges of the moving and fixed blades, respectively (°), λ is the critical angle for gripping the straw (°), and Φ₁, Φ₂ are the friction angles between the straw and the moving and fixed blades, respectively (°).

The middle part is a rotary tiller (Figure 3d, ②) with a tillage depth of 15 cm. A detailed description of this section is omitted for the sake of brevity, as its operating principle is identical to that of a conventional rotary tiller blade.

The rear part is a burying finger (Figure 3d, ③), which rotates in the forward direction. When the straw is being buried, the forces acting on it are shown in Figure 4c. Its working principle is to make the straw slide along the burying fingers for gradual burial. A force analysis, resulting in Equations (2) and (3), establishes the conditions required for this sliding motion. The analysis indicates that increasing the slide-cutting angle of the burying fingers reduces the force required for the straw to slide. Since a greater force is typically required to bury longer straw, the amount of straw buried into the soil versus that spread on the ground can be controlled by adjusting the slide-cutting angle of the burying finger in accordance with the length distribution of the chopped straw.

$$f_1\cos \phi +f_2\le F_{\mathrm{n}}\sin \phi$$

(2)

$$F\ge {\displaystyle \frac{{\mu }_{2}G}{(1-{\mu }_1{\mu }_2)\sin \phi -({\mu }_1+{\mu }_2)\cos \phi }}$$

(3)

where f₁ is the friction between burying finger and the stalk (N), f₂ is the friction between soil and the stalk (N), G is the gravity of the stalk (N), β is the slide-cutting angle of burying finger (°), φ is the angle between the tangent of burying finger and soil (°), F_n is the support force of soil on the stalk (N), F is the force of burying finger on the stalk (N), μ₁ is the coefficient of static friction between the stalk and burying finger, μ₂ is the coefficient of static friction between soil and burying finger.

To study the quality of seedbed and seeding, the ridge-forming machine and the seeding machine were also used in the DT and S, TART and S, and SRT and S modes. The main parameters of each tillage machine are shown in

Table 1. The working parameters of three types of tillage machines.

Working Parameters	Three Types of Tillage Machines
	The Rotary Plow	The Three-Axis Rotary Tiller	The Shallow Rotary Tiller
Traveling speed/m·s−1	0.5	0.5	0.5
Tillage depth/cm	35	20	15
Rotational speed/r·min−1	0	240	240

.

2.3. Experimental Design

Four experimental plots were set for the four CRM and ST modes, with each mode having 6 ridges. The length and width of each plot are 130 m and 3.9 m, respectively. A ridge was reserved between the adjacent plots to avoid the mutual interference between different modes. The operational procedures of the four CRM and ST modes during the test and the subsequent evaluation system are shown in Figure 5. It is noteworthy that the process of the ridges being created before seeding was not conducted in this study according to the no-tillage seeding technique requirements in Northeast China.

2.4. Indices

2.4.1. The Average Length of Roots and Straw in the Test Area

The proportion of roots and straw lengths in each category was calculated by Equations (4)–(6). The decay rate of roots and straw and the quality of seedbed are affected by the average length of roots and straw after being returned to the field. Usually, a slow decay rate would occur under the insufficiently broken roots and long straw [23]. Therefore, the roots and straw should be thoroughly broken, and their average length should be reduced as much as possible during the process of straw returning. During the test, after the ridges were formed, five rectangular plots, including one ridge of 2 m in length, were randomly chosen within each operational range. In each plot, all roots and straw were collected, and their length L_i were measured and averaged. The roots and straw were divided into three categories based on their length: L_i ≤ 5 cm, 5 < L_i ≤ 10 cm, and 10 cm < L_i.

$$P_1={\displaystyle \frac{N_{L_i\le 5}}{N}}\times 100\%$$

(4)

$$P_2={\displaystyle \frac{N_{5<L_i\le 10}}{N}}\times 100\%$$

(5)

$$P_3={\displaystyle \frac{N_{10<L_i}}{N}}\times 100\%$$

(6)

where P₁, P₂, P₃ are the proportion of roots and straw lengths in three categories, respectively (%), N_Li≤5, N_5<Li≤10, N_10<Li are the quantity of roots and straw in the three length categories, respectively, N is the total quantity of roots and straw in the plot.

2.4.2. The Weight of Roots and Straw in the Seedbed

In general, the lower the weight of roots and straw in the seedbed, the lower the possibility that seeds would be affected by the roots and straw during the process of seeding. However, this also means that more roots and straw would be buried deeply, and more soil would be disturbed during the process of tillage. It is not conducive to soil conservation. According to the relevant agronomic requirements, the depth of the soybean seeding in Heilongjiang Province is 3–4 cm, which would be increased by 1–2 cm in the dry years, as appropriate. Therefore, the top 5 cm of the soil on the ridge is defined as the seedbed. The shapes and dimensions of the ridge and furrow are shown in Figure 6. The division method of the plot is the same as that of the previous index. In each plot, all roots and straw in the seedbed were collected and weighed. The weight of roots and straw in the seedbed of the five plots was averaged.

2.4.3. The Uniformity of Roots and Straw Distribution in the Seedbed

The uniformity of roots and straw distribution in the seedbed was calculated using Equation (7). The spatial distribution of the roots and straw would be affected by the CRM and ST modes. A worse uniformity of roots and straw distribution in the seedbed means that the roots and straw are gathered together in the soil, and their decay rate would be reduced. Therefore, the uniformity should be improved as much as possible during the CRM and ST process. During the test, after the ridges were formed, five rectangular plots, including three ridges of 2 m in length, were randomly chosen within each operational range. Each ridge in each plot was divided into 5 areas along the longitudinal direction and 3 areas along the latitudinal direction according to the ridge. As a result, each plot was divided into 15 areas separately. All roots and straw in every area were collected and weighed. The coefficient of variation of the weight of roots and straw within each rectangular plot is calculated to characterize the uniformity of roots and straw distribution in the seedbed. The larger the coefficient of variation, the worse the uniformity of roots and straw distribution.

$$C{V}_{\mathrm{straw}}={\displaystyle \frac{\sqrt{\frac{{\displaystyle \sum _{i=1}^{25}{(x_i-\overline{x})}^2}}{n_{\mathrm{area}}}}}{\overline{x}}}$$

(7)

where CV_straw is the coefficient of variation of the weight of roots and straw, x_i is the weight of roots and straw in the area i (g), $\overline{x}$ is the average weight of roots and straw (g), n_area is the number of areas. Here, n_area = 15.

2.4.4. The Standard Deviation of Lateral Displacement of Seed

The standard deviation of lateral displacement of seed was calculated using Equation (8). After the straw is returned to the field, part of the roots and straw would be mixed and buried in the seedbed, which makes the environment of the seedbed complicated. The seeds would bounce back to the edge of the ridge or even fall into the furrow ridge as they collided with the roots or straw in the seedbed during the process of seeding. This could cause seeds to be missed and the crop yields to decrease. To measure the effects of the CRM and ST modes on the quality of seeding, five rectangular plots, including one ridge of 20 m in length, were randomly chosen within each operational range after the ridges were formed. To observe the status of the seeds in the seedbed conveniently, the press roller in the seeding machine that corresponded to the selected ridge was dismantled. One hundred seeds were chosen in each plot, and the standard deviation of lateral displacement of seed was measured based on the midline of the seed-furrow.

$$S{D}_{\mathrm{seed}}=\sqrt{{\displaystyle \frac{{\displaystyle \sum _{i=1}^{100}{(y_i-\overline{y})}^2}}{n_{\mathrm{seed}}}}}$$

(8)

where SD_seed is the standard deviation of lateral displacement of seed (mm), y_i is the lateral displacement of seed i (mm), $\overline{y}$ is the average lateral displacement variation of seeds (mm), n_seed is the quantity of seeds chosen. Here, n_seed = 100.

2.4.5. The Seed Germination Rate

The division method of the plot is the same as that of the previous index. After 18 days of seeding, the seed germination rate could be calculated by the following equation.

$$M={\displaystyle \frac{n_1}{n_2}}\times 100\%$$

(9)

where M is the seed germination rate (%), n₁ is the quantity of the soybean seedlings, n₂ is the quantity of the seeds sown, here n₂ = 133.

It is worth noting that there are variations in the size of the rectangular plots during the measurement of the five indices. When measuring the uniformity of roots and straw distribution in the seedbed, both the horizontal and vertical directions need to be considered simultaneously. When measuring the standard deviation of lateral displacement of seed and its germination rate, a sufficient quantity of the seeds needs to be obtained through a longer ridge.

2.4.6. The Harvest Yield

During the soybean maturity stage, the quantity of soybean plants per unit area, the average number of pods per plant, and the average number of grains per pod were measured. The quantity of soybean plants per hectare and the number of grains per plant were calculated. The mass of one hundred soybean grains was weighed multiple times to obtain an average 100-grain mass. Finally, the soybean yield under the different CRM and ST modes was calculated.

2.5. Data Analysis

The analysis of variance (ANOVA) was performed by using the statistical software (Design-Expert 8.0.6). Its core principle involves partitioning the total variation in a dataset into two components: between-group variation, which is attributable to the different groups or experimental treatments, and within-group variation, which results from random fluctuation within each group. The process evaluates this difference by calculating the F-statistic, defined as the ratio of the between-group variation to the within-group variation. If the between-group variation is substantially larger than the within-group variation, the resulting F-statistic will be large, which in turn yields a small p-value. When this p-value falls below a predetermined significance level, the null hypothesis (that all group means are equal) is rejected, leading to the conclusion that a statistically significant difference exists among the group means. The significance of the effects of different CRM and ST modes on each index was analyzed under the confidence coefficient of 95%.

3. Results

3.1. Effects of Different CRM and ST Modes on the Quality of Seedbed

In this study, we used three indicators (the average length of roots and straw in the test area, the weight of roots and straw in the seedbed, and the uniformity of roots and straw distribution in the seedbed) to evaluate the quality of the seedbed.

3.1.1. The Results of the Average Length of Roots and Straw in the Test Area

A significant effect of CRM and ST modes on the average length of roots and straw was observed (p < 0.001). As shown in Figure 7, the SF and NTS mode resulted in the longest average residue length (27.8 ± 3.4 cm). The average length under the DT and S mode was 24.6 ± 3.7 cm. The TART and S and SRT and S modes produced significantly shorter average lengths of 6.9 ± 2 cm and 5.32 ± 1.6 cm, respectively. The SRT and S mode also resulted in the highest proportion of residues shorter than 5 cm (53.8%), while the DT and S mode had the highest proportion longer than 10 cm (84.6%). Similar results are also evident from the surface conditions after tillage.

3.1.2. The Results of the Weight of Roots and Straw in the Seedbed

A significant effect on the weight of roots and straw in the seedbed was observed under the different CRM and ST modes (p < 0.001) (Figure 8). Under the DT and S mode, the weight of roots and straw was minimal. The SF and NTS mode resulted in the greatest weight. Under the TART and S and SRT and S modes, the weights were intermediate, with the SRT and S mode showing a lower weight compared with the TART and S mode.

3.1.3. The Results of the Uniformity of Roots and Straw Distribution in the Seedbed

The uniformity of roots and straw distribution in the seedbed was observed to be significantly different under the three conservation tillage modes (p < 0.001) (Figure 9 and Figure 10). Under the SF and NTS mode, the roots were concentrated in the position of the previous crop, resulting in poor uniformity. The SRT and S mode achieved the lowest coefficient of variation, indicating the most even distribution.

3.2. Effects of Different CRM and ST Modes on the Quality of Seeding

In this study, we used two indicators (the standard deviation of lateral displacement of seed and the seed germination rate) to evaluate the quality of seeding.

Significant effects of CRM and ST modes on the standard deviation of lateral displacement of seeds and the seed germination rate were observed (p < 0.001). The standard deviation can be sorted in descending order as follows: SF and NTS, TART and S, SRT and S, and DT and S (Figure 11a). The seed germination rate, sorted in descending order, was DT and S, SRT and S, TART and S, and SF and NTS (Figure 11b). Combining the indicators of the quality of seedbed, under the conditions of conservation tillage, the seed germination rate shows a negative correlation with the standard deviation of lateral displacement of seed, the average length of roots and straw, the weight of roots and straw in the seedbed, and the coefficient of variation of the weight of roots and straw in the seedbed (Figure 7, Figure 8, Figure 10 and Figure 11a,b).

3.3. Effects of Different CRM and ST Modes on the Harvest Yield

The final harvest yield is the ultimate indicator of the effectiveness of a CRM and ST mode. As shown in

Table 2. Parameter values of soybean yield were affected by different CRM and ST modes.

CRM and ST Modes	Grains Per Plant (Grains/Plant)	100-Grain Weight (g)	Plant Density (Plants·hm−2)	Yield (kg·hm−2)
DT and S	64.17 ± 16.78	19.24 ± 0.35	275,400 ± 578	3387.83
SF and NTS	54.83 ± 14.47	21.87 ± 0.51	262,100 ± 1356	3142.93
TART and S	59.83 ± 8.90	19.535 ± 0.29	269,300 ± 914	3147.52
SRT and S	61.33 ± 8.06	20.655 ± 0.69	271,100 ± 509	3434.22

, the SRT and S mode achieved the highest soybean yield (3434.22 kg/hm²), followed by DT and S (3387.83 kg·hm⁻²). The SF and NTS and TART and S modes produced the lowest soybean yields.

4. Discussions

This study demonstrates that in the system of maize–soybean rotation under ridge-tillage in the semi-arid regions of Northeast China, the choice of the Crop residue management and soil tillage (CRM and ST) mode is a critical factor determining the quality of both the seedbed and subsequent seeding. Our findings reveal a fundamental trade-off between achieving a low-residue seedbed for optimal seeding and adhering to the principles of conservation agriculture.

4.1. CRM and ST Modes Fundamentally Alter the Quality of Seedbed

Different CRM and ST modes created drastically different seedbed environments by altering the physical characteristics and spatial distribution patterns of roots and straw. The quality of the seedbed is mainly reflected in three key indicators: the weight, average length, and uniformity of distribution of roots and straw in the seedbed.

As a representative conventional tillage mode, the DT and S mode, through the rotary plow, achieved near-total burial of roots and straw, thus creating the cleanest seedbed with the lowest weight of roots and straw in it (Figure 8). This is because the rotary plow can completely invert the soil profile, burying surface residue such as roots and straw into the deep soil layers. According to a study by Xiao et al., a rotary plow can typically bury crop residue to a depth of 30 to 35 cm below the soil surface [24], and this depth far exceeds the seeding depth of major food crops. However, according to research by Tian et al. [25], in the cold region of Northeast China, burying straw too deeply inhibits the release of nutrients during the straw decomposition process. Specifically, when the straw returning depth was increased from 10 cm to 50 cm, carbon release in the first year decreased by 11%, while nitrogen release decreased by 25.9% [25]. At the same time, the advantages gained in improving seedbed quality may be weakened by the potential for soil erosion, runoff, and organic matter decline [26,27] caused by long-term deep tillage.

In contrast, all conservation tillage modes retained residue within the seedbed or on the surface. Among these, the SF and NTS mode, as a “minimal intervention” method where the residue remained untreated before seeding, resulted in the longest average length (Figure 7), the greatest weight in the seedbed (Figure 8), and, due to the concentration of residue in the original crop row positions, an extremely uneven distribution (Figure 9). Both the TART and S and SRT and S modes significantly reduced the average length of the residue. Of these, the SRT and S mode demonstrated the most superior performance; it not only produced the shortest average roots and straw (Figure 7) but also achieved the best uniformity of distribution (Figure 9 and Figure 10). This superior fragmentation and mixing effect can be attributed to its unique “supported cutting” mechanical design, which ensures a more effective and consistent chopping action compared with the three-axis rotary tiller that lacks this feature. This conclusion is consistent with the findings of previous studies. For example, Feng et al. found that supported cutting is highly effective at shredding stubble remaining in the soil, as it can achieve a qualified crushing rate of over 90% for root stubble with a low power consumption [28], while Hu et al. also confirmed that supported cutting can crush scattered straw with low power consumption [29].

On the other hand, a shorter average residue length and a more uniform distribution also mean that the pulverized roots and straw can be more thoroughly mixed with the soil. In a study conducted by Han et al., it was found that the degree of mixing between straw and soil is the most dominant factor affecting the decomposition rate of straw; through thorough mixing with the soil, the average decomposition rate of straw within 17 months increased by 60–160% [30]. Similar conclusions were also drawn in another study by Xi et al. [31]. This indicates that the SRT and S mode has the potential to bring long-term benefits to the soil.

4.2. Seedbed Conditions Directly Govern the Quality of Seeding

The quality of the seedbed had a direct and predictable impact on seeding performance. A clear, inverse relationship was observed in this study: as the amount and length of residue in the seedbed decreased and its distribution became more uniform, seeding accuracy (measured by the standard deviation of lateral displacement of seed) improved accordingly.

The DT and S mode, with its clean seedbed, provided a nearly ideal, obstruction-free condition for the seeder, thereby achieving the highest seeding accuracy (Figure 11a). Conversely, the large quantity of long, clumped residue in the SF and NTS mode constituted a severe physical barrier. We attribute this to seeds colliding with and bouncing off the rigid and irregularly shaped roots and straw during their descent (Figure 12), which is the main reason for inconsistent trajectories, large lateral displacement, and poor seeding quality. This hypothesis is consistent with the design philosophy of no-tillage seeders equipped with active residue-clearing devices. As shown in the study by Hou et al., temporarily clearing surface straw with a lateral residue-clearing device before no-tillage seeding can increase the qualified index of seed spacing by more than 30% [32]. Similarly, a 2025 review by Ma et al. also concluded that, “Crop stubble represents a significant constraint on seeding operations in straw-mulched fields, adversely affecting seed spacing accuracy, optimal depth placement, and overall seeding performance” [33].

Between the TART and S and SRT and S modes, the differences in the seedbed also translated into different seeding qualities. For the reasons mentioned above, the longer and unevenly distributed residue in the TART and S mode caused a greater lateral displacement of the seed compared with the shorter, uniformly distributed residue in the SRT and S mode. Therefore, the less disruptive seedbed environment under the SRT and S mode resulted in a seeding accuracy significantly superior to the other two conservation tillage modes. This again emphasizes that within conservation systems, managing the physical form of the residue is as important as managing its quantity.

4.3. Integrated Effects on Seed Germination and Harvest Yield

The seed germination rate served as a final, comprehensive indicator of overall operational quality, closely mirroring the trends in seeding accuracy. The highest germination in DT and S was a direct result of superior seed placement and optimal seed-soil contact in a residue-free environment. In the study by Fallahi et al. [34], the quality of the seeding operation was improved after clearing surface residue with specialized attachments. Concurrently, the seed germination rate also increased 16–23%, and its trend was consistent with the improvement in seeding quality [34]. This finding is highly consistent with the results of our study.

The differences in harvest yield can be explained by the interplay of yield components. The number of plants per hectare showed significant variation (p < 0.001), with the SF and NTS mode having the lowest plant density. This directly corresponds to its lower seed germination rate (Figure 11b). The other three modes maintained a high plant density with small discrepancies among them. Interestingly, while having the lowest plant density, the SF and NTS mode had the highest 100-grain weight. This is a typical example of yield component compensation, where fewer plants face less competition, leading to plumper individual seeds. This result is consistent with the finding of Liang et al. (2025) that appropriately increasing crop planting spacing enhances yield [35]. Conversely, the SRT and S mode struck an optimal balance. It maintained a high plant density, similar to DT and S and TART and S, but also achieved a significantly higher 100-grain weight than they did. This suggests that the superior seedbed quality under the SRT and S mode not only ensured good germination but also promoted better nutrient and water uptake during the seed-filling stage.

4.4. Comprehensive Analysis: Cost–Benefit Analysis

The cost–benefit analysis of the four SRT and S modes is detailed in

Table 3. The cost–benefit between different CRM and ST modes.

CRM and ST Modes	Cost (CNY·hm−2)	Revenue (CNY·hm−2)	Net Profit (CNY·hm−2)
DT and S	1500 + 600	8130	6030
SF and NTS	1050	7543	6493
TART and S	1200 + 600	7554	5754
SRT and S	1200 + 600	8242	6442

. Costs were estimated based on the prevailing practice in Northeast China, where farmers rent machinery from agricultural cooperatives at an all-inclusive price that covers rental, fuel, and labor. Based on 2023 statistics, the per-hectare rental prices ranged from CNY 1500–2250 for a rotary plow, CNY 1050–1800 for a no-tillage seeding machine, CNY 1200–1800 for a three-axle rotary tiller, and CNY 600–900 for a conventional seeder, with fluctuations mainly due to varying field conditions. For consistency, the lower end of each price range was used for all calculations. Since the SRT and S mode is not yet in widespread use, its cost was provisionally benchmarked against the three-axle rotary tiller. Revenue was determined by soybean yield and the national purchase price of CNY 2.4/kg.

The analysis reveals that the SF and NTS and SRT and S modes yielded the highest net profits, significantly outperforming the other two modes. While the SF and NTS mode was marginally more profitable, the SRT and S mode produced a greater soybean yield, and its potential contribution to national food security should not be ignored. Crucially, this analysis excludes the costs of pesticides and fertilizers. A comprehensive discussion of these expenses awaits long-term trials comparing the impacts of each mode on soil physicochemical properties, microbial communities, and the incidence of pests and diseases.

5. Conclusions

(1)

Conventional tillage (DT and S) achieved the highest seed placement accuracy and germination rate but yielded a lower net profit due to its high operational costs.

(2)

Among the conservation tillage modes, Shallow Rotary Tillage (SRT and S) created superior seedbed conditions, leading to the highest soybean yield and a high net profit, demonstrating the best overall performance.

(3)

No-tillage (SF and NTS) was highly profitable because of its low operational cost, but this was achieved at the expense of poor seeding quality and reduced crop yield.

(4)

A fundamental trade-off exists between the agronomic benefits of conventional tillage and the cost-effectiveness of conservation agriculture. Within conservation systems, the SRT and S mode provides the most advantageous and sustainable solution by effectively balancing these conflicting demands.