1 Introduction

Rapeseed (Brassica napus L.) is an important oil crop, widely planted around the world, mainly used to produce edible oil and biodiesel (Zhou, 2024). During the planting process, a large amount of straw is produced. In the past, these straws were often treated as waste. But now research has found that rapeseed straw has many benefits for the ecological environment.

Returning rapeseed straw to the field can not only increase soil organic matter, but also promote microbial growth and increase enzyme activity, all of which help improve soil health and fertility (Yang et al., 2020). In addition, rapeseed straw contains some special substances that have a "fumigation" effect, which can replace some chemical pesticides and reduce environmental pollution (Gardiner et al., 1999).

During the decomposition of straw, nutrients such as nitrogen (N), phosphorus (P) and potassium (K) will gradually be released, which is very important for the growth of the next crop. Especially potassium, it is released relatively quickly, plants can absorb it quickly, and can reduce the need for additional fertilization (Li et al., 2009). At the same time, returning straw to the field can also help to reduce heavy metal pollution in the soil, such as reducing the activity of cadmium (Cd) and lead (Pb), thereby alleviating pollution problems (Yang et al., 2020; Guo et al., 2023), and is also beneficial to crop growth.

This study mainly observed the decomposition process and nutrient release of rapeseed straw under field conditions. We measured the decomposition rate at different time points, analyzed the release patterns of nitrogen, phosphorus and potassium, and evaluated the changes in soil organic matter, microbial biomass and enzyme activity after returning to the field. At the same time, the role of rapeseed straw in reducing soil heavy metal pollution was also explored. It is hoped that through these studies, we can further understand the ecological value of returning straw to the field and provide some references for the development of sustainable agriculture.

2 Rapeseed Straw Composition

2.1 Biochemical components: cellulose, hemicellulose, lignin, and other nutrient contents

Rapeseed straw contains a variety of main components, such as cellulose, hemicellulose, lignin, and some basic nutrients, such as nitrogen, phosphorus and potassium. Cellulose is the most abundant component, which makes the straw stronger. After some treatment, the purity of cellulose in the straw can reach 85%, and the crystallinity is also very high (Svärd et al., 2018).Hemicellulose is also an important part, which is composed of some simple sugars, such as fucose, galactose, arabinose and rhamnose. These sugars help to decompose lignocellulose (Pei et al., 2016). The hemicellulose extracted from rape straw has a relatively high content of xylan and some galactoglucomannan (Svärd et al., 2015).Lignin is a complex component, mainly composed of syringyl and guaiacyl units. Different extraction methods affect their ratios (Svärd et al., 2018). Because lignin is difficult to degrade, it brings some difficulties to the saccharification of straw (Pei et al., 2016). In addition to these main components, rapeseed straw also contains some trace elements, such as calcium, potassium, sodium and silicon. These elements are also important for plant growth and biochemical reactions in the soil (Svärd et al., 2015). And nutrients such as nitrogen, phosphorus and potassium have a great influence on the use of straw in agriculture and industry (Wang et al., 2020).

2.2 Composition changes: influence of genotype and environment

The composition of rapeseed straw is not fixed and will vary significantly due to different varieties or different planting environments. Different types of rapeseed, such as common rapeseed (Brassica napus) and mustard rapeseed (Brassica juncea), have different cell wall structures and decomposition effects. Ordinary rapeseed has a lower lignin content and is more efficient in saccharification or ethanol production (Pei et al., 2016). This difference is mainly related to genes, because different varieties determine the ratio of cellulose, hemicellulose and lignin. Environmental conditions are also important, such as soil quality, climate conditions, fertilization methods, etc. These factors will affect the specific composition of straw. For example, the extraction effect of hemicellulose is related to the treatment method. Different methods (such as autohydrolysis or alkaline extraction) have different effects in different environments (Svärd et al., 2015). In addition, some pretreatment technologies, such as ultrasound or high-voltage discharge, can also change the content of lignin and cellulose in straw, thereby improving its degradation efficiency (Brahim et al., 2017).

3 Factors Influencing Decomposition

3.1 Environmental factors: temperature, moisture, and soil microbial activity

Temperature, humidity and microbial activity in the soil are the main environmental conditions that affect the decomposition rate of rapeseed straw. The higher the temperature, the stronger the activity of microorganisms and enzymes, and the faster the decomposition. Studies have found that when the temperature rises, the carbon in the straw decreases significantly, indicating that high temperature helps to accelerate decomposition (Cai et al., 2018). Humidity is also very important. Appropriate humidity is conducive to the growth of microorganisms on the surface of the straw, and can also increase their activity, thereby promoting decomposition. Studies have also found that when the humidity is high, the straw decomposes faster (Salam et al., 2019). Microorganisms in the soil, such as bacteria and fungi, are the key to whether straw can be decomposed quickly. After adding straw, the number and activity of microorganisms in the soil will increase, thereby accelerating the decomposition process. Studies have found that after adding straw, the number of fungi and bacteria has increased, and the decomposition has become faster (Zhao et al., 2019). In addition, different environments will also affect the type and structure of microorganisms, which will further affect the decomposition effect (Peng et al., 2016).

3.2 Straw characteristics: carbon-nitrogen ratio and lignin content

The composition of the straw itself also affects the decomposition rate, especially the carbon-nitrogen ratio (C/N) and the lignin content. The carbon-nitrogen ratio affects the balance of microorganisms in obtaining nutrients. When the carbon-nitrogen ratio is low, microorganisms can more easily obtain nitrogen and the decomposition rate will be faster. Studies have shown that straw with a low carbon-nitrogen ratio can be decomposed faster because nitrogen availability is higher (Cai et al., 2018). Lignin is another key component that is difficult to decompose. Lignin has a complex structure and is not easy for ordinary microorganisms to handle. Special enzymes are required to decompose it. If there is more lignin in the straw, the decomposition rate will be slower. Some studies have pointed out that the decomposition efficiency of lignin is much lower than that of cellulose and hemicellulose, indicating that it is more "stubborn" (Wang et al., 2021c). Therefore, high lignin content is often one of the main reasons for the slow decomposition of straw.

3.3 Field management: tillage, mulching and mixing

Field management methods, such as tillage methods, whether to cover, and how to deal with straw, will also affect the decomposition rate. Tillage can change the structure and ventilation of the soil, and also affect the activity of microorganisms. For example, no-tillage combined with straw mulching can increase the diversity and activity of microorganisms in the soil and promote straw decomposition. Studies have found that this practice can increase the organic carbon and total nitrogen content in the soil, indicating that nutrient cycling has become stronger (Luo et al., 2020). Mulching and mixing are also common management methods. Straw mulching can maintain soil moisture and temperature, providing a more suitable environment for microorganisms to move. Studies have shown that in covered plots, the decomposition rate of lignin and cellulose in straw is faster (Zhang et al., 2023). At the same time, mixing straw into the soil can also increase its contact area with microorganisms and accelerate decomposition. If decomposition agents are added, the decomposition efficiency will be higher (Wang et al., 2021c).

4 Decomposition Dynamics in Field Conditions

4.1 Stages of decomposition: early, intermediate, and late stages of straw breakdown

In the field environment, the decomposition of rape straw can be roughly divided into three stages: initial, intermediate and late stages. The initial stage generally occurs in the first 10 days. At this time, the decomposition speed is very fast, especially potassium (K) is released in large quantities, reaching 98.74% (Fengyu et al., 2009). This is because potassium is easily soluble in water and can be easily carried away by microorganisms. In addition, microorganisms have just begun to multiply in large numbers at this stage, so the decomposition is very fast. From the 10th day to about the 50th day, it is the intermediate stage. At this time, the decomposition slows down. The shape of the straw begins to become loose, and some soft parts such as parenchyma and vascular bundles begin to degrade, but the epidermis and harder parts are still relatively well maintained (Fengyu et al., 2009). This stage is mainly nitrogen (N) and phosphorus (P) slowly released, because they are not as easy to move as potassium and need longer to be decomposed by microorganisms. The late stage starts after the 50th day. The decomposition is even slower at this stage. What is left in the straw is mostly lignin and cellulose, which are not easily degradable and require some specific microorganisms to process. By the 120th day, the total decomposition rate of rapeseed straw can reach about 66.9% (Li and Zhong, 2021). This stage is critical for the release of remaining nutrients and the stabilization of soil organic matter.

4.2 The role of microorganisms: the interaction between microorganisms and straw

The decomposition of rapeseed straw depends largely on microorganisms in the soil. Bacteria and fungi can decompose various organic matter in straw and are important to the whole process. For example, some bacterial groups, such as Proteobacteria, Actinomycetes and Planctomycetes, have been found to have a strong relationship with the decomposition rate of straw and the nutrient content in the soil (Wang et al., 2021b). These bacteria secrete enzymes that can decompose cellulose, hemicellulose and lignin, allowing nutrients to be released faster. Some fungi, such as Marasmius tricolor 310b, work well in composting rapeseed straw, accelerating the decomposition of cellulose and lignin (Wang et al., 2023) (Figure 1). The presence of these fungi not only speeds up decomposition, but also makes the entire microbial community more stable and more interconnected. This cooperation allows straw to decompose continuously in various field environments. In addition, the types and numbers of microorganisms are also affected by fertilization methods and soil fertility. For example, adding straw to some soils that have been fertilized for a long time will significantly change the types of microorganisms and increase the number of fungi and bacteria (Zhao et al., 2019). Although actinomycetes are usually not numerous, they are particularly important for the decomposition of plant residues in poor soils (Bao et al., 2021).

5 Nutrient Release Dynamics

5.1 Release of major nutrients: patterns of nitrogen, phosphorus, and potassium release

The release of nitrogen (N), phosphorus (P) and potassium (K) from rapeseed straw is affected by the nutrient content of the straw itself and environmental conditions. The release process of nitrogen is generally: fast in the early stage and slow in the later stage. At the beginning, microorganisms will convert some nitrogen for plants to use, and then the release rate will slow down, but it will last for a long time (Khalsa et al., 2021; Niedziński et al., 2021). Compared with mineral fertilizers, organic fertilizers release nitrogen much more slowly. For example, 98% of the nitrogen in mineral fertilizers can be released in a short time (Niedziński et al., 2021). Phosphorus is released much slower than nitrogen and is more controlled. Its release rate is related to the stability of organic phosphorus and is also affected by the adsorption capacity of the soil. Sometimes phosphorus is released slowly for a long time, providing a relatively stable supply for plants (Saleem et al., 2021; Orden et al., 2022). Potassium is released the fastest. A lot of potassium is released early in the decomposition process (Brödlin et al., 2019; Khalsa et al., 2021), which can quickly meet the crop's potassium needs.

5.2 Time trend: fast release and slow release phase

The nutrient release of rapeseed straw can generally be divided into two stages: fast first and slow later. At the beginning, easily degradable components, such as soluble sugars and amino acids, will be quickly decomposed by microorganisms. At this time, nitrogen and potassium are released quickly (Khalsa et al., 2021; Suwardi et al., 2023), which can immediately provide nutrients to crops. After this stage, the release becomes slower. At this time, it is mainly the lignin and cellulose that are difficult to degrade that are slowly decomposed (Orden et al., 2022; Pan et al., 2023). This slow release phase is important for the entire growing season, which can continuously provide nutrients and help maintain soil fertility. The release of phosphorus is particularly affected because it is not easy to move. The release rate depends on microbial activity and the adsorption of minerals in the soil (Brödlin et al., 2019; Jarvie et al., 2020), so it will be slower but last longer.

5.3 Impact on soil fertility: Improvement of organic matter and nutrient pool

After rapeseed straw decomposes, it not only releases nutrients, but also greatly improves soil fertility. The organic matter it decomposes can increase the organic carbon in the soil, which is very helpful for improving soil structure, water retention capacity and microbial activity. The released nitrogen and phosphorus will also enter the soil organic matter, helping to expand the soil nutrient pool (Wang et al., 2022a; Tang et al., 2023) (Figure 2). At the same time, the release of these nutrients also makes various elements in the soil easier to be absorbed by crops. For example, the nitrogen released by organic fertilizers can increase the total nitrogen content in the soil and improve its effectiveness (Khalsa et al., 2021; Niedziński et al., 2021). Similarly, the phosphorus and potassium released by straw can increase the reserves of these two elements in the soil, making the nutrients more balanced and the fertility better (Saleem et al., 2021; Orden et al., 2022).

6 Case Studies and Comparative Insights

6.1 Comparisons with other crop residues: decomposition rates and nutrient release efficiency

The straw of different crops varies greatly in decomposition speed and nutrient release efficiency. For example, in a study conducted in the Jianghan Plain in central China, after 120 days, the decomposition rate of rice straw was 72.9%, wheat straw was 56.2%, and rape straw was 66.9%. This shows that rape decomposes a little faster than wheat, but slower than rice straw (Li and Zhong, 2021). Although the decomposition speed is different, the nitrogen release rate of the three straws is similar, 52.0%, 54.4% and 54.9% respectively. In another comparative experiment conducted in a rice field environment, the decomposition of wheat and rape was also different. After 100 days, the decomposition rate of wheat straw was 66.18%, and that of rape was 55.62%. At this time, the release rate of potassium is faster than that of nitrogen and phosphorus. Especially in the first 10 days, wheat straw released nearly 98.92% of potassium, and rapeseed was about the same, reaching 98.74% (Fengyu et al., 2009), indicating that the potassium in both straws is particularly easy to release. Other studies have shown that in tropical regions, legume residues decompose faster than non-legume residues. This is because legumes themselves have a high nitrogen content, which can promote microbial activity and allow them to release nutrients faster (Da Silva et al., 2021). Non-legume straw, such as sunflower, decomposes much more slowly and releases nutrients more slowly.

6.2 Field observations in different regions

Field experiments from different climate zones also help us better understand the decomposition characteristics of rapeseed straw. In the Red River Valley of the United States, a study compared the decomposition rates of several crop straws, including corn, dry beans, soybeans, potatoes, sugar beets and spring wheat. The results showed that sugar beets decomposed the fastest and corn the slowest. This is mainly related to the carbon-nitrogen ratio (C:N) of the straw. Straw with a low carbon-nitrogen ratio is easier to decompose (Chatterjee and Acharya, U., 2020). In northwest India, the decomposition and nitrogen release of corn, wheat and mung bean residues were studied. The study found that burying straw in the soil decomposes faster and releases more nitrogen than leaving it on the surface. This is because the moisture and temperature in the soil are more suitable for microbial activity, which is conducive to straw decomposition (Sandhu et al., 2022). In an experiment in China, the decomposition of rapeseed green manure at two different levels of returning to the field was compared. The results showed that when the amount of returning to the field was high, the decomposition was faster and the nutrient release was more. This may be because there are more nutrients in the soil, and the types and numbers of bacteria have increased, especially at high levels of returning to the field (Wang et al., 2022b) (Figure 3; Table 1).

7 Impact on Soil Health and Crop Productivity

7.1 Soil quality enhancement: organic matter improvement and microbial activity stimulation

Adding rape straw to the soil can significantly improve soil quality. The most important manifestation is that the organic matter has increased and the microorganisms have become more active. Studies have found that organic materials such as rape straw can increase soil organic carbon (SOC) and total nitrogen (TN), which are closely related to soil fertility. For example, someone has summarized that compared with using only chemical fertilizers, using organic materials can increase soil organic carbon by 38% and total nitrogen by 20% (Luo et al., 2018). After the increase of organic matter, not only can the soil structure become better, but also the soil's water retention capacity can be enhanced, making nutrients easier to be absorbed by crops, which is critical for the healthy growth of crops. At the same time, rape straw can also increase the number of microorganisms in the soil and increase the activity of enzymes. Studies have shown that organic farming methods such as returning straw to the field or composting can increase more microbial carbon and nitrogen than traditional agriculture (the increase ranges from 32% to 84%), and the activity of key enzymes such as dehydrogenase, urease, and protease is also stronger (Lori et al., 2017). These enzymes help break down organic matter, speed up nutrient cycling, and make the soil more fertile and more suitable for sustainable farming.

7.2 Impact on subsequent crops: growth, yield, and nutrient utilization

Rapeseed straw can not only improve the soil, but also make the crops planted later grow better and have higher yields. Studies have found that the addition of organic materials (such as rapeseed straw) has a significant effect on increasing crop yields. In the wheat-corn rotation system, the use of organic fertilizers increased wheat yields by 26.4% to 44.6%, and corn yields by 12.5% to 40.8%. However, the yield increase effect of plots that only used chemical fertilizers was not good (Zhou Zhiqiang et al., 2022). This may be because organic matter can improve soil structure and improve nutrient absorption efficiency. Long-term use of organic fertilizers is also helpful for soil fertility and crop production capacity. For example, a study conducted a 40-year experiment in red soil rice fields and found that the microbial community in the soil was more stable and the rice yield was higher in plots that had been using organic fertilizers and chemical fertilizers at the same time (Wang et al., 2021a). The combination of organic fertilizers and chemical fertilizers can not only make the nutrient release more even, but also continuously supply nutrients throughout the growing season, ultimately increasing the yield and making the yield more stable.

8 Ecological and Agricultural Implications

8.1 Benefits of straw retention: soil structure improvement and reduced synthetic fertilizer dependence

Leaving rape straw in the field has many benefits for soil and farming. It can improve soil structure and reduce the use of chemical fertilizers. After the straw rots, the organic matter in the soil will increase, and the soil will become softer and more breathable. In this way, water can penetrate better and retain moisture, and it is not easy for the soil to become hard or washed away by water (Hu et al., 2012; Zhang et al., 2023). In addition, when rape straw decomposes, it will slowly release some nutrients such as nitrogen (N), phosphorus (P) and potassium (K). These are key elements for crops to grow well. With them, less chemical fertilizers can be applied, which can save money and reduce environmental pollution (Feng et al., 2009; Wang et al., 2022b). In addition, straw can make the microorganisms in the soil more active. These little things will help decompose organic matter and recycle nutrients, which is also beneficial to crops. Studies have found that leaving straw can increase beneficial bacteria and fungi in the soil, helping to make the soil healthier (Peng et al., 2016; Zhao et al., 2019).

8.2 Challenges of decomposition: nutrient loss and residue accumulation

Although straw decomposition has many benefits, there are also some issues that need attention. A common problem is that nutrients will be lost, especially potassium, which can be easily washed away. For example, in places with heavy rain or frequent irrigation, potassium may be quickly carried away by water, which not only wastes nutrients but also may pollute the environment (Fengyu et al., 2009; Li and Zhong, 2021). This situation often occurs in the first few days of decomposition. Another problem is that the decomposition is too slow, and the straw is easy to pile up in the field. Some components, such as lignin and cellulose, decompose very slowly and will remain on the soil surface or in the soil. These residues will block seed germination and affect root growth, which is not good for crop growth (Hu et al., 2012; Zhang et al., 2023). Moreover, if there are too many residues in the field, it may disrupt the balance of microorganisms in the soil and make nutrient cycling worse (Zhao et al., 2019). Therefore, if you want to make good use of straw, you must have a reasonable management method that can help it decompose quickly and reduce these negative effects.

8.3 Optimization strategy: increase decomposition speed and improve nutrient utilization efficiency

In order to make the straw decompose faster and use nutrients better, you can try several methods. For example, bury the straw deeper. Studies have found that straw buried deep in the soil decomposes faster than straw placed directly on the surface, and plants can absorb nutrients more easily (Zhang et al., 2023). This can also release nutrients slowly and not be easily washed away by rain. In addition, you can also add some decomposers or mix some manure in. These things can make the microorganisms in the soil more active and help the straw to decompose faster. Some studies have shown that after adding these, the release rate of nitrogen and phosphorus has become faster, and the enzyme activity in the soil has also increased, which is very helpful for nutrient cycling (Guan et al., 2020). Finally, you can also arrange when and how to use straw according to the growth time of crops. In this way, the straw can be released just when the crops need nutrients most (Song et al., 2023). Not only can it be used, but it can also reduce waste.

9 Conclusion and Future Directions

The decomposition rate of rape straw is usually divided into two stages: fast in the early stage and slow in the later stage. Studies have shown that the decomposition rate of rape straw can reach 55.62% to 66.9% after about 100 to 120 days. During the decomposition process, potassium (K) is released the fastest, with nearly 98.74% released in the first 10 days. In contrast, nitrogen (N) and phosphorus (P) are released much slower. This decomposition process not only increases nutrients in the soil, but also changes the types of microorganisms in the soil, thereby improving soil fertility, which is very helpful for the development of sustainable agriculture.

Farmers are advised to return rape straw to the fields, which can improve the soil and reduce dependence on chemical fertilizers. Because potassium is released too quickly, the amount of potassium fertilizer should be adjusted according to the situation to avoid applying too much. In addition, different ways of returning to the field will affect the decomposition rate. Studies have found that placing straw on the soil surface is the best and decomposes faster. Farmers can also adjust management methods according to soil moisture, which is more conducive to decomposition and nutrient release, and can also help crops grow better and increase yields.

In this area, future research could focus more on the interactions of microorganisms during the decomposition of rapeseed straw. For example, fungi such as Thiobacillus, Azotobacter, and Pseudomonas may play a key role in the decomposition process. Long-term trials are also needed to observe the long-term effects of returning straw to the field on soil, nutrient cycling, and crop yields. It is also possible to study the differences in straw decomposition under different climatic conditions, which can help us develop more locally appropriate management methods.

Acknowledgments

The authors would like to thank the reviewers for their constructive comments and suggestions during the review process. These feedbacks have helped us refine the logical structure and research content of the paper, enhancing its overall quality.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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