Fine-Crush Straw Returning Enhances Dry Matter Accumulation Rate of Maize Seedlings in Northeast China

: In the conventional straw returning operation, the maize straw is broken into pieces of about 10 cm and degraded naturally in the farmland. Under the conventional straw returning mode, maize straw does not easily decompose quickly in cold climate conditions, which can cause a signiﬁcant decrease in the dry matter accumulation rate of next maize seedlings. Therefore, it is difﬁcult to popularize conventional straw returning in the maize-growing regions of Northeast China. In order to solve the above-mentioned problems, a new agronomic technology of straw returning is proposed in this study, and a corresponding Bionic Straw Fine Crusher is developed to match the agronomic requirements. The key function of ﬁne-crush straw returning is to signiﬁcantly increase the contact area between the straw pith and the external environment by signiﬁcantly shortening the crushing length, thus accelerating the rate of straw decomposition. In this study, the differences in operational effects between ﬁne-crush straw returning, conventional straw returning, and no returning are clariﬁed through 6 consecutive years of ﬁeld experiments. At the same time, statistical analysis of the experimental data reveals the inﬂuence of ﬁne-crush straw returning on the dry matter accumulation of maize at the seedling stage under the conditions of different returning modes, and determines the optimal agronomic parameter combination. The results of this study show that ﬁne-crush straw returning signiﬁcantly increased the decomposition rate, soil organic matter content, and soil accumulated temperature, thus creating a seedbed more favorable for maize seedling development. The experimental results showed that the optimal crushing length values of ﬁne-crush straw returning were 1.5 cm, 3 cm, and 1.5 cm under mulching returning, shallow burial returning, and deep tillage returning conditions, respectively. Compared with conventional straw returning and no returning operations, the ﬁne-crush straw returning operation can increase the maximum seedling dry matter accumulation of the maize crop by 5.1 g/plant and 2.8 g/plant (shallow burial), 4.2 g/plant and 1.8 g/plant (deep tillage), and 4.3 g/plant and 1.9 g/plant (mulching returning). The ﬁndings of this study may provide a viable new agronomic technology to accelerate the spread of straw returning in maize-growing areas of Northeast China.


Introduction
Although it is one of the most effective technologies for sustainable agricultural development, straw returning has never exceeded 10% of the area under maize cultivation in Northeast China [1,2]. Under conventional straw returning technology, the maize straw is broken into pieces 10 cm in length, and the pieces are completely retained in the field environment, thus inhibiting soil erosion [3,4]. Northeast China's maize-growing region is one of the only four remaining black-soil areas in the world, and its black-soil resources are formed under unusually harsh conditions, with a formation rate of 1 cm/400 years [5,6].
Currently, the black-soil layer in the area is being lost at a rate of 1 cm/year due to an inability to promote straw returning on a large scale [7]. Therefore, accelerating the promotion of straw returning in the region could have significant ecological benefits [8].
The main reason why conventional straw returning is difficult to promote in the maize-growing regions of Northeast China is that it significantly reduces the rate of drymatter accumulation at the maize seedling stage [9]. Dry-matter accumulation in maize seedlings is positively correlated with maize yield [10]; therefore, agricultural producers in the region have difficulty accepting the slow development of maize seedlings [11]. The rate of dry-matter accumulation in maize seedlings depends mainly on the pre-sowing soil organic-matter content and the soil temperature accumulated during the seedling period [12]. The cold climate in the maize-growing region of Northeast China makes it extremely difficult to decompose the large amount of straw fragments that remain in the farmland after conventional straw returning [13]. Yan et al. showed that a large amount of undecomposed straw fragments can cause slow soil warming, making it difficult for the soil accumulated temperature to meet the plant's developmental needs at the maize seedling stage [14]. Liu et al. showed that the decomposition rate of conventional straw returning was extremely low in Northeast China, thus making it difficult to rapidly increase soil organic-matter content in the short term [15].
In this paper, we propose a new agronomic technique that can accelerate the rate of straw decomposition, which we have named fine-crush straw returning, in order to reduce the negative impact of straw returning on dry-matter accumulation at the maize seedling stage. Maize straw consists of the epidermis and pith, and the pith is enclosed in the epidermis [16,17]. Straw returning can break the whole straw into a large number of pieces so that the pith leaks out from the ends of the pieces [18]. The core of straw fine crushing is to significantly increase the number of straw fragments by significantly reducing the crushing length (to less than 6 cm), which in turn increases the contact area between the pith and the external environment. Zhang et al. showed that straw relies mainly on the pith to absorb water with catabolic enzymes, etc. [19]. Stemmer et al. showed that the higher the rate of absorption of water, fungi, and enzymes by straw, the faster its decomposition rate is [20]. Therefore, it is highly likely that fine-crush straw returning will improve straw decomposition rates in the maize-growing region of Northeast China, which in turn will improve the dry matter accumulation of maize seedlings in this region.
The cutting resistance of a conventional straw returning machine is large [17], and it is always difficult to reach a minimum crushing length of under 6 cm [21]; therefore, it cannot realize fine-crush straw returning. Bionics engineering is a design method that can replicate superior biological functions, and it has been widely used in the field of agricultural engineering [22]. In recent years, locust mouthparts were found to be extremely efficient at cutting maize [23]. Zhao et al. found that the cutting resistance of locust mouthparts was only 23% that of the conventional straw-crushing tool, with a cutting efficiency more than 10 times higher [17]. In this study, we developed a new bionic straw fine crusher with a minimum crushing length of 1.5 cm by an engineering bionic design using locust mouthparts as the bionic prototype.
In summary, this paper proposes a way to reduce the negative impact of straw returning on maize seedling development in Northeast China through a combination of new agronomic techniques and new agricultural machinery. Through 6 consecutive years of field experiments, we studied the differences in operational effects between fine-crush straw returning, conventional straw returning, and no returning. Through statistical analysis of the experimental data, we reveal the influence of fine-crush straw returning on dry-matter accumulation at the maize seedling stage under different return mode conditions, and we determine the optimal fragmentation length value. The findings of this study can provide a new agronomic technology and its supporting implements to promote straw returning in the maize-growing regions of Northeast China and to guarantee sustainable agricultural development in the region.

Fine-Crush Straw Returning and Bionic Straw Fine Crusher
The fine-crush straw returning operation is composed of two parts: the fine crushing operation and the straw returning operation. The bionic straw fine crusher is mainly composed of a bionic straw fine-crushing mechanism and a tillage part hook-up mechanism.
The straw fine-crushing operation relies on the bionic straw fine-crushing mechanism. As shown in Figure 1a, the excellent cutting function of the locust mouthpiece is derived from its unique segmented, serrated structure with symmetrical movement [17][18][19][20][21][22][23][24]. As shown in Figure 1b-d, the bionic straw fine crusher can reduce the crushing length to 1.5 cm by restoring the structure and movement of locust mouthparts, and the effective control accuracy of crushing length can reach 1.5 cm [25,26]. The correspondence between the straw crushing length and the operating parameters of the bionic straw fine-crushing mechanism is shown in Table 1.

Fine-Crush Straw Returning and Bionic Straw Fine Crusher
The fine-crush straw returning operation is composed of two parts: the fine crushing operation and the straw returning operation. The bionic straw fine crusher is mainly composed of a bionic straw fine-crushing mechanism and a tillage part hook-up mechanism.
The straw fine-crushing operation relies on the bionic straw fine-crushing mechanism. As shown in Figure 1a, the excellent cutting function of the locust mouthpiece is derived from its unique segmented, serrated structure with symmetrical movement [17][18][19][20][21][22][23][24]. As shown in Figure 1b-d, the bionic straw fine crusher can reduce the crushing length to 1.5 cm by restoring the structure and movement of locust mouthparts, and the effective control accuracy of crushing length can reach 1.5 cm [25,26]. The correspondence between the straw crushing length and the operating parameters of the bionic straw fine-crushing mechanism is shown in Table 1. . bionic fine-crush returning blade design principle, (c). drive system operating principle of the bionic fine-crush returning blade design, (d). difference in operational effect between the bionic straw fine crushing mechanism and the conventional straw returning mechanism, (e). bionic straw fine crusher for mulching returning operation, (f). bionic straw fine crusher for shallow burial returning operation, and (g). bionic straw fine crusher for deep tillage returning operation. The straw returning operation relies on the tillage part hook-up mechanism. As shown in Figure 1e, when choosing not to carry the tillage component, the mulching finecrush straw returning operation can be achieved. As shown in Figure 1f, when the shallow Figure 1. Operating principle of fine-crush straw returning and bionic straw fine crusher. (a). Structure and movement of locust mouthparts, (b). bionic fine-crush returning blade design principle, (c). drive system operating principle of the bionic fine-crush returning blade design, (d). difference in operational effect between the bionic straw fine crushing mechanism and the conventional straw returning mechanism, (e). bionic straw fine crusher for mulching returning operation, (f). bionic straw fine crusher for shallow burial returning operation, and (g). bionic straw fine crusher for deep tillage returning operation. The bionic straw fine crusher has passed the quality inspection for agricultural machinery in the People's Republic of China. The data in the table are all derived from the actual data measured during the inspection by third-party inspection agencies, which can be found in the inspection report uploaded in the supporting materials.
The straw returning operation relies on the tillage part hook-up mechanism. As shown in Figure 1e, when choosing not to carry the tillage component, the mulching fine-crush straw returning operation can be achieved. As shown in Figure 1f, when the shallow burial harrow is selected, the shallow burial fine-crush straw returning operation can be realized. As shown in Figure 1g, when the deep tillage plow is selected, the deep tillage fine-crush straw returning operation can be realized.

Experimental Site
In this study, climatic conditions and soil environmental parameters were used as the selection criteria for the experiment site. The parameters at the experiment site should not be significantly different from the average level in the maize-growing region of Northeast China. As shown in Figure 2b, the maize-growing region of Northeast China is mainly concentrated within the black zone, which is known as the world's golden maize belt [27]. The final choice of experimental site was Wolong Village, Shuangyang District, Changchun City, Jilin Province (125.82 E, 43.72 N). As shown in Table 2, the values of all parameters at the experiment site for the past 20 years were not significantly different from the mean values for the black zone in Figure 2 (p > 0.05). Before the experiment, the experiment site had been planted with maize for 20 consecutive years (1995-2014) and the straw had never been returned to the field. burial harrow is selected, the shallow burial fine-crush straw returning operation can be realized. As shown in Figure 1g, when the deep tillage plow is selected, the deep tillage fine-crush straw returning operation can be realized.

Experimental Site
In this study, climatic conditions and soil environmental parameters were used as the selection criteria for the experiment site. The parameters at the experiment site should not be significantly different from the average level in the maize-growing region of Northeast China. As shown in Figure 2b, the maize-growing region of Northeast China is mainly concentrated within the black zone, which is known as the world's golden maize belt [27]. The final choice of experimental site was Wolong Village, Shuangyang District, Changchun City, Jilin Province (125.82 E, 43.72 N). As shown in Table 2, the values of all parameters at the experiment site for the past 20 years were not significantly different from the mean values for the black zone in Figure 2 (p > 0.05). Before the experiment, the experiment site had been planted with maize for 20 consecutive years (1995-2014) and the straw had never been returned to the field.

Experimental Method
The variety selected for the field experiment was Xianyu 335. This type of seed is the most commonly used variety in the region [31]. The specific parameters of this variety of maize straw are shown in Table 3. The amount of straw returning in the field experiment was 75,000 straw plants/hm 2 of cultivated land. As shown in Figure 2b, four crushing lengths of 1.5 cm, 3 cm, 4.5 cm, and 6 cm were set for fine-straw returning, as well as three

Experimental Method
The variety selected for the field experiment was Xianyu 335. This type of seed is the most commonly used variety in the region [31]. The specific parameters of this  Table 3. The amount of straw returning in the field experiment was 75,000 straw plants/hm 2 of cultivated land. As shown in Figure 2b, four crushing lengths of 1.5 cm, 3 cm, 4.5 cm, and 6 cm were set for fine-straw returning, as well as three returning modes of mulching, shallow burial, and deep tillage, for a total of 12 experiment combinations. The crushing length of the conventional straw returning was set at 10 cm, and three returning modes of mulching, shallow burial, and deep tillage were set. The no-straw returning operation was set up with three tillage patterns: no tillage, shallow burial, and deep tillage. Before the field experiment, the experimental site was divided into 18 experimental areas, and one experimental combination was used for each experimental area. To reduce the effect of climate change in different years on trial results, field experiments were conducted using 6 consecutive years of trials at the same location (October 2014-June 2020), with each planting year corresponding to one replicate trial. The strawreturning operation was conducted after each year's harvest operation (mid-to late October), and the experimental indicators were tested the following year before planting (early May) and throughout the maize seedling stage (early May to mid-June, approximately 40-45 days) [31].
Numerous studies have shown that long-term straw-returning operations can reduce the amount of nitrogen in the soil [32]. Therefore, the farmland was supplemented with N fertilizer [33] during the field experiment, and a ternary compound fertilizer of N, P, and K was applied as a base fertilizer (15-15-15, Hubei Qiansui Agricultural Materials Co., Ltd., Wuhan, China) at a rate of 120 kg/hm 2 each year before sowing. Additional fertilizer was applied once a year in June, with urea (Jinan Chuangtong Chemical Co., Ltd., Jinan, China) at a rate of 60 kg/hm 2 . All of the above fertilizer types are the most common varieties in the region [34], and numerous studies have shown that the above fertilization methods are effective in reducing the negative effects of declining nitrogen content on subsequent crops [35].
During the field experiment period, straw fine crushing was achieved using the Bionic Straw Fine Crusher. The conventional straw returning to the field was realized using the SR-100 straw-returning machine from Liaocheng Shengrong Machinery Co. (Liaocheng, China). The no-tillage operation was realized using the 9YQ-1.8A straw baler from Shandong Weizhi Machinery Co. (Jinan, China). The mulching returning mode with no tillage does not require any tillage parts. The shallow-burial returning mode relies on the 1BQ-4 shallow burial harrow from Shandong Dangkang Agricultural Equipment Co., Ltd. (Weifang, China). with a tillage depth of 20 cm. The deep tillage returning mode relies on the 1L-335 spade plow from Yucheng Huapu Machinery Co., Ltd. (Yucheng, China) with a tillage depth of 40 cm. All of the above tillage components are common models in the area [26].

Experimental Index, Experimental Methods, and Main Experimental Instruments
The main factor limiting the spread of straw returning in maize-growing regions of Northeast China is the reduction in the dry-matter accumulation rate that it causes at the maize seedling stage [9]. Therefore, the dry matter accumulation of maize at the seedling stage was selected as the core evaluation index in the field experiment. Meanwhile, three indicators, namely, the straw decomposition rate before sowing, soil organic matter content before sowing, and soil accumulated temperature at the maize seedling stage, may have significant effects on the dry-matter accumulation rate at the maize seedling stage [12][13][14][15][16]. Therefore, the above three indicators were selected simultaneously as auxiliary analysis indicators.

Straw Decomposition Rate (SDR) before Sowing (0-20 cm)
SDR was measured in the field experiment using the nylon bag method [36]. Each year, after the straw-returning treatment, a certain amount of chopped straw was selected for drying in each experimental area individually. The dried straw pieces were divided into groups of 40 g each. Each group of shredded straw pieces was packed into a nylon bag (200 × 300 mm). After tying a red nylon rope onto each nylon bag, it was placed back at the collection site of the straw fragments. In the shallow-burial returning and deep-tillage returning experiments, the nylon bags needed to be placed at the depth set in the experiment and the red nylon rope exposed to the surface. The following spring, the location of the nylon bag was determined according to the red nylon rope, and the nylon bag was retrieved. First, the straw was washed with distilled water to clean the soil adhering to the surface of the straw. Second, the straw was dried naturally and placed in an oven at 85 • C (temperature range: 50-200 • C, power: 500 W; CREE-5013B, Dongguan KERUI Instrument Co., Dongguan, China). Third, the straw was ground and weighed. The SDR value for each year is derived from Formula (1), and the 6-year average SDR value is derived from Formula (2).
Note: M 0 means the mass of straw before putting it into the soil; M t means the mass of dry matter of straw after treatment; i means the i-th year; and j means the j-th bag of chopped straw collected in a certain experiment plot.

Soil Organic Matter (SOM) Content before Sowing (0-20 cm)
Soil samples were collected separately from each experiment plot each spring before planting. Five soil sample collection sites were randomly selected according to the diagonal method. Since developing maize seedlings mainly absorb organic matter from the cultivated layer (0-20 cm) [37], soil samples were selected at a depth range of 0-20 cm. The collected soil samples were measured for organic matter content by the potassium dichromate titration method, based on the mass difference of the potassium dichromate solution before and after oxidation. The soil organic matter content can be calculated according to Formula (3) [38]. The SOM value for each year is derived from Formula (4), and the 6-year average SDR value is derived from Formula (5).
Note: X means soil organic matter content (%) of a soil sample; V 0 means volume of FeSO 4 standard solution consumed during blank titration, in mL; V means volume of FeSO 4 standard solution consumed during sample determination, in mL; c 2 means FeSO 4 standard solution concentration, in mol/L; 0.003 means 1/4 molar mass of carbon atoms, in mol/L; 1.724 means conversion factor; m means mass of dried specimen, in g; c1 means concentration of K 2 Cr 2 O 7 standard solution, in mol/L; V 1 means volume of K 2 Cr 2 O 7 standard solution aspirated, in mL; V 2 means volume of FeSO 4 standard solution consumed during titration, in mL; i means year i; j means the j-th soil sample of a certain experiment plot. The maize seedling stage is defined as the period from the start of the sowing operation until the first stalk node for more than 50% of the field is exposed above the ground by 2 cm, which is generally 40-45 days in Northeast China [39]. To facilitate comparison of SAT, a uniform 45-day experimental period was used for the field experiments. The soil-temperature measurement experiment was carried out using the 11,000 Temperature Gauge (Midwest company Antelias, Lebanon.; range: −40 to 150 • C accuracy: ±0.5 • C resolution: 0.1 • C battery: 1.5 V button battery). Soil temperature was measured annually from the first day after the completion of the sowing operation until the end of the seedling period. The measurements were taken five times daily between 08:00 and 16:00 with a 2-h interval. Soil-temperature experimental sites were selected randomly according to the diagonal method at five experiment sites. The growth of maize seedlings is mainly affected by the soil temperature in the cultivation layer, so the measurement depth was set to 0-20 cm. The SAT value for each year is derived from Formula (6), and the 6-year average SAT value is derived from Formula (7) [40].
Note: n means the number of days of continuous soil-temperature measurements; j means the j-th daily soil temperature collection; k means the k-th soil temperature experiment point collected within a given experiment plot.

Dry Matter Accumulation (DMM) of Maize at the Seedling Stage
At the end of the seedling stage each year, five maize plants were randomly selected in each experiment plot according to the S-curve method. First, the plants were divided into leaf and stalk parts. Second, the leaf and stalk parts were weighed for fresh weight. Third, the leaf and stalk parts were put into an oven (temperature range: 50-200 • C, power: 500 W; CREE-5013B, Dongguan KERUI Instrument Technology Co., Ltd., Dongguan, China.) and heated to 105 • C for 30 min, and then baked at 75 • C until constant weight. Fourth, after cooling to room temperature, the dry weight of each organ was determined separately using a balance (ECC2201; measurement range: 0.1-2200 g, linearity error: ±0.2 g, accuracy: ±0.5 • C; Nanjing Bernita Scientific Instruments Co., Nanjing, China.) The DMM value for each year is derived from Formula (8), and the 6-year average DMM value is derived from Formula (9) [41].
Note: m c means the total dry weight of each organ after drying; i means the i-th year; j means the j-th plant collected in a certain experiment plot.

Statistical Analysis Methods for Experimental Data
Analysis of variance (ANOVA) and least significant difference (LSD) were used in this study to analyze the statistical data. ANOVA was used to analyze the variance of the obtained data and determine whether the experimental factors significantly impacted the experimental indicators. LSD was mainly used to determine whether there were significant differences among the different levels of one experimental factor and to compare the averages of different levels. All experimental data were plotted and processed using Origin software.

Straw Decomposition Rate (SDR) before Sowing (0-20 cm)
As shown in Figure 3a-c, SDR showed a gradual increase with the shortening of the crushing length under the same conditions of the returning pattern. The strongest upward trend in SDR was observed when shallow-burial returning was used for field returning. As shown in Figure 3d-f, when the crushing length and returning pattern used were constant, there was no significant trend change in SDR as the number of years of returning operation increased. The experiment results showed that crushing length and returning pattern had significant effects on SDR (p < 0.05). Compared with conventional straw returning, fine-crush straw returning improved SDR by 2.3-6.7% (shallow burial), 1.3-5.7% (deep tillage), and 1.1-4.6% (mulching).

Statistical Analysis Methods for Experimental Data
Analysis of variance (ANOVA) and least significant difference (LSD) were used in this study to analyze the statistical data. ANOVA was used to analyze the variance of the obtained data and determine whether the experimental factors significantly impacted the experimental indicators. LSD was mainly used to determine whether there were significant differences among the different levels of one experimental factor and to compare the averages of different levels. All experimental data were plotted and processed using Origin software.

Straw Decomposition Rate (SDR) before Sowing (0-20 cm)
As shown in Figure 3a-c, SDR showed a gradual increase with the shortening of the crushing length under the same conditions of the returning pattern. The strongest upward trend in SDR was observed when shallow-burial returning was used for field returning. As shown in Figure 3d-f, when the crushing length and returning pattern used were constant, there was no significant trend change in SDR as the number of years of returning operation increased. The experiment results showed that crushing length and returning pattern had significant effects on SDR (p < 0.05). Compared with conventional straw returning, fine-crush straw returning improved SDR by 2.3-6.7% (shallow burial), 1.3-5.7% (deep tillage), and 1.1-4.6% (mulching).

Soil Organic Matter Content (SOM) before Sowing (0-20 cm)
As can be seen from Figure 4a-c, both the crushing length and returning pattern had significant effects on SOM (p < 0.05). As shown in Figure 4a-c, SOM showed a gradual increase with the shortening of the crushing length under the same conditions of the returning pattern. There was no significant difference (p > 0.05) between conventional straw returning and no returning under the same conditions of the returning pattern. The weakest upward trend in SOM was observed when deep tillage returning was used. The experiment results showed that crushing length and returning pattern had significant effects on SOM (p < 0.05). Compared with conventional straw returning, fine-crush straw returning improved the SOM by 1.3-5.1 g/kg (shallow burial), 0.7-3.7 g/kg (deep tillage), and 0.7-4.3 g/kg (mulching), respectively. Compared with no returning, fine-crush straw

Soil Organic Matter Content (SOM) before Sowing (0-20 cm)
As can be seen from Figure 4a-c, both the crushing length and returning pattern had significant effects on SOM (p < 0.05). As shown in Figure 4a-c, SOM showed a gradual increase with the shortening of the crushing length under the same conditions of the returning pattern. There was no significant difference (p > 0.05) between conventional straw returning and no returning under the same conditions of the returning pattern. The weakest upward trend in SOM was observed when deep tillage returning was used. The experiment results showed that crushing length and returning pattern had significant effects on SOM (p < 0.05). Compared with conventional straw returning, fine-crush straw returning improved the SOM by 1.3-5.1 g/kg (shallow burial), 0.7-3.7 g/kg (deep tillage), and 0.7-4.3 g/kg (mulching), respectively. Compared with no returning, fine-crush straw returning raised the SOM by 1.5-5.3 g/kg (shallow burial), 0.4-3.4 g/kg (deep tillage), and 1.1-4.7 g/kg (mulching), respectively. returning raised the SOM by 1.5-5.3 g/kg (shallow burial), 0.4-3.4 g/kg (deep tillage), and 1.1-4.7 g/kg (mulching), respectively. As shown in Figure 4d-f, the SOM showed a gradual increase with the increase in the number of years of operation when the fine-crush straw returning operation was used. There was no significant trend change in SOM with increasing number of years of operation when the conventional straw returning and no-returning operation were used. The experimental results showed that it was difficult to achieve organic-matter accumulation in soil in the short term (6 years) with conventional straw returning, which is similar to the findings of Zheng et al. [42] in Northeast China. Compared with the conventional-straw returning and no-returning operations, fine-crush straw returning is more beneficial to achieve organic-matter accumulation in soil.

Soil Accumulated Temperature (SAT) at the Maize Seedling Stage (0-20 cm)
From Figure 5a,b, it can be seen that under the conditions of shallow-burial returning and deep-tillage returning, the SAT showed a trend of increasing and then decreasing with the shortening of crushing length, as follows: no returning > fine-crush straw returning (3 cm) > fine-crush straw returning (1.5 cm) > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As shown in Figure 5c, the shorter the crushing length, the higher the SAT was under the mulchingreturn condition, as follows: no returning > fine-crush straw returning (1.5 cm) > finecrush straw returning (3 cm) > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As shown in Figure 5d-f, there was no significant trend change in SAT as the number of years of returning operations increased when the crushing length and returning pattern used were constant. The results of the experiment showed that crushing length and returning pattern had a significant effect (p < 0.05) on SAT. Compared with conventional straw returning, fine-crush straw returning obtained higher SAT, but it still had an inhibitory effect on soil heat-absorption efficiency. Compared with conventional straw returning, fine-crush straw returning improved SAT by 36.6-123.7 d·°C (mulching), 31.  As shown in Figure 4d-f, the SOM showed a gradual increase with the increase in the number of years of operation when the fine-crush straw returning operation was used. There was no significant trend change in SOM with increasing number of years of operation when the conventional straw returning and no-returning operation were used. The experimental results showed that it was difficult to achieve organic-matter accumulation in soil in the short term (6 years) with conventional straw returning, which is similar to the findings of Zheng et al. [42] in Northeast China. Compared with the conventional-straw returning and no-returning operations, fine-crush straw returning is more beneficial to achieve organic-matter accumulation in soil.

Soil Accumulated Temperature (SAT) at the Maize Seedling Stage (0-20 cm)
From Figure 5a,b, it can be seen that under the conditions of shallow-burial returning and deep-tillage returning, the SAT showed a trend of increasing and then decreasing with the shortening of crushing length, as follows: no returning > fine-crush straw returning (3 cm) > fine-crush straw returning (1.5 cm) > fine-crush straw returning (4.5 cm) > finecrush straw returning (6 cm) > conventional straw returning (10 cm). As shown in Figure 5c, the shorter the crushing length, the higher the SAT was under the mulching-return condition, as follows: no returning > fine-crush straw returning (1.5 cm) > fine-crush straw returning (3 cm) > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As shown in Figure 5d-f, there was no significant trend change in SAT as the number of years of returning operations increased when the crushing length and returning pattern used were constant. The results of the experiment showed that crushing length and returning pattern had a significant effect (p < 0.05) on SAT. Compared with conventional straw returning, fine-crush straw returning obtained higher SAT, but it still had an inhibitory effect on soil heat-absorption efficiency. Compared with conventional straw returning, fine-crush straw returning improved SAT by 36.6-123.

Dry Matter Accumulation (DMM) of Maize at the Seedling Stage
As can be seen from Figure 6a, under the shallow-burial returning condition, DMM showed a trend of increasing and then decreasing with the shortening of crushing length, as follows: fine-crush straw returning (3 cm) > fine-crush straw returning (1.5 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As can be seen from Figure 6b, the shorter the crushing length, the greater the DMM was under the deep tillage returning condition, as follows: fine-crush straw returning (1.5 cm) > fine-crush straw returning (3 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As can be seen from Figure 6c, the shorter the crushing length, the greater the DMM was under the mulching returning condition, as follows: fine-crush straw returning (1.5 cm) > fine-crush straw returning (3 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). The results of the experiment showed that crushing length and returning pattern had significant effects (p < 0.05) on DMM. Compared with conventional straw returning, the DMM rate was significantly increased by adopting any crushing length of straw. Only with the crushing length less than 3 cm was the DMM significantly higher than that without crushing.

Dry Matter Accumulation (DMM) of Maize at the Seedling Stage
As can be seen from Figure 6a, under the shallow-burial returning condition, DMM showed a trend of increasing and then decreasing with the shortening of crushing length, as follows: fine-crush straw returning (3 cm) > fine-crush straw returning (1.5 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As can be seen from Figure 6b, the shorter the crushing length, the greater the DMM was under the deep tillage returning condition, as follows: fine-crush straw returning (1.5 cm) > fine-crush straw returning (3 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). As can be seen from Figure 6c, the shorter the crushing length, the greater the DMM was under the mulching returning condition, as follows: fine-crush straw returning (1.5 cm) > fine-crush straw returning (3 cm) > no returning > fine-crush straw returning (4.5 cm) > fine-crush straw returning (6 cm) > conventional straw returning (10 cm). The results of the experiment showed that crushing length and returning pattern had significant effects (p < 0.05) on DMM. Compared with conventional straw returning, the DMM rate was significantly increased by adopting any crushing length of straw. Only with the crushing length less than 3 cm was the DMM significantly higher than that without crushing. Agronomy 2021, 11, x FOR PEER REVIEW 11 of 16 As shown in Figure 6d-f, the DMM showed a gradual increase with the increase in the number of years of operation when the fine-crush straw returning operation was used. There was no significant trend change in DMM with increasing number of years of operation when conventional-straw returning and no-returning operations were used. The results of the experiment showed that the use of a fine-crush straw returning operation could consistently improve DMM over a certain time frame (at least 6 years).

Discussion
Straw decomposition requires sufficient moisture, microorganisms with enzymes and other substances, and a suitable temperature environment [43]. The shorter the crushing length, the more straw fragments, provided the total amount of straw is constant. As shown in Figure 7a, straw is mainly composed of epidermis and pith [17]. Before the straw is broken, the pith has been wrapped by the epidermis, which makes it unable to absorb moisture, microorganisms and enzymes in the soil [23]. As shown in Figure 7b, only when the straw was broken could the pith come into contact with the abovementioned substances from both ends of each straw fragment [23][24][25][26]. Obviously, under the condition of a certain amount of maize straw, the greater the number of broken fragments, the higher is the total absorption efficiency of the pith to soil moisture, microorganisms and enzymes. Therefore, fine-crush straw returning can significantly improve the SDR, and the shorter the crushing length, the more obvious is the improvement effect. At the same time, the returning mode also has a significant effect on SDR. Under mulching-returning conditions, the straw fragments lie flat on top of the soil, making it difficult to fully contact the water, microorganisms, fungi, and decomposing enzymes inside the soil; therefore, the absorption rate of the above substances is low [26]. Under deep-tillage conditions, all straw fragments are buried in the 40 cm soil layer, where the temperature is low [44] and the microbial and enzymatic activities are severely inhibited. Under shallow-burial conditions, the vast majority of straw fragments are in the tillage layer (0-20 cm), where the straw fragments can be in full contact with the soil, and the temperature in this area is relatively suitable [45]. Therefore, under shallow-burial returning conditions, fine-crush straw returning is the most effective in improving the decomposition rate. As shown in Figure 6d-f, the DMM showed a gradual increase with the increase in the number of years of operation when the fine-crush straw returning operation was used. There was no significant trend change in DMM with increasing number of years of operation when conventional-straw returning and no-returning operations were used. The results of the experiment showed that the use of a fine-crush straw returning operation could consistently improve DMM over a certain time frame (at least 6 years).

Discussion
Straw decomposition requires sufficient moisture, microorganisms with enzymes and other substances, and a suitable temperature environment [43]. The shorter the crushing length, the more straw fragments, provided the total amount of straw is constant. As shown in Figure 7a, straw is mainly composed of epidermis and pith [17]. Before the straw is broken, the pith has been wrapped by the epidermis, which makes it unable to absorb moisture, microorganisms and enzymes in the soil [23]. As shown in Figure 7b, only when the straw was broken could the pith come into contact with the above-mentioned substances from both ends of each straw fragment [23][24][25][26]. Obviously, under the condition of a certain amount of maize straw, the greater the number of broken fragments, the higher is the total absorption efficiency of the pith to soil moisture, microorganisms and enzymes. Therefore, fine-crush straw returning can significantly improve the SDR, and the shorter the crushing length, the more obvious is the improvement effect. At the same time, the returning mode also has a significant effect on SDR. Under mulching-returning conditions, the straw fragments lie flat on top of the soil, making it difficult to fully contact the water, microorganisms, fungi, and decomposing enzymes inside the soil; therefore, the absorption rate of the above substances is low [26]. Under deep-tillage conditions, all straw fragments are buried in the 40 cm soil layer, where the temperature is low [44] and the microbial and enzymatic activities are severely inhibited. Under shallow-burial conditions, the vast majority of straw fragments are in the tillage layer (0-20 cm), where the straw fragments can be in full contact with the soil, and the temperature in this area is relatively suitable [45]. Therefore, under shallow-burial returning conditions, fine-crush straw returning is the most effective in improving the decomposition rate. It is through mineralization that straw can release organic carbon into the soil, which in turn increases the SOM [46]. By comparing Figure 3 with Figure 4, it can be seen that the trends for the decomposition rate and soil organic matter content are basically the same with a shorter crushing length, which indicates that the increase in decomposition rate can significantly accelerate the mineralization rate. Therefore, compared with conventional straw returning, fine-crush straw returning can significantly enhance SOM, and the shorter the crushing length, the more obvious the enhancement effect is. Meanwhile, due to the low SDR of conventional straw returning in Northeast China, the mineralization rate was severely reduced [47]. Therefore, the enhancement of SOM by conventional straw returning is not significant compared with no returning. Disturbance of the soil by tillage components can cause loss of soil organic matter [48]. When deep tillage is used to return the soil, the turning operation of the soil by the spar plow will cause the original organic matter within the soil to be turned to the soil surface and be lost in large quantities under the effect of wind and water erosion. Therefore, under deeptillage returning conditions, fine-crush straw returning has the weakest effect on the enhancement of soil organic matter content.
The average winter temperature in Northeast China is extremely low, and the soil freezes severely in winter [49]. The gradual warming of the temperature before sowing in spring causes the soil temperature to be much lower than the air temperature, and the soil urgently needs to absorb heat from outside [50]. Soil absorbs heat mainly through its own surface with large pores [51]. The undecomposed straw fragments before spring sowing will form an insulating layer and reduce the efficiency of heat absorption by the soil. At the same time, the undecomposed straw fragments also increase the macropore size of the soil, which promotes the absorption of heat by the soil. Under mulching returning conditions, the undecomposed straw fragments remain on the soil surface and cannot increase soil macroporosity, but only serve to impede soil heat absorption. Therefore, the shorter the crushing length, the higher the accumulated temperature of maize seedlings under mulching-returning conditions. Under the shallow-burial returning condition and the deep-tillage returning condition, the effect of undecomposed straw fragments on soil heat-absorption efficiency is two-sided. As the crushing length becomes shorter, the number of undecomposed straw fragments gradually decreases, which reduces the inhibitory effect of the insulation layer on the rate of soil heat absorption, but also reduces the macroporosity of the soil. Therefore, there is an extreme point where the highest soiltemperature accumulation at the seedling stage of maize can be obtained under both shallow-burial and deep-tillage returning conditions. DMM is mainly influenced by the SAT and SOM before sowing [6,12]; therefore, the crushing length has a significant effect on DMM. When mulching was used to return soil to the field, both SAT and SOM gradually increased as the crushing length was shortened. Therefore, the highest value of DMM under mulching-returning conditions was observed with a crushing length of 1.5 cm. When using shallow-burial returning and deep-tillage It is through mineralization that straw can release organic carbon into the soil, which in turn increases the SOM [46]. By comparing Figure 3 with Figure 4, it can be seen that the trends for the decomposition rate and soil organic matter content are basically the same with a shorter crushing length, which indicates that the increase in decomposition rate can significantly accelerate the mineralization rate. Therefore, compared with conventional straw returning, fine-crush straw returning can significantly enhance SOM, and the shorter the crushing length, the more obvious the enhancement effect is. Meanwhile, due to the low SDR of conventional straw returning in Northeast China, the mineralization rate was severely reduced [47]. Therefore, the enhancement of SOM by conventional straw returning is not significant compared with no returning. Disturbance of the soil by tillage components can cause loss of soil organic matter [48]. When deep tillage is used to return the soil, the turning operation of the soil by the spar plow will cause the original organic matter within the soil to be turned to the soil surface and be lost in large quantities under the effect of wind and water erosion. Therefore, under deep-tillage returning conditions, fine-crush straw returning has the weakest effect on the enhancement of soil organic matter content.
The average winter temperature in Northeast China is extremely low, and the soil freezes severely in winter [49]. The gradual warming of the temperature before sowing in spring causes the soil temperature to be much lower than the air temperature, and the soil urgently needs to absorb heat from outside [50]. Soil absorbs heat mainly through its own surface with large pores [51]. The undecomposed straw fragments before spring sowing will form an insulating layer and reduce the efficiency of heat absorption by the soil. At the same time, the undecomposed straw fragments also increase the macropore size of the soil, which promotes the absorption of heat by the soil. Under mulching returning conditions, the undecomposed straw fragments remain on the soil surface and cannot increase soil macroporosity, but only serve to impede soil heat absorption. Therefore, the shorter the crushing length, the higher the accumulated temperature of maize seedlings under mulching-returning conditions. Under the shallow-burial returning condition and the deep-tillage returning condition, the effect of undecomposed straw fragments on soil heat-absorption efficiency is two-sided. As the crushing length becomes shorter, the number of undecomposed straw fragments gradually decreases, which reduces the inhibitory effect of the insulation layer on the rate of soil heat absorption, but also reduces the macroporosity of the soil. Therefore, there is an extreme point where the highest soil-temperature accumulation at the seedling stage of maize can be obtained under both shallow-burial and deep-tillage returning conditions. DMM is mainly influenced by the SAT and SOM before sowing [6,12]; therefore, the crushing length has a significant effect on DMM. When mulching was used to return soil to the field, both SAT and SOM gradually increased as the crushing length was shortened. Therefore, the highest value of DMM under mulching-returning conditions was observed with a crushing length of 1.5 cm. When using shallow-burial returning and deep-tillage returning, the SOM showed that fine-crush straw returning (3 cm) < fine-crush straw returning (1.5 cm), and SAT showed that fine-crush straw returning (3 cm) > fine-crush straw returning (1.5 cm). The results of the experiment showed that the highest value of DMM (shallow-burial returning) was achieved when the crushing length was 3 cm. This indicates that soil temperature during the seedling stage has a more significant effect on DMM in shallow-burial returning. Similar conclusions were reached in a study by Andrade et al. [52]. The experimental results showed that the highest value of DMM in deep-tillage returning was obtained when the crushing length was 1.5 cm. This may be due to the fact that deep-tillage returning causes a large loss of SOM, resulting in a more significant effect of SOM on DMM. Compared with the no-returning mode, fine-crush straw returning's SAT was lower, although its disadvantage gradually decreased with the shortening of the crushing length and the advantage in SOM before sowing gradually expanded. Therefore, when the crushing length was shortened to less than 3 cm, fine-crush straw returning's DMM was significantly higher than that of no returning. Compared with conventional straw returning, the use of any crushing length of straw can significantly improve the DMM.
The experimental results showed that both SOM and DMM gradually increased with the number of years of the fine-crush straw-returning operation, which indicated that the cumulative positive SOM might have a significant effect on DMM. Both straw-returning operation and organic-fertilizer application in agricultural production can increase the content of soil organic matter [11,15,53], which needs to be consumed in large quantities during maize growth [49]. The annual amount of organic fertilizer application is calculated based on the amount of organic matter required for maize growth [54]. Therefore, the amount of organic matter replenished in the soil is approximately the same as the amount consumed when using conventional straw returning and no-returning operations. The higher soil mineralization rate of the fine-crush straw-returning operation causes the amount of organic matter replenished in the soil to be greater than the amount consumed. Therefore, when fine-crush straw operations are used, a certain amount of organic matter is carried over to the next year each year. Soil organic-matter content can be increased year by year, which can effectively improve the soil aggregation structure and thus obtain seedbed conditions more favorable for maize seedling development. Therefore, the finecrush straw operation can achieve continuous accumulation of soil organic matter, which can promote maize seedling development. Currently, straw-returning operations are generally not accepted in the maize-growing regions of Northeast China. The experimental site selected in this study has been planting maize continuously for 20 years and has never been subjected to straw-returning operations. The above conditions resulted in a large accumulation of soil microorganisms and enzymes capable of degrading straw in the soil of the experimental site. Obviously, in the first year after the fine-crush strawreturning operation, due to the large accumulation of raw materials required for straw degradation, shortening the crushing length can maximize the straw decomposition rate. Meanwhile, with the increase of experimental years, microorganisms and enzymes in the soil were consumed continuously, and the effect of different crushing lengths on the straw decomposition rate gradually decreased. The rate of straw decomposition directly determined the rate of soil organic matter supplementation by straw returning. Therefore, most of the differences in SOM between different experimental treatments originated from the first year of fine-crush straw returning.

Conclusions
This paper presents a new agronomic technique, fine-crush straw returning, whose core difference from conventional straw returning is a significant reduction in crushing length. This study found that by shortening the crushing length, the SDR, SOM and SAT can be significantly increased (special case: the maize seedling soil temperature at a crushing length of 1.5 cm is less than 3 cm when using shallow burial and deep tillage), thus obtaining more favorable seedbed conditions for maize seedling development. Based on the improvement of DMM, this study achieved optimal fragmentation length values of 1.5 cm, 3 cm, and 1.5 cm for fine-crush straw returning under mulching, shallow burial and deep tillage conditions, respectively.
Compared with conventional straw returning and no returning, when straw fine shredding is applied to the field with optimal shredding-length values, this can improve DMM by 5.1 g/plant and 2.8 g/plant (shallow burial), 4.2 g/plant and 1.8 g/plant (deep tillage), and 4.3 g/plant and 1.9 g/plant (mulching). In conclusion, fine-crush straw returning (crushing length set at 1.5 cm or 3 cm) can significantly improve the dry matter accumulation at the maize seedling stage, thus promoting the spread of straw regrading in maize-growing regions of Northeast China and ensuring the sustainability of agricultural development.