Experiments on the Influence of Corn Straw Morphological Combinations on Timely No-Tillage Sowing Soil Temperature and Moisture in Cold Regions

Abstract

After corn mechanical harvest in autumn in the cold region of northeast China, straw mulching leads to high soil topsoil water content and slow ground temperature rise in the spring sowing season, which may result in a no-tillage planter being unable to operate in time, delayed sowing date, poor seedling emergence quality, and low grain yield. Based on the problems of high moisture content and low temperature in soil topsoil in the spring sowing season in conservation tillage with straw mulching, an experimental study on timely sowing in the spring sowing season was carried out from the perspective of combination optimization of attribute parameters of straw mechanical treatment. Taking the four mechanical treatment attributes as experimental factors, including straw length, straw shape, mulching form (surface covering, inter-ridge covering, and inter-ridge mixed soil covering), and stubble height after corn harvest in autumn of 2019 as experimental factors, and soil topsoil moisture content and temperature during spring sowing in 2020 as evaluation indexes, a field plot experiment was carried out by four factors and three levels of orthogonal combination test method. The results showed that all the factors had significant influence on soil topsoil moisture content and soil topsoil temperature (p < 0.05). It is helpful to reduce the moisture content of soil topsoil and increase the temperature of soil topsoil by increasing the length of corn stalk, breaking the stalk moderately, covering the stalk between ridges, and increasing the stubble height of stalk. Design-Expert software was used to optimize the parameters. The results showed that when the length of straw was 150 mm, the shape of straw was half-cut (dividing straw into two parts evenly along the axis), the mulch was between ridges, and the stubble height was 600 mm, the moisture content of soil topsoil was 22~24% and the temperature of soil topsoil was higher than 8 °C during spring sowing. After corn harvest in autumn of 2021, corn stalks were treated in that year according to the results of parameter optimization combination. The soil topsoil moisture content and temperature were measured to be 24.4% and 8.2 °C, respectively, in the spring sowing season of 2022, which proved that the optimization results in this experiment were credible. The experimental results provide a technical reference for the implementation of conservation tillage with straw surface mulching in the cold region of northeast China.

1. Introduction

Straw mulching is an important part of conservation tillage, which creates a buffer layer for the interaction among water, heat, light, air, and soil, and has significant ecological and environmental effects in reducing soil water evaporation, regulating soil temperature, improving soil fertility ameliorating soil structure, and controlling soil erosion [1,2,3,4]. Heilongjiang Province, China belongs to a cold temperate continental monsoon climate, which has a long cold winter and abundant snowfall. After corn harvest in autumn, straw mulching on the surface easily leads to high water content of soil topsoil and the slow rise of ground temperature in the spring sowing season. This results in a delayed sowing date, poor emergence quality, and reduced grain yield [5,6,7]. Seeds need proper humidity and temperature to take root and emerge. According to different accumulated temperature zones, corn sowing is completed from mid-April to mid-May every year in the Heilongjiang Province [8]. Under the condition of conservation tillage with surface straw mulching, low temperature and humid soil delay the sowing date and inhibit early growth of crops; therefore, it is of great significance to take corresponding measures to increase soil temperature and reduce soil moisture content to ensure timely sowing [9].

The mechanical treatment properties of straw affect the radiation, heat, and water transfer between soil surface and atmosphere, thus affecting soil temperature and water content [10]. The results show that surface radiation, wind speed, and evaporation flux on soil surface are affected by crop stubble height [11]. The influence of stubble height on soil thawing and drying rate in the cold region in spring is greater than that of changing other physical characteristics of straw [12]. The snow cover in winter is affected by the height of corn stubble, and the enhanced snow cover on the soil with higher stubble reduces the infiltration of frost and accelerates the thawing of soil in spring [13]. Straw mulching thickness and stacking density are important physical characteristics affecting soil water evaporation [14,15,16,17], straw mulching thickness is related to straw yield and straw mulching mode, while straw stacking density is related to straw moisture content and geometry.

Mechanical treatment attributes such as corn straw geometry size, mulching form, mulching amount, and stubble height have different effects on soil moisture content and soil temperature in the spring sowing season in the cold region. These properties of straw mechanical treatment can be controlled by manual intervention (harvesting or sowing), so the study of straw mechanical treatment properties has great practical application value and scientific research significance [18]. The existing studies mainly focus on the effects of mulch form or stubble height on crop growth characteristics, crop yield, and soil chemical or physical characteristics [19,20]. There is no report on timely sowing in the spring sowing season in the cold region. Whether the interaction among mechanical treatment attributes of straw has significant influence on soil moisture content and soil temperature has not been studied. Moreover, the existing research has failed to provide a clear technical parameter value or value range for the mechanical treatment attributes of straw when implementing conservation tillage with surface straw mulching in specific areas, which is of weak significance for guiding actual production.

In this study, different combinations of four mechanical treatment attributes, straw length, straw shape, mulch form, and stubble height, were used to analyze the effects of the parameters and their interactions on soil moisture content and soil tillage temperature in the spring sowing season, and the parameter combination was optimized. It provides a theoretical reference for the treatment of straw in cooler areas during the spring sowing season in cold regions.

2. Materials and Methods

2.1. Test Materials

The experiment was carried out in Xinxing Village (48°14′ N, 125°40′ E), Beilian Town, Keshan County, Qiqihar City, Heilongjiang Province from September 2019 to May 2021. The climate of the pilot site was cold temperate continental monsoon climate, with annual average temperature of 2.4 °C, annual average effective accumulated temperature of 2400 °C, frost-free period of 120~124 days, annual average precipitation of 500 mm, rainfall concentrated in June, July, and August every year, annual average wind speed of 4 m/s, and strong wind weather from April to June and September to October every year, with the maximum wind force reaching Grade 8. The soil in the pilot site was clay soil, with organic matter 44.21%, alkali-hydrolysable nitrogen 203 mg/kg, available phosphorus 26.5 mg/kg, and available potassium 269 mg/kg in 0~100 mm soil layer. The corn variety was Hetian No. 4, which is sown from 23 April to 10 May and harvested from 20 September to 10 October every year under the condition of soil preparation in autumn. A total of 650 mm ridge spacing, and single row cultivation mode were adopted for each treatment, and the planting density was 70,000 plants/hm².

2.2. Test Methods

Four factors and three levels of orthogonal combination test method was adopted in the experiment. In order to consider the influence of interaction among factors on the evaluation index, L₂₇ (3¹³) orthogonal table was used to design the experiment. Straw length, straw shape, mulching form, and stubble height were taken as experimental factors, and soil topsoil moisture content and temperature as evaluation indexes. There were 27 groups of treatments. Among them, according to the relevant standards of GB/T 24675.6-2009 “Straw Crushing and Returning Machine for Conservation Tillage Machinery” and NY/T500-2015 “Operation Quality of Straw Returning Machine”, the minimum length of corn straw returning is not more than 100 mm, and 100 mm is set as the central level of straw length. Straw morphology is characterized by axial cutting times of straw, which can be divided into three forms: non-cut, half-cut, and opposite-cut (dividing straw into four parts evenly along the axis). Half-cut refers to dividing straw into two parts evenly along the axis, while opposite-cut refers to dividing straw into four parts evenly along the axis. The covering forms are surface covering, inter-ridge covering, and inter-ridge mixed soil covering, in which the surface covering is straw covering the ridge surface, the inter-ridge covering is straw covering between two ridges, and the inter-ridge mixed soil covering is straw and soil mixing covering between two ridges. According to the ear height of corn varieties and the highest cutting height of the corn harvester, 600 mm was set as the upper limit of stubble height. The test factor levels are shown in Figure 1 and

Table 1. Table of test factor levels for the influence of corn straw morphological combinations on soil temperature and moisture.

Levels	Test Factors
	Straw Length A/mm	Straw Shape B	Mulching Form C	Stubble Height D/mm
1	50	Non-cut	Surface covering	100
2	100	Half-cut	Inter-ridge covering	350
3	150	Opposite-cut	Inter-ridge mixed soil covering	600

.

2.3. Test Implementation

According to the test program, 27 test plots were planned in the experimental field, each plot had a ridge distance of 650 mm, a plot length (ridge length) of 15 m and a plot width of 3.9 m, with a total of six ridges, and a 1.3 m isolation belt was set between adjacent plots. On 21 September 2019, corn was harvested manually, and corn stalks were stubbled and cut, and then harvested and left the field according to the planning of the experimental community. After leaving the field, the corn straw was treated according to the length and shape of straw according to the test program, and the treated straw was manually laid in the planned test plot. Under the condition of soil preparation in autumn, the soil moisture content in topsoil is 22~24% [21], and the temperature of topsoil is stable above 8 °C [22,23], which can be used as the basis for sowing operation. On 28 April 2020, the soil topsoil moisture content and temperature within 0~100 mm of each treatment in the test plot were measured (the soil topsoil humidity and temperature met the requirements for sowing under the autumn soil preparation conditions from 24 to 27 April 2020). The temperature measurement time was 6:00 a.m., 10:00 a.m., 14:00 p.m., 18:00 p.m., and 22:00 p.m. on 28 April 2020, and 2:00 a.m. and 6:00 a.m. on 29 April 2020. The average value of the measured temperature at each time point was taken as the final value of the measuring point. The measurement and calculation methods of soil topsoil moisture content and temperature were as follows:

(1)

Measuring method of soil topsoil moisture content: The undisturbed soil in 0~100 mm soil layer was taken by ring knife and repeated three times. The total weight of fresh soil and aluminum box were weighed with JE1002 electronic balance (Shanghai Puchun Metrology Instrument Co., Ltd. Shanghai, China, resolution: 0.01 g). DG-101-1S electric constant temperature drying oven (Shaoxing Subo Instrument Co., Ltd. Shaoxing, China resolution: ±1 °C) was used to dry soil samples, and the total weight of dry soil and aluminum box was measured. According to Equation (1), soil topsoil moisture content was calculated.

$$\rho =\frac{1}{3}{\displaystyle \sum _{i=1}^3\frac{m_{\mathrm{si}}-m_{\mathrm{gi}}}{m_{\mathrm{gi}}-m}\times 100\%}$$

(1)

where m_si is the weight of fresh soil and aluminum box measured for the i-th time, g; m_gi is the weight of dry soil and aluminum box measured for the i-th time, g; and m_l is the weight of aluminum box, g.

(2)

Measuring method of soil topsoil temperature: The soil temperature data of 0~100 mm soil depth in each treatment was measured by TPJ-21 soil temperature recorder (Tuopu Yunnong Company, Zhejiang Province, China, resolution: 0.1 °C) and repeated three times. The average value was taken as the final test value.

The data were processed by Design Expert 8.0.6 software (Stat-Ease, Inc., Minneapolis, MN, USA), and the significant factors were analyzed by ANOVA. The significant level of all statistical differences was p < 0.05.

3. Results and Discussion

According to L₂₇ (3¹³) orthogonal table, each treatment is designed and planned, and the soil topsoil moisture content and temperature are obtained by measuring and calculating the test results, as shown in

Table 2. Table of test scheme and results for the influence of corn straw morphological combinations on soil temperature and moisture.

No.	A/mm	B	C	D/mm	Soil Topsoil Moisture Content/%	Soil Topsoil Temperature/°C
1	50	Non-cut	Surface covering	100	37.4	6.2
2	50	Non-cut	Inter-ridge covering	350	31.3	6.9
3	50	Non-cut	Inter-ridge mixed soil covering	600	29.4	7.5
4	50	Half-cut	Surface covering	350	35.3	6.5
5	50	Half-cut	Inter-ridge covering	600	30.3	7.8
6	50	Half-cut	Inter-ridge mixed soil covering	100	35.2	6.7
7	50	Opposite-cut	Surface covering	600	28.5	7.3
8	50	Opposite-cut	Inter-ridge covering	100	34.4	6.9
9	50	Opposite-cut	Inter-ridge mixed soil covering	350	33.2	7.1
10	100	Non-cut	Surface covering	350	36.3	6.7
11	100	Non-cut	Inter-ridge covering	600	27.3	8.2
12	100	Non-cut	Inter-ridge mixed soil covering	100	33.1	6.9
13	100	Half-cut	Surface covering	600	30.3	8.1
14	100	Half-cut	Inter-ridge covering	100	28.2	7.2
15	100	Half-cut	Inter-ridge mixed soil covering	350	30.4	7.8
16	100	Opposite-cut	Surface covering	100	35.1	6.6
17	100	Opposite-cut	Inter-ridge covering	350	29.3	7.7
18	100	Opposite-cut	Inter-ridge mixed soil covering	600	27.5	8.4
19	150	Non-cut	Surface covering	600	28.3	8.3
20	150	Non-cut	Inter-ridge covering	100	32.2	7.2
21	150	Non-cut	Inter-ridge mixed soil covering	350	31.4	7.9
22	150	Half-cut	Surface covering	100	27.2	6.8
23	150	Half-cut	Inter-ridge covering	350	27.4	7.6
24	150	Half-cut	Inter-ridge mixed soil covering	600	26.2	8.7
25	150	Opposite-cut	Surface covering	350	30.4	7.5
26	150	Opposite-cut	Inter-ridge covering	600	26.3	8.9
27	150	Opposite-cut	Inter-ridge mixed soil covering	100	31.1	7.4

. According to the statistics of the recent 10 years, the sowing date of corn in conventional autumn tillage in this area is from 20 April to 10 May. When the temperature of 0~100 mm soil topsoil reaches 8~10 °C and the soil moisture content reaches 22~24%, the soil temperature and moisture content meet the suitable sowing and mechanical operation conditions. According to the combined test results of some parameters obtained from the orthogonal test, there are no attribute combinations of straw mechanical treatment that meet the requirements of soil topsoil temperature and soil topsoil moisture content at the same time, but there are five parameter combinations in the combination of 600 mm stubble height, and the soil topsoil temperature exceeds 8 °C. It can be preliminarily seen that high stubble retention has a positive effect on improving soil topsoil temperature. However, in terms of data value, the soil topsoil temperature is generally at a lower level suitable for sowing temperature, and the highest soil topsoil temperature is 8.9 °C.

Four factors and three levels orthogonal combination test method were adopted in the experiment, L₂₇ (3¹³) orthogonal table was used to design the experiment. Data were analyzed using multi-factor ANOVA. Design Expert 8.0.6 software was used to analyze the variance of the test results, and the results are shown in

Table 3. Table of analysis of variance of test results for the influence of corn straw morphological combinations on soil temperature and moisture.

Evaluation Indexes	Source	Sum of Squares	df	Mean Square	F	p
Soil topsoil moisture content	A	66.13	2	33.06	18.72	0.0004
	B	15.16	2	7.58	4.29	0.0451
	C	27.14	2	13.57	7.68	0.0095
	D	96.97	2	48.48	27.45	<0.0001
	AB	19.87	4	4.97	2.81	0.0842
	AC	27.98	4	6.99	3.96	0.0353
	Residual	17.66	10	1.77
	Cor Total	270.91	26
Soil topsoil temperature	A	3.12	2	1.56	147.1	<0.0001
	B	0.23	2	0.12	11.05	0.0029
	C	1.43	2	0.72	67.69	<0.0001
	D	7.35	2	3.67	346.82	<0.0001
	AB	0.23	4	0.056	5.33	0.0146
	AD	0.26	4	0.065	6.12	0.0093
	Residual	0.11	10	0.011
	Cor Total	12.72	26

Note: 0.05 < p < 0.1 is significant effect, p < 0.05 is extremely significant effect.

. The interaction of straw length and straw form has a significant effect on soil topsoil moisture content, and the interaction of straw length and mulch form has a significant effect on soil topsoil moisture content. The primary and secondary relationship of each experimental factor on soil topsoil moisture content from large to small is stubble height, straw length, mulching form, and straw form. All factors have significant influence on soil topsoil temperature. The interaction between straw length and straw morphology has a very significant effect on soil topsoil temperature, and the interaction between straw length and stubble height has a very significant effect on soil topsoil temperature. The primary and secondary relationship of each experimental factor on soil topsoil temperature from large to small is stubble height, straw length, mulching form, and straw form.

On the basis of variance analysis, the model of soil topsoil moisture content and soil topsoil temperature was analyzed. As shown in Figure 2, the residual of the two models obeys the normal distribution law, which shows that the models established between soil topsoil moisture content, soil topsoil temperature, and experimental factors are reliable and reasonable, and can be used for subsequent parameter combination optimization and evaluation index prediction analysis.

3.1. Analysis of the Influence of Experimental Factors on Soil Topsoil Moisture Content

Through the analysis of variance, it can be seen that the interaction of straw length and straw morphology has a significant impact on soil topsoil moisture content. When the mulching form is surface mulching and the stubble height is 350 mm, the influence of straw length and straw form on soil topsoil moisture content is shown in Figure 3a. Under the condition of semi-split straw form, the soil topsoil moisture content gradually decreased with the increase in straw length. Studies have shown that the increase in straw length accelerates water infiltration [24,25], which promotes the drying of soil surface and reduces the water content of soil topsoil. With the increase in straw length, the density of straw layer decreased, the gap between straws increased, and the evaporation rate of soil water accelerated, which promoted the drying of soil, which was consistent with experimental results. Under the condition of non-cut and opposite-cut straw, the soil topsoil moisture content increased first and then decreased with the increase in straw length. When the length of straw is 50 mm, the moisture content of topsoil is the smallest under the condition of opposite cross-section and the largest under the condition of semi-cross-section. When the length of straw is 100 mm and 150 mm, the moisture content of soil topsoil is the smallest under the condition of half-cut straw and the largest under the condition of non-cut straw. The results showed that in the actual operation process, the mechanical treatment of straw morphology should be moderate, and it is possible to increase soil moisture content if it is too large or too small [26].

When the mulching form is inter-ridge mulching (inter-ridge mixed soil mulching) and the stubble height is 350 mm, the influence of straw length and straw form on the soil topsoil moisture content, as shown in Figure 3b,c. Under any straw form condition, the soil topsoil moisture content gradually decreases with the increase in straw length. When the length of straw is 50 mm, the moisture content of soil topsoil is the smallest under the condition of opposite cross-section and the largest under the condition of semi-cross-section; when the length of straw is 100 mm and 150 mm, the moisture content of soil topsoil is the smallest under the condition of half-cut straw and the largest under the condition of non-cut straw; therefore, the length of straw can be appropriately increased under the actual operation conditions, thus accelerating soil drying.

Analysis of variance showed that the interaction of straw length and mulch form had significant influence on soil topsoil moisture content. When the straw shape is half-cut and the stubble height is 350 mm, the influence of straw length and mulching form on the water content of soil topsoil is shown in Figure 4. When the straw length is 50 mm and 100 mm, the soil topsoil moisture content is the smallest under the condition of ridge mulching and the largest under the condition of surface mulching; when the length of straw is 150 mm, the moisture content of topsoil is the smallest under the condition of inter-ridge mulching and the largest under the condition of inter-ridge mixed soil mulching. Under the condition of inter-ridge mulching and inter-ridge mixed soil mulching, most of straw is located at the bottom of ridge, and there is no straw mulching at the top and side of ridge, which can improve the evaporation rate of water and accelerate the drying of soil topsoil, which is consistent with the experimental results. The moisture content of soil topsoil under the condition of inter-ridge mulching is lower than that under the condition of inter-ridge mixed soil mulching, which may be due to the fact that the density of straw mixed with soil after winter precipitation is higher than that of pure inter-ridge mulching, and it has stronger ability to store water and prevent water evaporation. The results are consistent with the research of Jiangtao Qi et al. (2021) [27]. The results show that straw mulching between ridges has a good effect on reducing soil topsoil moisture content in the cold region of northeast China.

The moisture content of soil topsoil gradually decreases, as shown in Figure 5. The main reason is that the biomass of corn straw is mostly concentrated in 500 mm above the root. When the stubble height reaches 600 mm, the actual amount of straw covered on the surface is less, and the exposed area of the surface increases, which is beneficial to increase the contact area between soil and flowing air and sunlight, accelerate the drying speed of soil, and reduce the moisture content of soil topsoil. It may be due to the increase in stubble height that the flow area of airflow passing through corn stubble group decreases, which promotes the increase in surface wind speed, accelerates the evaporation of topsoil water, and further reduces the water content of topsoil [28].

3.2. Analysis of the Influence of Experimental Factors on Soil Topsoil Temperature

Analysis of variance shows that the interaction between straw length and straw morphology has a significant impact on soil topsoil temperature. When the mulching form is ridge mulching and the stubble height is 350 mm, the influence of straw length and straw form on soil topsoil moisture content is shown in Figure 6a. Under any straw morphology, the soil topsoil temperature increased with the increase in straw length. According to the change law of soil topsoil moisture content, under this condition, soil topsoil moisture content decreases, soil heat transfer enhances, and soil topsoil temperature increases, which is consistent with Xi Juan et al. [29]. When the length of straw is 50 mm, the soil topsoil temperature is the lowest under the condition of non-cut straw form, and the highest under the condition of opposite cutting straw form; when the length of straw is 100 mm, the soil topsoil temperature is the lowest under the condition of non-cut straw form, and the highest under the condition of half-cut straw form; when the length of straw is 150 mm, the soil topsoil temperature is the lowest under the condition of half-cut of straw, and the highest under the condition of opposite section of straw [30,31]. From the measured data, it can be found that under the condition of any straw length, the temperature difference of soil topsoil is in the range of 0.1~0.3 °C when the straw morphology is half-cut and opposite-cut; it can be considered that both half-cut and opposite-cut straw morphology have the ability to improve ground temperature, which may be related to the thermal conductivity of corn straw husk and inner pulp.

As can be seen from Figure 6b, when the straw form is half-cut and the mulching form is inter-ridge mulching, the soil topsoil temperature increases with the increase in stubble height under the condition of any straw length [32]. This may be due to the strong snow fixation ability in winter, while snow cover helps to delay the heat loss and frost infiltration in the topsoil, and the heat retained in the topsoil in spring helps to accelerate the thawing in spring, thus increasing the temperature of the topsoil [33]. Although stubble height is helpful to thaw soil topsoil quickly, there is a difference in temperature compared with tillage without straw mulch.

When the straw length is 100 mm, the straw form is half-cut and the stubble height is 350 mm, the soil topsoil temperature under the conditions of inter-ridge mulching and inter-ridge mixed soil mulching is higher than that under the conditions of surface mulching, and there is no significant difference between inter-ridge mulching and inter-ridge mixed soil mulching. Compared with surface mulching, the surface coverage rate of straw is lower, and more soil is exposed to light, which accelerates the temperature rise of topsoil [34]. Through the above analysis, it can be seen that increasing the length of corn straw, moderately breaking straw, covering straw between ridges, and increasing the stubble height of straw are all helpful to improve the soil topsoil temperature.

3.3. Parameter Combination Optimization

The factors in this experiment are straw mechanical treatment attributes, and the level values of each factor can be obtained by adjusting the relevant parameters of the harvester or straw returning machine, but the treatment of each parameter is related to the operation efficiency, energy consumption, and complexity of machine restructuring. This optimization is based on the basic principles of improving operation efficiency and reducing energy consumption, which is maximizing straw length and reducing straw fragmentation on the premise of meeting soil topsoil moisture content and temperature. Design Expert 8.0.6 software (Stat-Ease, Inc., Minneapolis, MN, USA) was used to optimize the parameters, and the optimization goal is that the soil topsoil moisture content is 22~24%, and the soil topsoil temperature is higher than 8 °C. The final optimization parameters are: the length of straw is 150 mm, the shape of straw is half-cut, the covering form is ridge covering, and the stubble height is 600 mm. On 30 April 2021, the temperature of 0~100 mm soil topsoil prepared by conventional tillage is stable above 8 °C. The soil topsoil in the plot planned by optimized combination parameters is measured, and the results show that the soil topsoil moisture content is 24.4%, and the soil topsoil temperature is 8.2 °C, which is consistent with the optimized results.

4. Conclusions

Stubble height, straw length, mulching form, and straw form have significant influence on soil topsoil moisture content and soil topsoil temperature. When the corn straw length is 150 mm, the straw shape is half-cut, the mulching form is ridge mulching, and the stubble height is 600 mm, the timely no-tillage sowing operation can be realized under the protection tillage condition of corn straw mulching in the spring sowing season. The soil topsoil moisture content is between 22% and 24%, and the soil topsoil temperature is higher than 8 °C.

It is helpful to reduce the moisture content of soil topsoil and increase the temperature of soil topsoil by increasing the length of corn stalk, breaking the stalk moderately, covering the stalk between ridges, and increasing the stubble height of stalk. It provides a basis for the implementation of conservation tillage based on straw mulching in the cold regions of northeast China and provides a reference for the development of corresponding agricultural machinery.

This study is carried out under specific soil, climate, and cultivation conditions. For other environmental and working conditions, whether the optimized parameter combination meets the requirements needs further confirmation.