A High Amount of Straw Pellets Returning Delays Maize Leaf Senescence, Improves Dry Matter Accumulation and Distribution, and Yield Increase in Northeast China

Abstract

Enhancing chlorophyll retention in maize leaves and prolonging the grain-filling duration constitute critical strategies for yield improvement in agricultural production systems. This study investigated the mechanistic relationship between yield enhancement pathways and the leaf senescence process induced by high-input straw pellets amendment. We analyzed the impact mechanisms of green leaf area dynamics and dry matter redistribution on yield during late reproductive stages, establishing theoretical foundations for yield optimization through intensive straw pellets incorporation. The study used the maize variety Jingnongke 728 as the experimental material. Based on previous research, four treatments were set up, including no straw returning (CK), chopped straw (15 t/ha) returning to the field (FS1), a large amount of chopped straw (75 t/ha) returning to the field (FS5), and a large amount of pelletized straw (75 t/ha) returning to the field (KL5), with four replicates. A two-year experimental design systematically assessed green leaf area index (GLAI), dry matter accumulation, distribution, translocation, yield components, and grain yield to explore the differences among various treatments under different straw returning amounts and returning forms. The study detected no significant differences between FS1 and CK. Although KL5 and FS5 delayed leaf senescence, FS5 significantly depressed green leaf area index (GLAI) at the R1 stage (silking), which results in it not having more effective photosynthetic area during late phenological phases. In dry matter dynamics, KL5 exhibited 5.52–25.71% greater pre-anthesis accumulation, 2.73–60.74% higher post-anthesis accumulation, and 9.48–25.76% elevated ear dry matter allocation relative to other treatments. KL5’s post-anthesis assimilates contributed 2.43–17.02% more to grain development, concurrently increasing ear-to-total biomass ratio. Yield analysis ranked KL5 as the superior treatment with 0.68–25.15% yield advantage, driven by significantly enhanced kernel number per ear and 100-kernel mass, whereas FS5 displayed the lowest kernel count among all treatments. Returning 75 t/ha of straw pellets to the black soil area in Northeast China can significantly delay the senescence of maize leaves and increase the accumulation of dry matter after anthesis by maintaining the effective photosynthetic area of leaves in the later stage of growth, thereby achieving the goal of increasing yield. The research can offer a practical and novel approach for straw return in the black soil region of Northeast China and provide a new technological pathway for enhancing crop productivity.

1. Introduction

Maize is the largest crop in the world and also in China, playing an important role in ensuring national food security [1]. Preliminary 2024 agricultural census data indicate China’s maize cultivated area reached 44.733 million hectares, demonstrating a national average yield of 6.59 t/ha based on grain dry weight standards [2]. The rapidly increasing maize production process consumes a large amount of water and fertilizer and produces a large amount of straw [3]. Straw is an agricultural waste rich in organic carbon, containing a large amount of nitrogen, phosphorus, potassium and other micro-nutrients for crop growth [4]. It plays a very important role in releasing carbon, nitrogen, and phosphorus elements after returning to the soil, and can regulate the imbalance of soil nutrients [5]. However, due to long-term cultivation and unreasonable cultivation management, there is a significant loss of soil organic carbon, intensified soil erosion, and damage to soil structure [6]. The total area of black soil in China is about 1.03 million km², mainly distributed in four provinces and cities: Heilongjiang, Jilin, Liaoning, and Inner Mongolia. It is one of the four famous black soil areas in the world, with a typical black soil area of about 170,000 km², mainly distributed in a crescent shape in Heilongjiang and Jilin provinces [7]. Among them, Heilongjiang Province, the largest grain producing province in China, currently has 57.44% of the country’s black soil arable land area and over 133.33 million hectares of grain crops [8,9]. In the analysis of soil nutrients in Northeast China from 1985 to 2020, it was found that the content and density of organic carbon decreased by 3.07 g/kg and 6.71 Mg C/ha, respectively. The organic carbon storage decreased by 0.32 Pg in the past 35 years, and nearly 64% of farmland soils showed negative changes in organic carbon content [10]. The excessive use of black soil and unreasonable fertilization structure has resulted in a 50% to 60% decrease in soil organic carbon content compared to before cultivation [11]. Many studies have found that returning straw to the field can optimize microbial communities and have a positive effect on promoting soil organic matter, nutrient cycling, and improving soil fertility [12,13,14]. The degradation of straw in returning straw to the field is a mineralization and decay process involving microorganisms and enzymes [15]. Therefore, although straw returning to the field is one of the most effective technologies for achieving sustainable agricultural development, straw returning to the field has never exceeded 10% of the maize planting area in Northeast China [16].

Returning straw pellets to the field is a method that has emerged in recent years. It involves highly chopping straw and producing straw pellets with a volume reduced to 1/10–1/15 and a density increased by 7–10 times [17]. Due to its high degree of fragmentation, straw pellets can be better mixed with soil after returning to the field. Compared with chopped straw, the content of cellulose, hemicellulose, and lignin in straw pellets decreased by 7.78%, 30.80%, and 10.06%, respectively. The half-decomposed and fully-decomposed periods of straw pellets are 30 days and 14 days shorter than those of chopped straw, significantly reducing the straw decomposition period [18]. Using the nylon mesh bag burial method to decompose straw, the average decomposition rate of straw pellets increased by 31.68% compared to the control during the first 60 days of cultivation; After 300 days of cultivation, the cumulative decomposition rate reached 80.81%, which was 8.7% higher than the control, and the improvement effect was significant [19]. Many studies have also shown that returning straw pellets to the field can rapidly increase the organic carbon, soluble organic carbon, and alkaline nitrogen content of the topsoil [19,20,21]. It can also increase the activity of soil sucrase, urease, catalase, alkaline phosphatase, and increase the number and diversity of bacteria [22]. Returning straw pellets to the field can also improve soil’s physical properties. The combination of straw pellets returning and rotary tillage treatment can reduce soil bulk density in the 0–20 and 20–40 cm soil layers, improve water holding capacity and soil aggregate stability [23]. Returning straw pellets to the field also changes soil respiration. As the input of organic material pellets increases, the proportion of material carbon converted into soil organic carbon in dryland ecosystems does not decrease but remains stable [24]. Compared with conventional chopping straw returning, straw pellets returning can significantly increase soil available nutrients, wheat biomass, and plant total nitrogen, phosphorus, and potassium contents [25]. And it also has a significant yield increasing effect on maize [26,27]. It has been found that returning straw pellets to the field increases the iron, manganese, magnesium, and amino acid content in maize grains compared to returning chopped straw, improving the quality of the grains [28].

Conventional straw returning is often limited by the amount of straw returned, while straw pellets returning is different. Due to its good degree of decomposition, it can often accommodate more straw pellets in the same soil without affecting the growth of aboveground crops [29]. A study has found that the proportion balance between soil organic carbon and nutrient elements, soil humus composition and structure, soil physical properties and aggregate distribution, and soil microbial community structure of straw pellets buried deeply in the field are all better than those of straw pellets buried deeply in the field at low levels [7]. The increase of nutrient element ratio by straw pellets has a short-term and rapid effect compared to chopped straw, and can significantly improve the nutrient content of sub cultivated soil [29]. Moreover, the high amount of straw granulation returning to the field also showed significant effects on crop yield increase. Compared with the nonreturning treatment, it increased the number of ears, kernels per ear, and 100-kernel weight per unit area, and increased maize yield by 5% to 23% [26]. Therefore, the high amount of straw granulation returning to the field can achieve stable and high yields while thickening and fertilizing the black soil layer, providing a feasible method for solving the problem of straw returning to the field in Northeast China.

Previous studies on returning straw pellets to the field have mostly focused on the soil, and research on the growth and development of above-ground crops has been limited to the level of yield. Although the important role of straw pellets returning to the field in improving soil and its impact on crop yield have been clarified, the reasons for the increased yield of straw pellets returning to the field are still unclear. In previous studies, it was found that maize leaves treated with continuous high volume and deep burial of straw pellets had better green retention, delayed leaf senescence process, and increased grain filling period [27]. Leaf senescence constitutes the final stage of leaf development and is crucial for plant adaptability. The transfer of nutrients from leaves to grains is achieved through this process [30]. The most obvious characteristics of leaf senescence are chlorophyll degradation and leaf yellowing. The decrease in chlorophyll and net photosynthetic rate, as well as the reduction in green leaf area, ultimately leads to leaf senescence in maize after silk emergence [31]. By monitoring the changes in the leaf area index of maize after silk emergence, the senescence process of maize leaves in the later stage of growth can be determined. Leaf Area Index (LAI) refers to the total area of plant leaves per unit of land area [32]. It is commonly used to assess crop growth. LAI quantifies leaf area in ecosystems and is a key variable in processes such as photosynthesis, respiration, and precipitation interception [33]. The Green Leaf Area Index (GLAI) is often used to limit the definition of LAI to green areas where photosynthesis and transpiration are active [34]. It is expressed as the total green leaf area of plant leaves per unit land area [35]. By monitoring GLAI, the senescence cycle of maize leaves can be quantified and its patterns can be explored. Leaf growth affects the accumulation of dry matter and grain yield of crops [36]. More than 70% of the assimilated substances in maize grains come from the production of photosynthetic substances during leaf grouting, delaying the senescence of leaves after anthesis and increasing the length of green leaves can increase the accumulation of dry matter, thereby increasing the yield of maize grains [37]. The senescence rate of maize leaves after the anthesis period is closely related to the yield of mature grains. Returning straw to the field can increase crop yield, but whether the reason for the increase in yield is closely related to the degree of greenness of the leaves still needs further research. Therefore, our research will explore the influence of straw pellets returning to the field on maize yield and yield composition from the perspective of the senescence of maize leaves and the accumulation and transport of maize dry matter after anthesis.

2. Materials and Methods

2.1. Soil and Climate Characteristics of the Experimental Site

The field experiment was designed in the Modern Agriculture Demonstration Zone of Heilongjiang Academy of Agricultural Sciences in Harbin, Heilongjiang Province, China (45°50’54” N, 126°50’12” E, altitude 128 m). Heilongjiang Province is located in northeastern China and belongs to the continental monsoon climate of cold temperate and temperate regions. Its climate characteristics are characterized by long and cold winters and mild and rainy summers. Harbin City belongs to the first temperate zone in Heilongjiang Province, with an active accumulated temperature (≥10 °C) of over 2700 °C and an annual frost free period of 135–145 days. The average annual temperature is 3.5–4.5 °C, the annual rainfall is 400–600 mm, the annual fluctuation is severe, and the seasonal distribution is unstable. The annual rainfall in 2023 is 519.40 mm, and the annual rainfall in 2024 is 588.10 mm (

Table 1. Meteorological conditions of maize growing season at the experiment site (2023–2024).

Month	Average Temperatures (°C)		Precipitation (mm)		Sunshine Hours (h)
	2023	2024	2023	2024	2023	2024
April	8.13	10.50	22.50	11.90	230.10	242.20
May	16.35	15.12	42.70	78.00	285.30	212.20
June	22.21	19.69	14.10	248.50	236.20	178.60
July	23.01	24.54	221.60	116.00	233.20	195.80
August	21.81	23.58	199.50	104.70	224.50	196.30
September	17.26	16.30	19.00	29.00	200.00	243.50

). The soil type of experimental soil is chernic horizon. The basic characteristics of the soil layer from 0 to 20 cm before the experiment are: soil organic carbon (SOC) content of 16.16 g/kg, total nitrogen (TN) content of 1.47 g/kg, total phosphorus (P) content of 0.82 g/kg, total potassium (K) content of 16.86 g/kg, alkaline hydrolyzable nitrogen (AN) content of 158.2 mg/kg, rapidly available phosphorus (AP) content of 28.9 mg/kg, rapidly available potassium content (AK) of 113.8 mg/kg, and soil pH value of 6.41.

2.2. Experimental Material

The experiment variety Jingnongke 728 has excellent quality, resistance to dense planting, lodging, comprehensive resistance, good stability, fast dehydration, early maturity, suitable for mechanized harvesting, and is suitable for planting in areas with an effective accumulated temperature of about 2500 °C [38,39].

2.3. Experimental Design

The experiment is based on a long-term positioning experiment arranged and implemented in the autumn of 2018, which has been ongoing for 6 years. Design four experimental treatments, including no straw return (0 t/ha, CK), chopped straw returning to the field (15 t/ha, FS1), a large amount of chopped straw returning to the field (75 t/ha, FS5), and a large amount of pelletized straw returning to the field (75 t/ha, KL5). Random block design repeated 4 times, with a small area of 16 m² (4 m × 4 m). The experimental maize straw pellets are produced using a “field self-propelled straw picking and pellets making integrated machine”. The diameter of the maize straw pellets is about 9 mm, and the density is about 1.14 g/cm³ (Figure 1). Both straw pellets and chopped straw are manually buried to a depth of 30–40 cm in the soil layer. Planting maize every year, sowing on 30 April 2023, and harvesting on 26 September; Sow on 27 April 2024; harvesting on 27 September. The planting density is 67,500 plants/ha, the planting row spacing is 65 cm, and 600 kg/ha of maize specific slow-release fertilizer (N-P₂O₅-K₂O=24-12-12) is applied during sowing. Other management is the same as the conventional field.

2.4. Data Collection

2.4.1. Green Leaf Area Index (GLAI)

When maize enters R1, three representative plants with uniform growth are selected from each plot and collect all their green leaves. The length width coefficient method is used to measure the total area of green leaves per live plant and calculate its green leaf area (GLA) [40]. The calculation formula is as follows:

$$\mathrm{GLA}({\mathrm{m}}^2)=0.75\times \sum _{\mathrm{i}=1}^{\mathrm{n}}\left(\mathrm{L}\times \mathrm{W}\right)$$

In the equation, 0.75 is the correction factor, L and W are the maximum length and width of the blades, and n is the number of blades.

Select two rows of plants with no missing seedlings in the community to count the number of plants. Convert the actual density to the actual number of plants in each community, and calculate the Green Leaf Area Index (GLAI) [41]. Calculate using the following formula:

$$\mathrm{GLAI}={\displaystyle \frac{\mathrm{GLA}\times \mathrm{P}}{\mathrm{GA}}}$$

In the equation, P represents the actual number of plants in each community, GA represents the land area (m²).

2.4.2. Leaf Senescence Fitting

When maize enters the R1, record the silk emergence time of maize and record this period as the starting time of leaf senescence. From then on, record the degree of leaf senescence of maize every seven days, and record that more than half of the leaves are withered or yellow as senescence until the seeds are ripe and harvested. Fit the dynamic changes of green leaf area index throughout the entire senescence cycle using a quadratic equation and observe the senescence differences of maize leaves under different treatments through the fitting model. Fit the dynamic changes in the green area of maize leaves to the logistic equation [42]:

$$\mathrm{y}={\displaystyle \frac{\mathrm{a}}{1+{\mathrm{e}}^{-\mathrm{b} \left(\mathrm{t} - \mathrm{c}\right)}}}$$

In the equation, t is the number of days after silk emergence (d), y is GLAI, a is the theoretical initial value of GLA, b is a constant describing the senescence rate, c is the time to reach the maximum senescence rate.

2.4.3. Accumulation and Transportation of Dry Matter

At the R1 and R6 of maize, select three representative and uniformly growing aboveground plants from each plot, and classify the aboveground plants. At R1, it was divided into three parts: leaves, stem (stem + leaf sheaths), and spike. At R6, it was divided into four parts: leaves, stem (stem + leaf sheaths), bract leaves (bract leaves + spike axis), and grains. The samples are then placed in an oven at 105 °C for 30 min and then dried at 80 °C to constant weight. After cooling to room temperature, they are weighed using a one percent balance to calculate the contribution rate of nutrient organ dry matter transport to grain dry matter accumulation during anthesis [43]. Calculate using the following formula:

$${\mathrm{DMAR}}_{\mathrm{before}}(\%)={\displaystyle \frac{{\mathrm{T}}_{\mathrm{R}1}}{{\mathrm{T}}_{\mathrm{R}6}}}\times 100$$

In the equation, DMAR_before is the dry matter accumulation rate before anthesis T_R1 is the dry matter accumulation in R1; T_R6 is the dry matter accumulation in R6.

$${\mathrm{DMA}}_{\mathrm{after}}(\mathrm{g}/\mathrm{plant})={\mathrm{T}}_{\mathrm{R}6}-{\mathrm{T}}_{\mathrm{R}1}$$

In the equation, DMA_after is the dry matter accumulation after anthesis.

$${\mathrm{DMAR}}_{\mathrm{after}}\left(\mathrm{\%}\right)={\displaystyle \frac{{\mathrm{DMA}}_{\mathrm{after}}}{{\mathrm{T}}_{\mathrm{R}6}}}\times 100$$

In the equation, DMAR_after is the dry matter accumulation rate after anthesis.

$$\mathrm{DMPR}(\%)={\displaystyle \frac{{\mathrm{W}}_{\mathrm{i}}}{{\mathrm{W}}_{\mathrm{total}}}}\times 100$$

In the equation, DMPR is the dry matter partitioning ratio; W_i is the dry weight of a certain organ; W_total is the total dry weight of the plant.

$$\mathrm{DMTA}(\mathrm{g}/\mathrm{plant})={\mathrm{W}}_{\mathrm{R}1}-{\mathrm{W}}_{\mathrm{R}6}$$

In the equation, DMTA is the dry matter translocation amount; W_R1 is the dry matter mass of nutritional organs in R1; W_R6 is the dry matter mass of nutritional organs in R6.

$$\mathrm{DMTE}(\%)={\displaystyle \frac{\mathrm{DMTA}}{{\mathrm{W}}_{\mathrm{R}1}}}\times 100$$

In the equation, DMTE is the dry matter translocation efficiency.

$${\mathrm{CRDMG}}_{\mathrm{after}}(\mathrm{g}/\mathrm{plant})=1-\left({\displaystyle \frac{{\mathrm{W}}_{\mathrm{transferred}}}{{\mathrm{W}}_{\mathrm{grain}}}}\right)\times 100$$

In the equation, CRDMG_after is the contribution rate of dry matter to grains after anthesis; W_transferred is the dry matter mass transported from nutrient organs to grains; W_grain is the dry weight of the grain.

2.4.4. Determination of Yield and Components

Harvest and measure yield during the R6 stage of maize. Retrieve all two rows in the middle of each community, harvest all ears, weigh them, calculate the average single ear weight, select 10 uniform ears from each treatment according to the average single ear weight, and ensure that the selected 10 ears’ weight = average single ear × 10, air dry and experimental the seeds, record the number of rows and kernels in each ear, calculate the number of kernels per ear, measure the 100-kernel weight and grain moisture content (Japanese Kett grain moisture analyzer PM-8188-A, Tokyo, Japan), and calculate the grain yield (calculated based on 14.0% moisture content).

2.5. Statistical Analysis

Microsoft Excel 2016 software was used for data entry and calculation, SPSS 26 software was used for one-way analysis of variance, the LSD method was used for significant difference analysis at the p < 0.05 level between treatments, Origin2021 software was used for plotting, and the chart data were all mean ± standard deviation.

3. Results

3.1. The Effect of the High Amount of Straw Pellets Returning to the Field on GLAI Fitting Curve of Maize

As shown in Figure 2, through the fitting analysis of the senescence process of maize leaves, it can be seen that the fitting curves of GLAI in all four treatments show a parabolic shape. The fitting curves of FS1 and CK are close to overlapping; Compared with FS5, the GLAI of FS5 was generally lower than CK within 28 days after silk emergence, and was 10.51%, 13.17%, 12.50%, 11.54%, and 6.11% lower than CK at 0 d, 7 d, 14 d, 21 d, and 28 d, respectively. The difference in GLAI between the two groups decreased after 28 days, and was 2.78%, 5.40%, 1.74%, and −7.70% lower than CK at 35 d, 42 d, 49 d, and 56 d, respectively. The fitting curve of FS5 gradually overlapped with CK; Compared with CK, the GLAI of KL5 was higher than that of CK by 4.28% since the R1 period, and the difference in senescence rate was small in the early stage of slow senescence, and the difference in senescence rate was different from 35 d, and the GLAI of KL5 at 35 d, 42 d, 49 d, 56 d, and 63 d was higher than that of CK 9.01%, 11.74%, 18.96%, 49.88%, and 336.97%, respectively. It can be seen that the GLAI of FS1 was similar to that of CK. The time of GLAI in FS5 was significantly lower than that of other treatments from R1 to the following month. However, the GLAI of KL5 from silking to grain maturity was higher than that of the other three groups, and the senescence rate slowed down significantly when maize leaves entered the rapid senescence stage (Figure 2).

3.2. The Effect of the High Amount of Straw Pellets Returning to the Field on Senescence Rate of Maize Leaves

As shown in

Table 2. The differences in leaf senescence rate of maize leaves for straw returning with different amounts and morphologies after R1.

Treatments	FS1	FS5	KL5	CK
0 d	0	0	0	0
7 d	−4.78	−6.44	−3.32	−4.19
14 d	−1.56	−1.29	−1.48	−1.88
21 d	−1.66	−0.41	−1.16	−1.27
28 d	−3.43	−0.77	−3.20	−5.59
35 d	−8.14	−4.01	−6.53	−7.03
42 d	−15.59	−17.11	−12.87	−15.00
49 d	−13.57	−16.02	−13.70	−18.93
56 d	−63.08	−57.29	−51.18	−61.25
63 d	−96.87	−97.15	−89.55	−96.42

, by comparing the rate of decrease in maize leaf area index, the senescence rate of leaves in all four treatments showed a significant increase from 35 days after maize entered the silk emergence stage, and the leaf function approached complete loss at 63 days. Taking 35 days as the boundary between slow and rapid leaf senescence, the leaf senescence rates of the four treatments showed the same pattern and similar decline before 35 days. GLAI remained relatively stable before 28 days and began to accelerate at 28 days. GLAI in the FS1, FS5, KL5 and CK treatments decreased by 10.97%, 8.74%, 8.87% and 12.36%, respectively. The decline trend of straw returning treatment in all three groups was lower than that in CK; After 35 days, the senescence rate of the leaves rapidly increased, and the GLAI of the FS1, FS5, KL5 and CK groups decreased by 18.23%, 12.40%, 14.82% and 18.52%, respectively. The deceleration of GLAI in the four groups still showed the same trend, but there were differences in the magnitude of the decrease; Compared with CK, the senescence rate of FS1 leaves showed the same pattern; The magnitude of leaf senescence rate in FS5 was lower than that in CK before 56 days, and the GLAI of FS5 and CK decreased by 73.95% and 78.25% respectively at 56 days. However, in the last seven days before maturity, FS5 showed a rapid downward trend, and the GLAI of FS5 and CK was 99.26% and 99.22% respectively at 63 days. The decrease in GLAI of KL5 was more stable compared to the other three groups. At 56 days, the GLAI of the FS1, FS5, KL5 and CK groups was 22.03%, 26.05%, 31.27% and 21.75% of that of the R1 group, respectively. GLAI was still significantly higher than the other three groups. At 63 days, GLAI was still 3.27% higher in the R1 group than 0.69%, 0.74% and 0.78% in the FS1, FS5 and CK groups. It can be seen that FS1 has no significant effect on the senescence rate of maize leaves; Both FS5 and KL5 affect reducing the senescence rate of leaves, with KL5 having a more significant delaying effect and being able to maintain it until the final stage of maize growth (Table 2).

3.3. The Effect of the High Amount of Straw Pellets Returning to the Field on Dry Matter Accumulation Before and After Maize Anthesis

As shown in

Table 3. The differences in dry matter accumulation of maize before anthesis and after anthesis for straw returning with different amounts and morphologies.

Year	Treatments	DMAbefore (g/Plant)	DMAafter (g/Plant)
2023	FS1	147.12 ± 4.20 c	152.29 ± 11.21 ab
	FS5	164.84 ± 9.16 b	115.66 ± 13.75 b
	KL5	180.27 ± 5.58 a	185.92 ± 23.00 a
	CK	170.85 ± 13.00 ab	135.79 ± 26.24 b
2024	FS1	161.30 ± 3.05 b	164.57 ± 1.59 a
	FS5	160.63 ± 10.52 b	145.43 ± 11.34 b
	KL5	201.93 ± 8.95 a	169.07 ± 7.47 a
	CK	166.93 ± 4.96 b	155.97 ± 11.19 ab

Notes: Data are expressed as mean ± standard deviation. Different letters within the same column indicate significant difference at the 5% level.

, there were significant differences in the accumulation of pre and post-anthesis dry matter in maize under different treatments (p < 0.05). The pre-anthesis dry matter accumulation of the four treatments in 2023 showed a pattern of KL5 > CK > FS5 > FS1, with KL5 being 22.54%, 9.37% and 5.52% higher than FS1, FS5 and CK treatments, respectively. In 2024, the pre-anthesis dry matter accumulation of KL5 was significantly higher than the other three treatments, with KL5 being 25.19%, 25.71% and 20.97% higher than FS1, FS5 and CK treatments, respectively. The accumulation of post-anthesis dry matter in all four treatments showed a trend of KL5 > FS1 > CK > FS5. In 2023, KL5 was significantly higher than FS1 and significantly higher than CK and FS5. KL5 was 22.86%, 36.92% and 60.74% higher than the FS1, CK and FS5 treatments, respectively. In 2024, KL5 and FS1 were significantly higher than CK and FS5 treatments, and KL5 was 2.73%, 8.40% and 16.25% higher than FS1, CK and FS5 treatments, respectively. It can be seen that over the course of two years, FS1 significantly increased the accumulation of dry matter before and after the maize anthesis, while FS5 had a relatively small effect on the accumulation of dry matter before and after the maize anthesis. However, the accumulation of dry matter after the maize anthesis was significantly inhibited, while KL5 significantly increased the accumulation of dry matter before and after the maize anthesis compared to the other three treatments (Table 3).

3.4. The Effect of the High Amount of Straw Pellets Returning to the Field on Dry Matter Accumulation and Distribution in Different Parts of Maize R1 Stage

As shown in Figure 3, the four treatments in the R1 stage of maize were mainly based on the proportion of stems and leaves. In 2023, the proportion of stems and leaves in FS1, FS5, KL5 and CK treatments was 76.98%, 77.55%, 74.23% and 78.75%, respectively. In 2024, the proportion of stems and leaves was 76.49%, 77.57%, 70.08% and 76.49%, respectively; Among them, the proportion of stems was the largest, with the proportions of stems treated with FS1, FS5, KL5 and CK in 2023 being 53.24%, 53.65%, 51.33% and 53.93%, respectively; The proportion of stems in 2024 was 51.83%, 53.06%, 46.29% and 51.28%, respectively. In different treatments, the proportion of KL5 ears was higher than the other three groups (p < 0.05). In 2023, it was 2.75%, 3.31% and 4.51% higher than the FS1, FS5 and CK treatments; In 2024, it was 6.41%, 7.50% and 6.41% higher than FS1, FS5 and CK treatments. The dry matter accumulation of KL5 ears was also significantly higher than the other three treatments. In 2023, it was 11.92%, 14.76% and 21.24% higher than FS1, FS5 and CK treatments, and in 2024, it was 27.27%, 33.42% and 27.26% higher than FS1, FS5 and CK treatments. It can be seen that the two treatments FS1 and FS5 have no significant effect on the increase of dry matter accumulation in various parts of maize during the R1 period, while KL5 has a significantly higher dry matter accumulation in the R1 period than the other three treatments, and can effectively increase the proportion of dry matter in the ear during the R1 period, with a significant effect on improving the dry matter accumulation in the ear (Figure 3).

3.5. The Effect of the High Amount of Straw Pellets Returning to the Field on Dry Matter Accumulation and Distribution in Different Parts of Maize R6 Stage

As shown in Figure 4, there were significant differences among the four treatments (p < 0.05). Among them, the proportion of mature maize was mainly composed of grains. In 2023, the grain proportions of FS1, FS5, KL5 and CK treatments were 54.08%, 52.76%, 55.15% and 53.63%, respectively. In 2024, the grain proportions were 53.37%, 51.23%, 52.33% and 53.34%, respectively. The allocation proportion was grain > stem > bract leaves, leaf. Among them, the grain proportion of KL5 in 2023 was the highest, higher than FS1, FS5 and CK at 1.97%, 4.53% and 2.82%, respectively. In 2024, the grain proportion of KL5 was lower than FS1 and CK, and the grain proportion of FS5 was the lowest; However, from the perspective of grain accumulation, KL5 was significantly higher than the other three treatments. In 2023, the grain accumulation of KL5 was 22.32%, 34.66% and 25.92% higher than that of FS1, FS5 and CK, respectively. In 2024, the grain accumulation of KL5 was 9.48%, 24.76% and 13.28% higher than that of FS1, FS5 and CK, respectively. Over the course of two years, other leaves, stems, bracts, and cob parts of maize also showed a significant increase in KL5 compared to the other three treatments; Overall, the four treatments showed a trend of KL5 > FS1 and CK > FS5. It can be seen that during the R6 period, from the perspective of dry matter accumulation and distribution in various organs, there was no significant difference between FS1 and CK. FS5 has significantly lower dry matter accumulation in all organ parts compared to the other three treatments. KL5 can significantly increase the dry matter accumulation in various organ parts, especially in the grains area, with a more significant effect (Figure 4).

3.6. The Effect of the High Amount of Straw Pellets Returning to the Field on Dry Matter Transport in Maize

As shown in

Table 4. The differences in dry matter transport and grains contribution of maize for straw returning with different amounts and morphologies.

Year	Treatments	DMTA (g/Plant)		DMTE (%)		CRDMGafter (%)
		Leaf	Stem	Leaf	Stem
2023	FS1	11.34 ± 1.72 bc	9.84 ± 3.29 b	32.96 ± 5.78 ab	13.62 ± 5.83 b	86.18 ± 3.46 a
	FS5	15.07 ± 2.74 ab	22.57 ± 4.93 a	32.77 ± 5.46 ab	26.16 ± 5.72 a	78.97 ± 2.98 b
	KL5	10.01 ± 2.02 c	8.67 ± 1.95 b	25.14 ± 5.37 b	9.93 ± 2.92 b	89.77 ± 2.81 a
	CK	16.46 ± 2.65 a	21.56 ± 0.82 a	37.05 ± 3.48 a	22.55 ± 1.55 a	76.71 ± 0.35 b
2024	FS1	4.98 ± 0.93 b	11.13 ± 0.71 ab	12.47 ± 1.86 c	12.66 ± 1.89 a	91.14 ± 0.94 ab
	FS5	8.97 ± 0.90 a	12.28 ± 4.19 a	27.59 ± 4.89 a	16.07 ± 3.21 a	85.15 ± 2.45 c
	KL5	5.47 ± 1.12 b	7.27 ± 0.23 b	11.72 ± 2.58 c	7.86 ± 0.66 b	93.35 ± 0.63 a
	CK	7.43 ± 0.80 a	11.57 ± 1.06 ab	19.16 ± 1.96 b	13.03 ± 1.37 a	89.03 ± 1.22 b

Notes: Data are expressed as mean ± standard deviation. Different letters within the same column indicate significant difference at the 5% level.

, there are significant differences in the dry matter transport of maize under different treatments (p < 0.05). From the perspective of blade transport, the two treatments of FS5 and CK are significantly higher in blade transport volume than FS1 and KL5. In 2023, FS5 was higher than FS1 and KL5 at 32.89% and 50.50%, respectively; And in 2024, it was higher than FS1 and KL5 at 80.24% and 64.02%, respectively. In 2023, CK was higher than FS1 and KL5 at 45.15% and 64.38%, respectively; And in 2024, it was higher than FS1 and KL5 at 49.25% and 35.82%, respectively. In terms of leaf transport rate, CK had the highest leaf transport rate in 2023, which was 4.09%, 4.28% and 11.91% higher than FS1, FS5 and KL5, respectively; In 2024, the leaf transport rate of FS5 was the highest, higher than that of FS1, KL5 and CK treatments by 15.12%, 15.86% and 8.42%, respectively. It can be seen that the leaf transport rate under FS5 maintained a high level for two years, while the leaf transport rate under KL5 was significantly lower than the other three treatments. In terms of stem transport, the trend of stem transport volume and stem transport rate was the same. In 2023, FS5 and CK were higher than FS1 KL5, interms of stem transport, FS5 was higher than FS1, KL5, and CK by 129.26%, 160.38% and 4.67%, respectively. In terms of stem transport rate, it was higher than 12.54%, 16.23% and 3.61%, respectively; In 2024, KL5 was significantly lower than other treatments. In terms of stem transport, KL5 was 34.73%, 40.80% and 37.18% lower than FS1, FS5 and CK, respectively. In terms of stem transport rates, KL5 was lower than 3.87%, 8.21% and 5.17%, respectively. The overall contribution rate of post-anthesis dry matter accumulation to grain showed a trend of KL5 > FS1 > CK > FS5. In 2023, KL5 was higher than FS1, FS5 and CK by 4.17%, 13.68% and 17.02%, respectively; In 2024, they were higher than 2.43%, 9.63%, and 4.85% respectively. From the perspective of transport, the pattern of FS5 in leaf transport and stem transport was similar to that of CK, while the trend of FS5 and KL5 was closer. In comparison, the FS5 and CK treatments have higher leaf transport, stem transport than FS5 and CK treatments KL5; However, in terms of the contribution of post-anthesis dry matter accumulation to grains, KL5 has a higher contribution rate compared to other treatments (Table 4).

3.7. The Effect of the High Amount of Straw Pellets Returning to the Field on Maize Yield and Components

As shown in

Table 5. The differences in maize yield and yield components for straw returning with different amounts and morphologies.

Treatments	2023				2024
	Ear Number (Ear/ha)	Kernel Number (ear−1)	100-Kernel Weight (g)	Yield (t/ha)	Ear Number (Ear/ha)	Kernel Number (ear−1)	100-Kernel Weight (g)	Yield (t/ha)
FS1	67,949 ± 2937.43 a	535 ± 15.67 ab	33.72 ± 0.88 bc	12.25 ± 0.32 a	66,026 ± 2220.49 a	521 ± 30.85 ab	33.58 ± 0.96 b	12.22 ± 0.03 b
FS5	66,667 ± 4003.04 a	461 ± 18.67 c	34.67 ± 0.93 ab	11.23 ± 0.29 b	63,462 ± 1923.00 a	464 ± 44.69 c	34.14 ± 0.25 b	11.07 ± 0.47 c
KL5	69,231 ± 6661.47 a	568 ± 13.90 a	35.85 ± 0.94 a	12.34 ± 0.72 a	66,026 ± 2220.49 a	546 ± 31.96 a	36.21 ± 0.37 a	13.86 ± 0.17 a
CK	68,590 ± 2937.43 a	519 ± 29.69 b	32.73 ± 0.43 c	11.25 ± 0.18 b	66,026 ± 1110.24 a	490 ± 26.62 bc	33.70 ± 1.72 b	12.59 ± 0.25 b

Notes: Data are expressed as mean ± standard deviation. Different letters within the same column indicate significant difference at the 5% level.

, different treatments had no significant effect on the number of maize ears. The number of kernels per spike showed a trend of KL5 > FS1 > CK > FS5. In 2023, KL5 was 6.25%, 23.28% and 9.43% higher than FS1, FS5 and CK treatments, respectively; In 2024, they were higher than 4.65%, 17.56% and 11.33% respectively. There was a significant difference in 100-kernel weight, with KL5 having the highest 100-kernel weight in 2023, significantly higher than FS1, FS5 and CK, which are 6.32%, 3.41% and 9.54%, respectively, with CK having the lowest 100-kernel weight; In 2024, KL5 was significantly higher than other treatments, 7.81%, 6.04% and 7.43% higher than FS1, FS5 and CK treatments, respectively. There was no significant difference among the other three groups. There were also differences in the yield of the four treatments over the course of two years, with KL5 having the highest yield and FS5 having the lowest yield overall; In 2023, KL5 was 0.68%, 9.82% and 9.68% higher than FS1, FS5 and CK treatments, respectively. In 2024, KL5 was 13.39%, 25.15% and 10.06% higher than FS1, FS5 and CK treatments, respectively. It can be seen that the different forms and amounts of straw returning to the field have little effect on the number of ears per mu of maize, but have a significant impact on the number of kernels per ear, 100-kernel weight, and yield; KL5 and FS1 can effectively increase the number of kernels per ear in maize, with KL5 showing the most significant increase in grain number per ear, but FS5 showing a significant decrease in grain number per ear; KL5 can significantly increase 100-kernel weight, while FS1 and FS5 have no significant effect on the increase in grain weight; In terms of yield, the KL5 treatment demonstrated the best performance, achieving significant increases of 9.68% and 10.06% compared to the CK treatment over the two-year period. However, the yields of the two chopped treatments exhibited fluctuations across years under varying temperature and rainfall conditions. Specifically, FS1 showed an increase in yield in 2023, while FS5 experienced a decrease in yield in 2024 (Table 5).

4. Discussion

4.1. High Amount of Straw Pellets Returning to the Field Increased GLAI of Maize and Delayed Senescence Rate of Leaves

Leaf senescence is the last stage of plant development, in which chlorophyll, protein, and other substances are rapidly decomposed, and other macromolecular substances are degraded and assembled in a highly orderly manner and are transported to young leaves or reproductive organs [44,45]. Many studies have shown that the photosynthetic rate of post-anthesis populations can be increased and prolonged by delaying the senescence of maize leaves, maintaining leaf greenness and prolonging the photosynthetic time of leaves [46,47]. It has been found that straw mulching no-tillage promoted the mobilization of antioxidant and cell osmotic regulatory responses, optimized endogenous hormone signaling, maintained a high source of photosynthesis and maintained green color during the grain filling stage, thereby delaying the senescence of wheat leaves [48]. Returning straw to the field after anthesis can effectively maintain high photosynthetic capacity, slow down the degradation of chlorophyll content and reduce the senescence rate of maize leaves [49]. However, some argue that returning straw to the field does not always positively impact the leaves. Research indicates that maize canopy leaves are prone to nitrogen deficiency due to the influence of straw returning, which affects photosynthesis [50]. Through GLAI analysis of maize post-R1 stage, this article discovered that conventional straw returning does not significantly affect leaf senescence. Nevertheless, increasing the quantity of straw returned and altering its morphology have a notable impact on the senescence process of maize. When maize is in R1, its GLAI reaches the peak of the entire growth cycle [51]. Comparing the GLAI of the four treatments, it can be found that the GLAI of FS1 is closer to that of CK; As the amount of chopped straw increased, GLAI decreased, the GLAI of FS5 was significantly lower than the other three treatments; But the GLAI of KL5, which is also returned to the field at a 5-fold rate, is the highest among the four groups; Comparing the GLAI processed by the four groups, it can be found that the GLAI of FS1 is closer to that of CK; As the amount of chopped straw increased, GLAI decreased, the GLAI of FS5 was significantly lower than the other three treatments; But the GLAI of KL5, which is also returned to the field at a 5-fold rate, is the highest among the four groups; For straw in chopped form, an increase in the amount of return to the field will result in a decrease in GLAI in maize R1, but changing the return form to granules greatly improves the above situation. The leaf area index of the treatment with nitrogen fertilizer under straw returned reached its maximum at R1, while the leaf area index of the treatment without nitrogen fertilizer reached its maximum at VT. It is evident that the deficiency of nitrogen in crops leads to premature senescence of leaves [52]. Returning straw to the soil will affect the carbon nitrogen ratio of the soil and the level of nitrogen in the soil will also affect the performance of the leaves [53]. The reason for this situation may be due to the low temperature in winter, which inhibits the physiological activity of soil microorganisms in Northeast China, reduces the decomposition rate of returning straw and leads to insufficient release of effective nutrients [54,55]; The excessive amount of straw returned to the field hinders decomposition and competes with crop growth for nutrients, leading to plants experiencing adverse conditions and premature leaf senescence [56]. The pellets morphology results in a higher degree of straw fragmentation, thereby enhancing the ability to improve soil nutrients. Changes in soil nutrients affect changes in crop nutrients, which in turn influence changes in GLAI.

Through the analysis of GLAI fitting curves and senescence rates, it can be seen that the four treatments have a similar downward trend in GLAI during the slow senescence process of leaves in the first 35 days. The form and amount of straw returned to the field do not affect the speed of early leaf senescence; After 35 days, when the leaves entered the rapid senescence process, there were differences in the senescence process among the four treatments. The GLAI fitting curves of FS1 and CK had a high degree of overlap, the senescence rate trend of the leaves was consistent; The GLAI fitting curves of FS5 and CK have a high degree of overlap, but the senescence rate trend of the leaves is slightly higher than that of CK; The GLAI fitting curve of KL5 is smoother and the senescence rate is lower. The impact of returning straw to the field on the leaf senescence process is more concentrated in the rapid senescence stage after 35 days. The difference in straw dosage has no significant effect on leaf senescence rate, while the change in straw returning morphology is an important reason for the difference. From the above analysis, returning straw pellets to the field can delay the senescence of maize leaves, mainly reflected in increasing GLAI at R1 and delaying the senescence process of the middle and later stages of maize leaves. Previous studies have shown that returning straw pellets to the field has a significant impact on the green retention of maize leaves during the maturity stage. It has been found that in the third year of returning to the field, returning high amounts of straw pellets to the field has higher total nitrogen content, moderate soil carbon nitrogen ratio, lower bulk density and suitable soil pH value compared to returning chopped straw to the field [27]. Some studies also suggest that the amount of chopped straw returned to the field should not be too high. When the amount of straw returned to the field is ≥3%, the improvement effect of straw pellets on the activity of nitrogen cycling enzymes in various soils is significantly better than that of chopped straw [57]. Returning straw pellets to the field can significantly increase the activities of soil urease, alkaline phosphatase, sucrase and catalase, as well as the microbial biomass carbon and nitrogen content [22]. It can be seen that returning straw pellets to the field is highly likely to indirectly prolong the senescence rate of maize leaves by affecting the carbon and nitrogen nutrients in the soil and the activity of various enzymes in the soil.

4.2. High Amount of Straw Pellets Returning to the Field Enhances Accumulation of Dry Matter, Which Is Beneficial for the Distribution of Dry Matter to the Ear Parts of the Fruit

The yield of crops is determined by the accumulation, distribution and transport of dry matter during the growth period, there is a significant positive correlation between dry matter accumulation and yield [58]. Crop yield depends not only on the accumulation of photosynthetic products but also on their distribution to grains [59]. Maximizing the effective accumulation of pre-anthesis storage substances and their rational distribution among various organs, allocating more dry matter to grains and increasing the accumulation of dry matter after silk emergence are key factors for high crop yield [60]. Many studies suggest that inhibiting senescence to maintain leaf photosynthesis and accumulate more nutrients is an effective measure to improve crop yield and quality [61,62]. Through analysis of the accumulation of dry matter before maize anthesis, it was found that KL5 had a significantly higher accumulation of dry matter before maize anthesis than the other three groups, while the difference between the other three groups was not significant; The distribution of dry matter in different parts of maize in the four treatments of R1 was significantly higher in the KL5 treatment than in the other three groups in terms of the proportion of ear parts and dry matter accumulation. From the perspective of dry matter accumulation and distribution before maize anthesis, the difference in the amount of straw returned to the field does not have a significant impact on the accumulation and distribution of dry matter before maize anthesis, while the morphology of straw significantly affects the accumulation of dry matter. Compared with chopped straw returned to the field, the accumulation of dry matter in straw pellets is higher, the accumulation of the ear part is higher and the proportion of the ear part is higher. The early dry matter accumulation rate of maize mainly depends on the soil organic matter content before sowing and the early soil temperature accumulation [63]. Studies have found that the shorter the length of straw crushing, the higher the accumulated temperature of maize seedlings under returning conditions. Setting the straw crushing length to 1.5 cm or 3 cm can significantly improve the early dry matter accumulation of maize [16], which is consistent with the results of this study.

Through the analysis of dry matter accumulation after maize anthesis, it was found that KL5 was slightly higher than FS1, but significantly higher than CK and FS5. In the distribution of dry matter in each part of maize in the four groups at R6, except for the leaves, there was no significant difference, the KL5 of stem, bract leaves and grains was significantly higher than that of the other three groups, the dry matter of FS5 was lower than that of the other three groups. From the perspective of dry matter accumulation and distribution after maize cobs, the amount of conventional returning to the field can enhance the accumulation of dry matter in maize. However, as the amount of return to the field increases, it will cause a decrease in dry matter accumulation and affect the dry matter accumulation in grain parts. Changing the shape of chopped straw to granules and then increasing the amount of returning to the field not only does not reduce the accumulation of dry matter but also significantly increases the amount of dry matter in various parts. Combining with the process of leaf senescence, KL5 has the slowest leaf senescence rate and the highest accumulation of dry matter after anthesis; In the early stage of FS5, GLAI was the lowest and there was no significant delay in senescence rate. The accumulation of dry matter after anthesis was significantly lower than the other three treatments; Compared with CK, FS1 has a similar senescence rate and slightly higher accumulation of dry matter after anthesis. Some studies suggest that conventional straw returning can slow down the degradation rate of chlorophyll in the later stage of plant growth and increase the intensity of photosynthesis in plants [64]. The higher accumulation of post-anthesis dry matter in FS1 may be because returning straw to the field can increase dry matter accumulation by increasing post-anthesis chlorophyll content and enhancing photosynthesis. However, the increase in the amount of returning farmland has placed a burden on the absorption of nutrients by plants, resulting in a decrease in the accumulation of dry matter after FS5 anthesis. Many studies have found that maize needs to absorb more nitrogen after the silking stage to meet the plant’s absorption needs in the later stages of growth [65,66]. The cold climate in the maize planting areas of Northeast China often makes it difficult for a large number of straw fragments in the soil to decompose after returning straw to the field [67]. Excessive returns will further aggravate the difficulty of decay. However, returning straw pellets to the field is relatively easier to decompose and can promote rapid decomposition and nutrient release of straw, shortening the decomposition period of straw [18], which ensures the demand for dry matter accumulation after maize anthesis.

4.3. High Amount of Straw Pellets Returning to the Field Enhances Contribution of Post-Anthesis Dry Matter Accumulation to Grain

The formation of yield mainly comes from the transport of pre-anthesis storage substances and the accumulation of photosynthetic products in functional leaves after anthesis [68]. The redistribution ability of stems and leaves plays a crucial role when crops enter the growth stage [69]. Previous studies have shown that the formation of grain yield is mainly due to the transfer of dry matter from nutrient organs [70]. However, the high dry matter transport rate of maize nutritional organs can affect the production of photosynthetic products and the low dry matter transport rate is not conducive to grain filling [71]. This study found that FS5 and CK have higher levels of leaf and stem transport compared to FS1 and KL5. In the form of chopped straw, a low returning amount will reduce the dry matter transport in maize stem and leaf parts; In the form of straw pellets, a high return to the field can also reduce the dry matter transport in maize stems and leaves. From the perspective of the contribution rate of post-anthesis dry matter accumulation to grain, its pattern is opposite to the dry matter transport in maize stem and leaf parts, with straw pellets morphology contributing the highest to post-anthesis dry matter accumulation and grain contribution; Under the form of chopped straw, an increase in the amount of returning farmland will lead to a decrease in the contribution rate of post-anthesis dry matter accumulation to the grain. The contribution of dry matter to grains before silk emergence is limited, there is a negative correlation between grain yield and the rate of dry matter transfer from nutrient organs [50]. At high yield levels, the contribution of post-anthesis dry matter to the final grain yield is much greater [72]. Some studies suggest that premature senescence of leaves may lead to more nutrients in the leaves being reused, there is a trade-off between yield and nutrient reuse during the senescence process of leaves [73]. However, excessive transport of nutrients after crop anthesis can affect the production of photosynthetic products in the later stages of crop leaves, leading to accelerated leaf senescence, reduced grain filling rate and limiting yield improvement [74]. This is consistent with the research findings of this article, that crops produce a large amount of dry matter accumulation during the grain filling stage. If the nutrient organs produce excessively high dry matter transport rates during the early stages of reproductive growth, it may not have a positive effect on increasing yield.

4.4. High Amount of Straw Pellets Returning to the Field Increases Number of Kernels per Ear, 100 Kernel Weight, Ensuring Stable and High Crop Yields

The ultimate goal of delaying leaf senescence is to increase yield. Crop yield was affected by different tillage and straw management methods [75]. By analyzing the yield composition of these four treatments, it can be seen that the three treatments under straw returning to the field have no significant effect on the number of maize ears and are more affected by changes in the number of kernels per ear and 100-kernel weight, which is consistent with previous research results [26]. KL5 has a significant effect on increasing the number of kernels per ear and 100-kernel weight, while FS5 significantly reduces the number of kernels per ear of maize. Returning straw pellets to the field has a strong fertilizing effect on the soil. Studies have shown that granulation of straw can significantly improve its returning properties and enhance the soil’s ability to absorb straw [19]. Converting straw into straw pellets and returning them to the field can effectively improve soil tillage structure, enhance soil fertility and create a favorable growth environment for crops [76]. The increase in the amount of farmland returned did not increase the difficulty of decomposition but instead avoided the impact of excessive farmland volume on crop nutrient absorption. The increase in the amount of chopped straw returned to the field has a significant impact on the number of corn kernels per ear. Some studies suggest that the reduction in yield of chopped straw is due to the significant disturbance of microbial community structure caused by the application of a large amount of chopped straw [77]. The changes in soil community structure can affect the growth and development of aboveground crops. Combined with GLAI analysis, it was found that premature senescence of leaves can lead to insufficient accumulation of photosynthetic products in the early stage of grain formation, affecting the number of kernels in the ear and ultimately having adverse effects on yield.

Through the analysis of the yield data of the four treatments, it can be seen that the yield of KL5 is significantly higher than the other three treatments over the course of two years, the yield increasing effect is stable. In Dezhou City, Shandong Province, China, returning straw pellets to the field can effectively increase wheat and maize yields [78]. The combination of straw pellets and deep plowing can also achieve a dual improvement in the quality and yield of tobacco [79]. These are consistent with our experimental results. From the perspective of leaf senescence, the decrease in leaf senescence rate significantly increases crop yield. Someone found through correlation analysis between the senescence index of maize ear position leaves and yield that the degree of senescence of maize ear position leaves directly affects the yield of maize [80]; Some people have also found that the duration of grain filling in spring wheat is positively correlated with the duration of leaf area [81]. Returning straw pellets to the field can significantly increase the leaf area index, flag leaf chlorophyll content and net photosynthetic rate during the filling stage; And improve the flag leaf photosynthetic performance after anthesis [82]. This is consistent with our experimental results. The yield of the other three treatments showed a trend of FS1 being higher than CK and FS5 over the course of two years. FS1 can increase yield but the effect is unstable, while FS5 often leads to reduced yield. These two treatments did not fundamentally affect the senescence rate of leaves and the yield changes were probably more due to the photosynthetic properties of leaves and the changes in leaf area index at R1. The temperature and rainfall in different years under straw returning to the field affect the ecological environment of the soil, thereby affecting the decomposition of straw [83]. The alternating wet and dry conditions resulting from precipitation can influence the activity of decomposers and cause a shift between anaerobic and aerobic bacteria within the microorganisms decomposing straw. An increase in temperature within an optimal range boosts microbial biomass and activity, thereby accelerating the decomposition of straw [84]. The decomposition rate of straw will also affect the growth and development of aboveground plant leaves, thereby affecting yield [85]. Analysis of regional precipitation patterns in Heilongjiang Province (1961–2023) reveals a mean annual precipitation of 535.2 mm [86], with 2023 classified as a drought year and 2024 as a high-rainfall year. Monthly comparisons demonstrate significantly elevated rainfall in May-June 2024 versus 2023, alongside lower mean temperatures. These climatic divergences exerted differentiated impacts on chopped straw decomposition dynamics, particularly affecting nutrient release synchrony and microbial mineralization efficiency. At the same time, the increase in straw returning amount makes decomposition more difficult and easier to reduce yield; However, returning straw in granular form did not encounter the above-mentioned problems and the increase in returning amount did not affect the leaf area index of the leaves. Therefore, it can be seen that the change in straw form is the main reason for delaying leaf senescence and increasing crop yield.

5. Conclusions

Returning 75 t/ha of straw pellets to the black soil area in Northeast China can significantly delay the senescence of maize leaves and increase the accumulation of dry matter after anthesis by maintaining the effective photosynthetic area of leaves in the later stage of growth, thereby achieving the goal of increasing yield. The research can offer a practical and novel approach for straw return in the black soil region of Northeast China, provide a new technological pathway for enhancing crop productivity.