Effects of corn straw cover years on soil organic matter and compactness in typical black soil areas of Songnen Plain

China is a huge agricultural country with rich straw resources. However, the problem to be solved urgently is to find means to use crop straw effectively and reasonably. To study the influence of no-tillage straw mulching measures on soil compactness and soil organic matter, taking the black soil straw mulching no-tillage test field in Gaojia Village, Lishu County, Jilin Province as the research object, the characteristics of soil compactness and organic matter in different tillage layers under different cover years of no-till straw are discussed. The results show the following: (1) in the 0–50 cm tillage layer, the highest treatment method of soil organic matter is NT1 (no-tillage corn stover mulching for one year), with an average value of 2.62%, and the soil organic matter content of NT1 treatment increased by 57.46% compared with RT (ridge tillage). (2) The compactness of surface soil (0–20 cm) no-tillage soil is higher than RT, with an increase of more than 6.87%, and the compactness of deep soil (20–50 cm) no-tillage soil is lower than RT, with a decrease of more than 23.48%. (3) No-tillage can improve the agglomeration structure of the surface soil, increase the moisture content of the surface soil, reduce the volume of the surface soil, and improve the organic matter content of the surface soil, thus increasing the firmness of the surface soil. This study can provide a theoretical basis for promoting the comprehensive utilization of straw resources and developing eco-agriculture.


Introduction
The Northeast Black Land Area is an important agricultural product base in China, mainly growing corn, with a total cultivated land area of about 1.853 × 10 7 hm 2 (Wang et al. 2021). Therefore, the use of straw after a maize harvest cannot be ignored. Maize straw is an important renewable resource with a high utilization value (Feng et al. 2011;Hoogmoed 2009). Most of the crop straws in China have been removed or burned in situ, reducing soil water, and subsequently exacerbating soil impoverishment and environment degradation (Bagriacik et al. 2021;Shinde et al. 2021).
Straw cover to the field can effectively reduce soil pressure, slow down soil wind and water erosion, and increase soil water storage and moisture retention capacity (Sadeghpour et al. 2014;Wang et al. 2020). Especially for rain-fed agricultural areas, it can effectively improve soil water retention and ensure stable grain yield (Diatta et al. 2020).
In recent years, the application research of straw cover to farmland has achieved remarkable results (Kaija et al. 2016). Xu et al. (2015) shows that straw cover to the field and loosen soil measures can not only increase the nutrient content of soil but also increase the yield of corn seeds and straw. Zhao et al. (2015) show that soil deep pine combined with straw to return to the field can reduce soil capacity and increase soil microbial number and enzyme activity. Fernández-Ugalde et al. (2009) shows that straw covering farmland can improve soil structure characteristics and increase soil moisture utilization, thus increasing crop yields. At present, the research on straw cover to the field mainly focuses on soil improvement, crop growth, crop yield, etc. There are few studies straw cover years to the soil organic matter and soil compactness.
Black soil is recognized as the most fertile and extremely precious high-quality land in the world. It is mainly characterized by high fertility and suitable farming. The typical black soil area in northeast China is an important grain production area and commercial grain base, which plays a decisive role in ensuring national food security. Therefore, this paper takes the black soil straw mulching test field in Gaojia Village, Lishu County, Jilin Province as the research object and analyzes the soil organic matter content and soil compactness of maize straw cover under the condition of no tillage conditions and discusses the characteristics of soil compactness and organic matter at different tillage depths. It aims to provide theoretical reference for promoting the comprehensive utilization of straw resources and the sustainable development of ecological agriculture.

Study area
The study was conducted in a protective farming research and development base of Shenyang Institute of Applied Ecology, Chinese Academy of Sciences (Gaojia Village, Lishu County, Jilin Province; 43°18′51″-43°19′12″N, 124°14′26″-124°14′31″E) (Fig. 1). Before the long-term field positioning began, the experimental field was a corn planting area with many years of traditional ridge farming history. The area has a temperate semi-humid monsoon climate with four distinct seasons and sufficient light. The average annual temperature of 5.8 °C, average annual sunshine of 2 698.5 h, and average annual precipitation of 577.2 mm; the precipitation is mainly concentrated in June-August, which is characterized by the typical black soil area of the Songnen Plain. The soil parent matter in the experimental area is loamy clay, and the soil type is medium-level black soil (Du et al. 2015).

Experimental design
The test adopts a random group design, and five different treatments of corn straw are carried out in the sample area, as follows: Ridge tillage (RT), no-tillage without corn stover mulching (NT 0 ), no-tillage corn stover mulching for one year (NT 1 ), no-tillage corn stover mulching for two years (NT 2 ), and no-tillage corn stover mulching for three years (NT 3 ). Each group of experimental treatments was repeated four times. A total of 20 communities were designed in a random area, with an area of 8.7 × 30 m 2 . The sample area is generally located on the northwest side (Fig. 2).
Traditional ridge tillage leaves about 15 cm of stubble every autumn harvest, and all the straws are removed from the farmland without cover. In the spring of the following year, the stubble was extinguished and ploughed and ridged (18-25 cm). Under no-tillage conditions, the treatment method of corn straw is to leave about 30 cm of stubble after the autumn harvest every year. According to the corn straw cover years, the corn straw is evenly covered on the surface, and all the remaining straw will be removed. The soil was further disturbed only for sowing and fertilization. The ridge height maintained was 15 cm, and the ridge spacing was 60 cm. The maize seeds used in the study area was the same as the amount of fertilization. The Jilin Kangda 2BMZF-4 no-tillage seeding machine was used to sow the seeds, which can complete sowing, fertilization, and other works simultaneously under the condition of surface straw cover.

Experimental method
The measurement date of soil compactness is from June to July 2020 to 2022. The date of soil compactness measurement is June-July 2020-2022. The instrument used is the SC900 soil compactness meter of spectrum in the United States. The soil compaction was measured at a depth of 0-45 cm, at a minimum spacing of 2.5 cm. The soil compaction meter represented the measured soil compaction based on the theoretical kPa value of the manometer. The "triangle" three-point measurement method is used to measure the soil compactness value (Peng et al. 2019). Repeat 3 times for each measurement position, taking the average value as the soil compactness value at that point. When measuring, the edge of the sample area was avoided. Finally resampled into six gradients (0-5 cm, 5-10 cm, 10-20 cm, 20-30 cm, 30-40 cm, and 40-50 cm).
The five-point sampling method is used to collect soil layer soil with 0-5 cm, 5-10 cm, 10-20 cm, 20-30 cm, 30-40 cm, and 40-50 cm. Each soil sample is put into a polyethylene self-sealing bag, labeled, and then brought back to the laboratory. Remove the impurities in the sample and put the sample into the indoor vent for drying. Finally, the content of soil organic matter was determined by potassium dichromate-external heating volumetric method (Elliott et al. 2016;Jiménez et al. 2017). The data were analyzed and plotted using Excel 2016 and DPS software.

Changes in soil organic matter content in different crop layers
Soil organic matter is an important index of soil fertility, and the straw cover can improve soil fertility and improve soil structure. The treatment of different straw cover years has a great impact on the change of soil organic matter content in different cultivated layers. As can be seen from Fig. 3, under the treatment of the same test, the content of soil organic matter gradually decreases with the increase of soil depth. In the 0-30 cm tillage layer, the soil organic matter content of different straw cover years is NT 1 > RT > NT 0 > NT 3 > NT 2 ; in the 30-50 cm tillage layer, the soil organic matter content of different straw cover years is NT 1 > NT 0 > RT > NT 3 > NT 2 .

Changes in soil compaction in different crop layers
Soil compactness is an important index reflecting the physical characteristics of soil, and tillage has a significant effect on soil compactness. According to the comprehensive analysis in Fig. 4, the compactness of the no-tillage soil in the 0-20 cm cultivated layer is higher than that in RT, and the compactness of the no-tillage soil in the 20-50 cm cultivated layer is lower than that in RT. In the 0-20 cm tillage layer, the soil compactness of different straw covers increases with the increase of soil depth. The size of soil compactness is NT 3 > NT 2 > NT 1 > NT 0 > RT. 20-30 and 40-50 cm tillage layer, the soil compactness of different straw cover years is RT > NT 3 > NT 2 > NT 1 > NT 0 . 30-40 cm tillage layer, the soil compactness of different straw cover years is RT > NT 3 > NT 1 > NT 0 > NT 2 .
Compared with RT, in the 0-10 cm tillage layer, the soil compactness of NT 0 is significantly higher than that of RT, and the soil compactness increased by 343 and 436 kPa, respectively, an increase of 165.70% and 208.61%, respectively. In the 10-20 cm tillage layer, the soil compactness of NT 0 does not change much compared with RT. In the 20-50 cm tillage layer, RT increases significantly with the increase of soil depth, and the soil compactness of NT 0 does not change significantly with the increase of soil depth. This may be because traditional farming loosened the soil surface, resulting in a smaller soil compactness. The deep soil is compacted by mechanical tillage, resulting in increased soil compactness. Compared with RT, NT 1 , NT 2 , and NT 3 increased soil compactness in the 0-5 cm tillage layer by 169.08%, 184.54%, and 188.89% compared with RT. The soil compactness of NT 1 , NT 2 , and NT 3 in the 5-10 cm tillage layer increased by 389.95%, 423.44%, and 437.32%. NT 1 , NT 2 , and NT 3 in the 10-20 cm tillage layer, and the soil compactness does not change much. NT 1 , NT 2 , and NT 3 decreased by 25.11%, 24.25%, and 23.48% in the 20-30 cm tillage layer. NT 1 , NT 2 , and NT 3 decreased by 45.35%, 50.49%, and 41.46% in the 30-40 cm tillage layer. NT 1 , NT 2 , and NT 3 decreased by 40.28%, 39.88%, and 39.43% in the 40-50 cm tillage layer. This shows that under no-tillage conditions, the straw cover can significantly improve the compactness of the surface soil and make the soil loose.

Effects of soil physical properties on soil compactness and organic matter
Soil moisture content is an important feature that affects soil compactness and soil organic matter. Under no-tillage conditions, the soil moisture content changes significantly. As shown in Fig. 5, in the 0-10 cm tillage layer, the soil moisture content varies greatly under no-tillage conditions, and the soil moisture content increases with the increase of straw covers. In the 10-50 cm tillage layer, the difference in soil moisture content is small, and with the increase of soil depth, the overall moisture content generally shows a downward trend, while RT soil moisture content shows a trend of increasing first and then decreasing. This is mainly due to the profound impact of soil moisture content on soil physical, chemical and biological processes. Straw cover can effectively reduce the contact between the soil and the air, thus reducing the emission of water. Soil moisture content also affects the organic matter content of the soil surface, improves the soil structure, thus changing soil compactness, and promoting the potential nutrient release of the soil.
Soil capacity is an important physical indicator of soil (Dong et al. 2013). As can be seen from Fig. 6, the change trend of soil capacity and soil compactness is consistent, which shows that soil capacity has a certain impact on soil compactness. In the same tillage layer, with the increase of straw coverage, the soil capacity shows a downward trend. RT shows a tendency to gradually increase with the increase of depth in the tillage layer. Under no-tillage conditions, the soil capacity increases first and then decreases with the increase of depth under different treatment methods. Among them, the soil capacity in the 10-20 tillage layer reaches the largest, with NT 0 is 1.47 g/cm 3 , NT 1 is 1.42 g/cm 3 . This may be because different straw coverage years significantly reduce soil capacity and effectively reduce the runoff of soil moisture in the tillage layer, thus significantly reducing soil compactness and hardness of 0-20 cm, while the impact on soil compactness of 20-30 cm is relatively small.
Soil aggregate is an individual formed by gel accumulation of soil particles (Su et al. 2017). As shown from Fig. 7, the distribution of granular aggregates on the surface of the soil is basically the same, all of which are small particle aggregates (0.25-2 mm) > microparticle aggregates (0.053-0.25 mm) > large particle aggregates (> 2 mm) > powder + clay (< 0.053 mm). With the increase of straw covers, microparticle aggregates and small particle aggregates transform into large particle aggregates, increasing the large particle aggregates. This may be due to the increase in the content of surface soil organic matter after the straw covers the soil under no-tillage conditions. At the same time, the cementing effect of soil organic matter promotes the formation of the soil granular structure, which is conducive to the aggregation of large particle aggregates in the soil. After the granular structure comes into water, it is not easy to collapse, so as to improve the pore

Effect of straw cover years on soil organic matter
As an important indicator to evaluate soil quality, soil organic matter can not only improve the effectiveness of soil nutrients but also improve soil water storage capacity, ventilation, and permeability (Hamza and Anderson 2005;Lv et al. 2015). This study found that under no-tillage conditions, NT 1 is conducive to improving soil nutrients and promoting the accumulation of organic matter on the soil surface, which is consistent with Hamdi et al.'s research results (Hamdi and Srasra 2013). NT 1 treatment reduces soil capacity and improves soil permeability. At the same time, the return of straw to the field increases the porosity of the soil. Soil water seeps down the soil pores, causing soil organic matter to enter the deep layer of the soil, which is also confirmed by Lu et al. studies (2015). Qian and Cai (2003) studies have shown that straw mulching treatment better protects the soil environment and significantly improves the soil moisture content, which is conducive to the reproduction and activities of microorganisms, thus promoting the decomposition of various organic residues in the soil and significantly improving the content of soil organic matter. This study also shows that soil moisture content affects soil organic matter content, improves soil structure, and thus affects soil compactness. Zhou et al. (2018) also analyzed the impact of straw return to the field on the diversity of microbial functions. They also found that soil organic matter decreased with the increase of straw return to the field, which may be closely related to the return time of straw, the physical and chemical properties of soil foundation and climatic conditions, which is consistent with the results of this study.

Effect of straw cover years on soil compactness
Soil compaction is an important criterion that determines the soil structure, and it affects the conversion and utilization of soil nutrients and the growth and development of crop roots (Hassan et al. 2007;Nawaz et al. 2013). Different straw covers under no-tillage conditions are an important factor affecting soil compaction. Straw mulching without tillage treatment has a great impact on the surface of the soil and has a certain improvement effect on the deep structure of the soil, which is consistent with the results of Nie et al (2021). This study found that in the 0-20 cm tillage layer, straw cover treatment is compared with traditional farming methods, the compact soil surface has the function of fixing the root system, which can enhance the resistance of crops to fall. At the same time, the return of straw to the field to cover the surface, which also reduces the evaporation of water in the soil and increases the moisture content of the surface soil. In the 20-50 cm tillage layer, compared with RT, the soil compactness is significantly reduced, which is equivalent to the effect of deep pine. The soil is loose, the proportion of pores is appropriate, and water, fertilizer, gas, and heat and other factors are coordinated, which is not only conducive to the growth of the root system and the formation of a larger root system but also conducive to the absorption of deep nutrients and moisture by soil roots (Ma et al. 2019;Zhang et al. 2015). Tian et al. (2013) believes that after straw covers the surface, the organic matter produced by its decomposition is conducive to the formation of soil aggregates, improves the soil structure, and reduces soil compactness. With the increase of no-tillage straw returning years, the surface runoff decreases and the infiltration volume increases, which can lead to the compaction of the soil, which is consistent with the conclusions of Sacco et al.'s studies (Sacco et al. 2012).

Relationship between soil organic matter and soil compactness
Straw cover can enhance soil microbial activity, thus affecting soil compactness and soil organic matter content . In this study, under the condition of no-tillage treatment, the soil compactness is relatively uniform, and the oxygen content on the surface of the soil is more sufficient. Therefore, the decomposition of nutrients by microorganisms is mainly concentrated on the surface layer. The content of soil organic matter in different straw returns to the field shows a tendency to decrease with the increase of soil depth, which is consistent with Zhang et al. research results (Zhang et al. 2016). Wu et al. studies have shown that traditional ploughing causes organic matter soil and crop residues enriched by 0-10 cm tillage layer into the 10-20 cm tillage layer, so that the soil organic matter is evenly distributed (Wu et al. 2012). Tillage has a great impact on the deep soil. Due to mechanical compaction, the soil compaction increased, which is not conducive to the downward migration of nutrients, so that the soil organic matter in the soil depth is basically reduced, which is consistent with the research results of this paper.

Conclusions
In the 0-30 cm tillage layer, the soil organic matter content treated with different straw coverage years is NT 1 > RT > NT 0 > NT 3 > NT 2 ; in the 30-50 cm tillage layer, the soil organic matter content treated with different straw coverage years is NT 1 > NT 0 > RT > NT 3 > NT 2 . In the 0-50 cm tillage layer, the average value of NT 1 organic matter reaches 2.62%. The soil organic matter content of NT 1 treatment increased by 57.46% compared with RT. NT 1 treatment is conducive to soil organic matter conservation and soil structure improvement, while NT 2 and NT 3 soil organic matter loss is relatively fast. The compactness of no-tillage soil in the 0-20 cm arable layer is significantly higher than that of RT, in which the increase in soil compactness in the 0-10 cm is more than 165.70%; the soil compactness at 10-20 cm does not change much. The compactness of no-tillage soil in the 20-50 cm is lower than RT, and the soil compactness at 20-50 cm tillage layer has decreased by more than 23.48%. Straw cover can significantly improve the compactness of the surface soil and make the 0-20 cm cultivated soil loose.
No-tillage can improve the structure of surface soil aggregates and promote the transformation of surface microparticle aggregates and small particle aggregates into large particle aggregates. The increase of large particle aggregates on the surface of the soil reduces the porosity of the soil surface, reduces the evaporation of soil surface moisture, and increases the soil moisture content, which promotes the up and down transport of water and nutrients in the soil and reduces the surface weight of the soil. Therefore, the content of organic matter on the surface of the soil is improved, thus increasing the firmness of the surface soil.
Author contribution The first author (Chunwei Song): writing-original draft preparation, writing-review and editing, formal analysis, validation, visualization, software, and investigation. Corresponding author is the second author (Huishi Du): conceptualization, methodology, resources, data curation, supervision, project administration, and funding acquisition. The authors read and approved the final manuscript.
Funding This research was funded by the Natural Science Foundation of Jilin Province (No. 20210101398JC).

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflict of interests The authors declare no conflict of interest.
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