Five-Dimensional Straw Utilization Model and Its Impact on Carbon Emission Reduction in China

Abstract

Enormous quantities of straw in China are burnt in open fields or discarded randomly, leading to a serious waste of biomass resources and environmental pollution. To accelerate sustainable development, straw resources must be used efficiently and reduce carbon emissions. Based on a systematic literature review, this study summarizes China’s latest development in straw utilization. It analyzes the accounting methods, carbon emission reduction effects and potential of straw utilization. The study highlights that straw utilization in China can be categorized into five-dimensional straw utilization models (FDSUM). The cost of collection, storage and transportation, and straw utilization technology are the main factors affecting straw utilization. FDSUM contributes greatly to agricultural carbon reduction. Straw-to-fertilizer has the highest contribution, and straw-to-fuel has the largest carbon emission reduction potential. The carbon emission reduction from straw–to-fuel utilization in 2020 was 63.43 × 10⁹ kg CO₂. In addition, China has not developed a standardized carbon accounting method for straw utilization. China needs to prioritize straw-to-fertilizer and straw-to-fuel conversion and develop low-carbon production technologies. This study will serve as a reference to further improve the utilization of straw in China and provide preliminary ideas for establishing a unified national carbon accounting system for straw utilization.

1. Introduction

Crop straws are valuable biomass resources in the agricultural ecosystem, and China has a high straw production as an agricultural country. According to the national statistics, the total amount of straw production in China is 8.56 × 10⁸ tons (t) in 2020, of which 7.22 × 10⁸ t can be collected and recycled. The comprehensive utilization rate of straw reached 87.6%, but about 8.9 × 10⁷ t of straw was not effectively utilized [1]. Previous studies have pointed out that most unused crop straws are sent for burning or simply random stacking, which can generate many harmful gases such as carbon dioxide (CO₂), nitrogen oxides, benzene and polycyclic aromatic hydrocarbons. These gaseous emissions endanger not only human health, but also cause environmental pollution, exacerbate climate changes, and even lead to environmental disasters [2,3,4]. Improving the comprehensive utilization rate of straw in China can reduce greenhouse gas (GHG) emissions caused by the open field burning of straw and random stacking, effectively replace fossil energy, and reduce environmental pollution. Straw recycling can also improve soil carbon sequestration and soil quality, which is critical for agricultural carbon sequestration and emission reduction.

China has set the goal to achieve carbon peaking by 2030 and carbon neutrality by 2060. Agricultural emission reduction and carbon sequestration are of great importance and have the potential for carbon peaking and carbon neutralization at a state level. According to the United Nations (UN) Food and Agriculture Organization (FAO), global GHG emissions from agriculture and food production have increased by 17% in the past 30 years. About 31% of CO₂ emissions came from agricultural production [5]. In 2020, China’s carbon emissions will reach 9.899 billion tons, accounting for 30.7% of global carbon emissions [6]. In 2020, about 8% of China’s total carbon emissions came from agricultural production [7], among which, the GHG emissions from open burning of straws reached 3.8 × 10⁷ t CO₂ [8]. The comprehensive utilization of straw can reduce GHG emissions by partially replacing fossil fuels, save forest wood resources by replacing wood as raw material and increase the carbon sink by replacing grass as a feed. The utilization and recycling of crop straws can reduce the input of other primary resources and decrease carbon emissions in China [9,10].

Straw utilization has formed a certain mode in different regions in China. There are mainly five directions to recycle crop straw, including straw-to-fertilizer, straw-to-feed, straw-to-fuel, straw-to-base material and straw-to-raw material utilization. These straw utilization methods are closely integrated into China’s agricultural production and local conditions. This can effectively improve the comprehensive straw utilization level and facilitate the reduction in carbon emissions. First, the utilization of straw as a fertilizer can improve soil carbon sequestration capacity and increase the soil carbon sink. Secondly, straw-to-feed directly or indirectly provides a source of feed for animal husbandry. Straw can be used to produce briquette fuel and biogas/natural gas and serve as an alternative energy supply replacing fossil fuels such as coal, which can reduce GHG emissions. In addition, straw can be used as a base material for mushroom production and seedling breeding, or raw material for papermaking, producing plates, braids and degradable wares, reducing deforestation and improving forest carbon sinks. Overall, the comprehensive utilization of straw has great potential and space to contribute to agricultural emission reduction and carbon sequestration.

According to statistics, the usage of straw as fertilizer, feed, fuel, cultivation material and raw material in 2020 was 448.68 × 10⁶ t, 111.01 × 10⁶ t, 61.14 × 10⁶ t, 4.99 × 10⁶ t and 7.18 × 10⁶ t, respectively. This corresponds to 62.1%, 15.4%, 8.5%, 8.7% and 1.0% of the total straw, respectively, accounting for 87.6% of the total comprehensive straw utilization rate [1]. Given this, straw-to-fertilizer, feed and fuel are the major utilization approaches in China and are also the focus of most academic research works investigating crop straw utilization. Few studies have been completed to investigate the utilization of straw as a base material or raw material. According to Cong et al., China’s straw resources have generally shown a ladder-like distribution characteristic of “high in the east and low in the west, high in the north and low in the south”, and straw is mainly used as a fertilizer in agricultural production [11]. Shi et al. found that in 2015, the total amount of straw-to-fertilizer, feed and fuel accounted for 91.6% of the total utilization of straw in China. However, significant differences in the comprehensive utilization rate and structure of different straw types have been observed [12]. In addition, existing research on the impact of straw utilization on carbon emissions has mainly focused on two aspects of straw-to-fuel and straw returning to the field. The impact of comprehensive straw utilization on carbon emission reduction in China is still unclear. Ma et al. found that from 2008 to 2019, the annual average carbon emission of straw burning in China was 8.74 million t (mt) CO₂e with an average annual decrease rate of 17.3% since 2014. Using straws to produce solid briquette fuel contributes to the largest carbon emission reduction in straw-to-fuel utilization. The carbon sink of straw returning to the field is generally increasing yearly [13]. Huo et al. showed that China’s net GHG emission reduction contribution of the comprehensive utilization of straw was 7.0 × 10⁷ t CO₂e in 2020. Straw-to-fertilizer and fuel contributed the most to an emission reduction of 7.9 × 10⁷ t CO₂e and 3.8 × 10⁷ t CO₂e [8]. Wang et al. claimed that processing agricultural waste into biochar to enrich soil instead of burning it directly effectively reduces GHG emissions, improves soil quality and promotes crop growth [14]. Moreover, studies by Cátia et al. showed that GHG emissions from straw utilization also depend on the chemical composition of the straw used for the composting and composting with crop straw with higher total organic N (TON)and lignin content has low carbon dioxide emissions [15].

Based on the FDSUM in China and the current accounting method of carbon emissions from straw utilization, this paper provides a systematic review of the current status of the technology and carbon emissions in straw utilization. This study also considers the real-world straw utilization levels and methods in different regions in China and summarizes the research gaps in straw utilization. The national and regional straw utilization and its carbon emission reduction potentials are also assessed and discussed. The paper proposes the key technologies and directions for future research. The ultimate goal of the study is to provide guiding information for the optimization of straw utilization methods and schemes and future research directions for carbon emission reductions of straw utilization.

2. Materials and Methods

2.1. Methodology and Flowchart

According to the classification method widely used in China, this study divided the straw utilization methods into five categories: straw-to-fertilizer, feed, fuel, base material and raw material, referred to as the “five-material utilization” scheme (Figure 1). A literature search was carried out by searching the “China National Knowledge Infrastructure (CNKI)”, “Web of Science”, “Springer” and “Bing” with the keywords of “straw”, “five-material utilization”, “five-dimensional straw utilization “, “technology and status quo”, “carbon emissions”, “carbon accounting methods” and “China”. The search results were limited to publications in English and Chinese, and priority was given to articles published during 2010–2021. This paper focuses on the utilization technology and status of the five straw utilization methods in China, the accounting method of carbon emissions, and the impact of the five straw utilization methods on carbon emissions. The aim was to identify promising research directions and key technologies and propose suggestions for improving China’s straw utilization and reducing carbon emission.

2.2. Research Regions

China was divided into six regions for this research based on farming systems, the climatic conditions and the zoning method adopted in the straw resources survey during “China’s second pollution source census”. The regions were Northeast, Northwest, North, Southeast and Southwest China, and the middle and lower reaches of the Yangtze River Region (

Table 1. Research regions.

Regions	Northeast	Northwest Region	Northern Region	Southeast Region	Southwest Region	Middle and Lower Reaches of the Yangtze River
Provinces and cities	Liaoning Jilin Heilongjiang Inner Mongolia	Shaanxi Gansu Qinghai Ningxia Xinjiang	Beijing Tianjin Hebei Shanxi Shandong Henan	Fujian Guangdong Guangxi Hainan	Chongqing Sichuan Guizhou Yunnan Tibet	Shanghai Jiangsu Zhejiang Anhui Jiangxi Hubei Hunan

).

3. Straw Utilization Methods

3.1. FDSUM and Carbon Emissions

This section highlights the FDSUM and the associated carbon reduction potential.

3.1.1. Straw as Fertilizer

Straw is rich in nitrogen, phosphorus, potassium, calcium, magnesium and a large amount of organic matter [16]. It is a good biomass resource and can be used as fertilizer for returning to the field. Straw fertilizer utilization technology mainly includes two categories of direct and indirect straw-returning technology. Straw directly returning to the field mainly includes the ploughing and deep ploughing technology, the rotary tillage mixed burying technology and the no-tillage mulching technology. Straw returning to the field has been widely applied in many large agricultural regions in China (

Table 2. Comparison of straw fertilizer utilization technologies.

Utilization Technologies	Advantage	Shortcoming	Suitable Application Area
Ploughing and deep ploughing	Can process a large amount of straw with a high processing efficiency, which can break the bottom of the plough, and effectively increase the soil organic matter, fertilize soil, improve soil water retention capacity and eliminate pests and diseases.	Cost is relatively high. A certain amount of nitrogen fertilizer needs to be applied to alleviate the problem of the microbial decomposing of straw and crop growth competing for nitrogen sources.	Suitable for large-scale land with relatively flat plots, such as returning corn stalks to fields in Northeast China, returning cotton stalks to fields in Northwest China, and returning straws to fields in North China.
Rotary tillage and mixed burial	The process is simple, and the operation is convenient. Shorter processing time; relatively low cost.	Leads to loose soil, decreased bulk density and poor soil structure; affects seed germination and rooting and leads to poor drought resistance and lodging resistance of the crops; increases the probability and degree of pests and diseases.	Suitable for the rice–wheat rotation and rice–rape rotation double cropping area in the middle and lower reaches of the Yangtze River; the wheat–maize rotation double cropping area in North China. Not suitable for sloping dry land with serious soil erosion.
No-tillage mulching	The process is simple, with a reduced number of times of mechanical entry. Low cost. The soil is covered with straw and solidified with stubble to reduce wind erosion, water erosion and ineffective water evaporation. It improves the utilization rate of natural rainfall.	Measures such as chemical spraying, mechanical/manual weeding and crop rotation are needed for weed and pests control.	Suitable for rainfed agricultural corn planting areas in Northeast China, Northwest China, North China, Southeast China and Southwest China.
Rapid ripening in the field	Effectively promotes the decomposition of straw; not limited by seasons and locations; saves processing time and work; easy to apply and reduces crop diseases.	High operating costs in mountainous and hilly areas.	Suitable for double cropping areas with high amounts of straw production, such as the southeast area, southwest area, and the middle and lower reaches of the Yangtze River; unsuitable for the northwest area and cold areas with arid soil and poor soil entropy.
Bioreactor	Converted into heat, organic matter and nutrients required for crop growth, which can effectively improve soil structure, soil moisture and reduce pest and disease damage	Requires high ground and reactor shed temperature which requires an enhanced ventilation system and moisture removal management.	It can be applied in places with abundant straw resources, which is more suitable for greenhouse crop cultivation.

Data source: Wang et al. [17].

). Except for the straws with serious diseases and pest infestation or continuous cropping obstacles, straw returning to the field is a feasible solution. Indirect straw returning to the field mainly includes field rapid decomposing, bioreactor technologies, retting and carbon-based fertilizer production technology. In the indirect returning to field process, the straws are first collected, stacked or buried in soil pits near the field, then decomposed by applying decomposing inoculants, microbial strains, human and livestock manure or by pyrolysis and finally returned to the fields as fertilizers.

It has been claimed in most existing studies that using straw as fertilizer is the most direct, economic and ecological way of straw utilization [18]. Straw-to-fertilizer can effectively increase soil organic matter content and improve soil structure and soil fertility. It is significant for alleviating the imbalance of nitrogen, phosphorus and potassium ratios in China’s soil and mitigating the environmental issues caused by excessive chemical fertilizers and straw burning. However, some studies have indicated that returning straw as fertilizer may have negative effects. For instance, straw fertilizer has a low nutrient content and a slower release rate than chemical fertilizers. The effect of increasing the crop yield is not obvious and even leads to a decrease in crop yield in the short term [19]. In addition, too high an amount of straw returning to the field, an insufficient degree of straw crushing and returning depth, too high stubble and unsuitable nitrogen fertilizer can also cause poor emergence and yellowing of crops [20]. The pathogens, insect eggs and grass seeds carried in the straw can also cause pests and weed problems [21,22]. Many experimental studies have shown that the main reason for these problems is the lack of understanding of the law of straw decomposition and regulation methods [23,24]. There is a need to further understand the principles of the transformation, distribution and turnover of straw after returning to the field, the impact factors of straw returning to the field and the causes of inducing diseases, pests and weeds. Overall, the negative impact of returning straw to the field can be effectively reduced by proper adjustment of the amount of straw returning to the field [25], application of livestock manure [26] and chemical fertilizers [27] and other agricultural management measures.

With the highest amount, straw-to-fertilizer is China’s most important straw utilization method. In the sub-regions, nearly half of the straw in China’s six major regions was utilized as fertilizer in 2020. Among them, the proportion of straw-to-fertilizer utilization in North China, South China and the middle and lower reaches of the Yangtze River was 73.8%, 73.1% and 69.4% [1]. They were all higher than the national average level. The proportion of fertilizer utilization in the southwest, northwest and northeast regions is relatively low, with a utilization rate of 55.2%, 49.1% and 47.2% [1], respectively. In the northwest and northeast regions, a low winter temperature, less summer rainfall and less soil microbial content make the straw less susceptible to decay after returning to the field, leading to a relatively low utilization rate of straw as a fertilizer. For the southwest area, the utilization rate of straw fertilizer is low because the terrain is dominated by hills and mountains, which are unsuitable for agricultural machinery operations and thus have a high labor cost.

The conclusions from existing studies on improving soil organic carbon storage and soil carbon sequestration by straw returning are consistent. Straw returning to the field has been widely recommended as a typical approach for carbon sequestration in farmland [28,29,30,31]. After the straw is returned to the field, 8% to 35.7% of the organic carbon is stored in the soil carbon pool in the form of soil organic carbon [32,33]. The straw-returning experiment started in 1986 at the Lausanne Experiment Station in the United Kingdom (UK) and showed that straw returning to the field can significantly increase the accumulation rate of soil organic carbon [34]. The double-cropping rice experiment in India showed that the combined application of rice straw and inorganic nitrogen fertilizer was the most effective for soil carbon sequestration, and the annual carbon sequestration amount reached 0.35 tons per hectare (t/ha) [35]. Jin et al. carried out a meta-analysis, estimated the soil carbon sequestration of different farmland management methods in China and found that the annual carbon sequestration amount of straw returning to the field could reach 0.597 t/ha [36]. Their study also found that the results of the method were different from those of the literature data by comparing it with the Intergovernmental Panel on Climate Change (IPCC) inventory guidelines [37], indicating that there are problems in its practical application in China and that further research is needed. Chen et al. presented a Life Cycle Assessment (LCA). The authors found that the implementation of conservation tillage in the Guanzhong Plain of Shaanxi Province had significant carbon sequestration and emission reduction potential. Each hectare of farmland could reduce 5093.09 kg of CO₂ equivalent emissions per year [38]. Ma et al. used the emission factor method to calculate the annual average carbon sink of straw returning to the field in China to be 271 mt CO₂-e with an increasing trend by year from 2008 to 2019 [13]. Kern et al. estimated that if 57% of the arable land in the United States (US) adopted conservation tillage techniques, the carbon sequestration capacity of US soil could reach 80–129 Tg, and this value could reach 286–468 Tg if 76% of arable land adopted conservation tillage measures [39]. It can be seen that conservation tillage has great potential for carbon emission reduction. Other studies have shown that the contribution of straw-to-fertilizers has a relatively larger contribution to carbon emission reduction in the five major straw utilization methods. It is estimated that the straw-to-fertilizer contribution to carbon emissions and sequestration will reach 8.2 × 10⁸ t CO₂e and 8.7 × 10⁷ t CO₂e in the year 2030 and 2060, accounting for 40.7% and 31.9% of the total carbon emission reductions of comprehensive utilization of straw [8].

Although various studies have affirmed the carbon sequestration ability of straw-to-fertilizer utilization, its overall trend and effects on soil carbon content in China still require further investigation considering the complex practical conditions in China. China has a vast territory with a complex terrain and diverse natural conditions, resulting in different straw-returning technologies and straw types suitable for different regions. Therefore, further research is needed on the carbon sequestration potential of straw returning under different natural conditions and straw-returning technologies.

3.1.2. Straw as Feed

Crop straw is mainly composed of cellulose, hemicellulose and lignin. Although its protein content is not high (3–6%), it can meet the basic nutritional needs of general livestock after supplementing with an appropriate amount of roughage [40]. For ruminants, the microorganisms in their intestines can use enzymes to decompose cellulose and hemicellulose into volatile fatty acids such as acetic acid, propionic acid and butyric acid, which can provide 60% to 70% of their energy and a carbon framework for protein synthesis in the body [41]. Therefore, straw-based roughage occupies a very important position as the feed source of ruminant livestock. China is rich in straw resources, and Chinese farmers have had the tradition of using straw as feed for herbivorous livestock since ancient times. Currently, the straw used as feed in the country accounts for about one-fourth of the total amount of straw that can be collected and utilized in the country, which has made an important contribution to the steady development and strategic structural adjustment of China’s aquaculture industry. The promotion of straw as feed is conducive to the resource utilization of agricultural waste. It can also provide a good source of roughage for dairy cows, beef cattle, sheep and other livestock and poultry. Using advanced technology and methods to process straw can retain trace elements and proteins in straw, increase its palatability and nutritional content and facilitate transportation and storage [42]. Unprocessed straw as feed has low nutritional value and poor taste, which can easily lead to a low absorption rate of livestock. Processed straw can feed different animals, and its multiple advantages are gradually attracting attention. In China, straw feed utilization technology is mainly divided into three types, namely the physical, chemical and biological approaches, covering straw green (yellow) storage technology, straw alkalization/ammonification technology, straw briquette feed processing technology, straw cutting and kneading technology, straw extruding–bulking technology, and straw puffing technology (

Table 3. Comparison of straw as a feed utilization technology.

Technology	Advantages	Disadvantages
Green (yellow) storage technology	Less nutrient loss, high feed conversion rate, easy long-term storage, sterilization, disinfection effect, etc.	It cannot be used as a single feed for a long time, which can cause diarrhea, complicated production management and large differences in the nutrition of green (yellow) stored feed.
Straw Alkalization/Ammonification Technology	Improve the quality of straw feed; increase the feed intake rate and digestibility; improve the nutritional level of straw; improve the return rate of feed; improve the efficiency of breeding; increase the income of farmers, etc.	It cannot be directly used as feed and needs to be treated with ammonia release. It cannot meet the growing needs of livestock when used as a single feed. Generally, it can only be used as feed for ruminant livestock such as adult cattle and sheep to prevent ammonia poisoning, etc.
Straw briquette feed processing technology	Small size, large specific gravity; convenient transportation and not easy to deteriorate; easy for long-term storage; good palatability; high feed intake rate and convenient feeding; low-priced, etc.	Relatively higher requirements for the degree of agricultural mechanization and technicians are high, and the nutrition of the briquette straw feed is relatively insufficient, etc.
straw cutting and kneading technology	The simplest process with the highest efficiency and the lowest cost; extend the residence time of cellulose, hemicellulose and lignin in the rumen, which is beneficial to the digestion and absorption of livestock and improves the straw feeding rate.	Straw kneading and cutting machine has the disadvantages of large feeding resistance, fast wear of the moving blade, high cost, and inability to adjust the gap between the moving and fixed knives.
straw extruding-bulking technology	Improved content of soluble components, digestible and absorbable components and palatability; improved feeding value; improved indicators such as feed intake rate, digestibility, feed rate of return, and daily weight gain.	Environmental factors such as temperature, moisture, pressure, friction, etc., can cause the content loss of vitamins, enzymes, microbial inhibitors, proteins and amino acids in straw feed.

Data source: Wang et al. [43].

). Among them, straw green (yellow) storage technology, straw cutting and kneading technology and straw extruding–bulking technology are widely used due to the advantages of simple technology, high feed conversion rate, good palatability, low cost and the fact long-term storage is easy. Processed straw as feed has the advantages of less nutrient loss, high feed conversion rate, good palatability and long storage time. More importantly, processed straw has antiviral properties and contributes to the health of the animals. With the continuous development of modern agricultural facilities and biotechnology, more and more compound methods will be used to produce straw-based feed. In general, these methods are all developing in the direction of low production cost, the high nutritional value of feed and long storage time.

The utilization of straw for feed is the second largest utilization method of straw in China. The northwest region ranks first, reaching a straw-to-feed utilization rate of 35.7% [1]. It is followed by the northeast (21.8%) and southwest (21.1%) regions [1]. The high straw-to-feed utilization rate is closely related to the more developed animal husbandry industry in these three regions. In North China, South China and the middle and lower reaches of the Yangtze River region, the development of animal husbandry is relatively limited and affected by various factors such as regional economic, social and environmental conditions. The straw-to-feed utilization rates in these three regions are at relatively low levels of 10.5%, 8.0% and 5.6%, respectively [1]. In addition, as these region’s economic and technological levels are relatively developed, the straw collection, storage and transportation system and agricultural mechanization are relatively high. Consequently, more crop straws are collected and processed into fertilizers and fuels. The use of straw as feed is mainly to meet the forage needs of herbivorous animals such as cattle and sheep. Its utilization matches the development of China’s animal husbandry industry.

Animal husbandry is an important source of methane emissions. Ruminants such as dairy cows, beef cattle and sheep produce a large amount of methane during digestion. The total amount of methane released by enteric fermentation reaches 86 mt per year [44]. Methane emissions from ruminants can be reduced by adding feed supplements, lactic acid bacteria or adjusting feed ratios to straw feed [45,46,47,48]. Studies by Azlan et al. have shown that the ammoniated straw can effectively reduce methane emissions by 32% [49]. Zhang et al. showed that using pre-treated straw as feed can effectively reduce methane emissions by 16.55% [50]. In addition, when processed into high-quality roughage with comprehensive scientific technologies, straw fed to cattle and sheep and returned to the field after being digested has a better carbon sequestration effect than direct straw returning to the field [51,52,53]. Li used the direct measurement method to conduct four-year experimental research and found that although straw returning to the field after being digested increased the greenhouse gas emissions of animal husbandry, it can also increase the soil carbon sequestration capacity. Compared with the direct return of straw to the field, indirect straw return’s carbon sequestration and emission reduction ability was increased by 3.47 times [54]. Through field experiments combined with the actual measurement method and material balance algorithm research, Li et al. found that the corn straw returning to the field after serving as feed and being digested by animals had the largest carbon sequestration and emission reduction effect. The CO₂ and NO₂ emissions were significantly lower than those of direct straw returning and non-straw returning fields [55]. Ma et al. used the actual measurement method to show that under the same nitrogen application rate (260 kg/ha), the treatment of straw returning to the field after being fed to and digested by the cow could significantly increase the carbon sequestration of wheat plants compared with direct straw returning to the field, thereby enhancing the carbon sequestration ability of wheat fields and the net ecosystem productivity (NEP) was 0.14 kg/m² [56].

To sum up, straw is a good feed source for regions in China with a more developed livestock and poultry breeding industry, and it serves as a reference for enhancing straw utilization techniques and reducing greenhouse gas emissions. In addition, straw as feed provides an excellent ecological material and energy cycle in agriculture. The authors suggest that, in the future, China should develop a straw-feeding technology that is less expensive and cleaner and retains more nutrients in straw suitable for the growth of livestock and poultry. Depending on the soil conditions, it is also necessary to re-treat the straw residues from livestock and poultry to improve the soil’s physical and chemical properties and fertility.

3.1.3. Straw as Fuel

Straw is an important source of biomass energy. Straw contains 40% carbon, and the energy density can reach 14.0–17.6 MJ/kg [55]. In other words, the thermal energy value of 2 t of straw can replace 1 t of standard coal. Therefore, promoting the use of straw can effectively reduce primary energy consumption, provide efficient clean energy, and contribute to the country’s environmental construction and CO₂ emission reduction. Therefore, using straw as fuel is gradually becoming a hotspot of scientific research. The main technologies of straw-to-fuel utilization in China can be summarized as “Four transformations and one electrification mode”, namely solidification, carbonization, gasification, liquefaction and used for power generation. These utilization methods have been well developed with matured technical systems and well-established standard systems and industrial models. According to different utilization technologies, it can be subdivided into straw baling and direct combustion heating (heat) technology, straw curing technology, straw carbonization technology, straw-to-biogas technology, straw-to-cellulosic ethanol production technology, straw pyrolysis gasification and other gasification technologies, straw direct combustion (co-firing) power generation technology and straw cogeneration technology [57,58,59]. In general, there are various means to utilize straw as fuel, each with its characteristics and advantages (

Table 4. Comparison of different straw-to-fuel utilization methods.

Technologies	Types	Advantages	Disadvantages
Straw curing technology	Screw extrusion, piston stamping, circular mould compressing, roll-in stalk forming.	High thermal efficiency; wide range of applications; easy storage, transportation and use.	High power consumption, insufficient mechanical wear resistance, short life span and high production cost.
Pyrolysis carbonization technology	Straw charcoal.	Low cost; high calorific value; wide range of uses; high efficiency.	The high technical difficulty, fast wear-out rate of parts, and high energy consumption of raw materials; require strict storage conditions against fire and moisture.
Straw Gasification Technology	Bio-gasification, pyrolysis gasification.	High production controllability and low impact from natural conditions; significantly improves thermal efficiency; wide range of uses.	Slow development; low benefits when scaling up, low operating rate; high scrap rate.
Straw Liquefaction Technology	Hydrolysis liquefaction, pyrolysis liquefaction.	Recycling, easy to use; low transportation cost and high added value.	Long fermentation time; low gas generation rate; high cost of raw material storage, transportation and pre-treatment.
Straw Power Generation Technology	Direct combustion power generation	Environment-friendly distributed power generation technology.	High investment, high cost and low efficiency compared to coal-fired power plants.
	Co-fired power generation	Flammable, low-cost, low-risk renewable energy utilization.	Ash deposition affects system operation.
	Gasification power generation	Flexibility, low cost, compact equipment with less pollution,	Slow development and low power generation rate.
	Cogeneration	generate electricity and provide heat; high heat energy utilization efficiency.	Low heat price and no obvious economic advantage.

). Among these means, straw-to-fuel by carbonization (especially straw biochar) and straw solidification are promising for future development in China due to their advantages such as good environmental performance, energy-saving potential, high production efficiency and potential to be deeply processed into high value-added products [60].

At present, straw-to-fuel utilization has been limited in China because it has not yet formed a complete system for the straw-to-fuel utilization industry. In addition, the cost of straw collection, storage and transportation is still very high, the quality of machinery processing for straw packing and transportation is relatively low and the equipment related to straw-to-fuel utilization is yet not well established [61]. Corn straw, wheat straw and rice straw have become the main sources of straw-to-fuel utilization due to their straw yield, collectible amount, storage and transportation costs. According to the statistics, in 2020, the straw-to-fuel utilization rate of these three main crops accounted for about 88.5% of the total straw-to-fuel utilization in China [1]. Regarding the different regions, the proportion of straw-to-fuel utilization in Northeast China ranked first, reaching 14.9% [1]. The straw-to-fuel utilization rates in the middle and lower reaches of the Yangtze River and the southwest region were 10.4% and 10.1%, respectively, which are higher than the national average [1]. The straw-to-fuel utilization rates in Northwest China, South China and North China were all 4.1% [1]. Due to the long wintertime and low temperature in the Northeast China region, the demand for straw-to-fuel utilization is relatively high. Among the top ten provinces for large-scale straw-to-fuel utilization, the three provinces are in the northeast region, accounting for 36.09% of the national total straw-to-fuel utilization [1] (Figure 2). In addition, the straw fuel utilization in the middle and lower reaches of the Yangtze River represented by Anhui and Jiangsu showed a high level. Among the 112 enterprises with an annual straw-to-fuel utilization volume of more than 50,000 t corresponding to 36.61% were from the middle and lower reaches of the Yangtze River [1]. A total of 39.22% of the 51 enterprises with annual straw-to-fuel utilization of more than 100,000 t were from the middle and lower reaches of the Yangtze River region [1]. Different from the northeast region and the middle and lower reaches of the Yangtze River, due to the relatively low rural development level in the southwestern region, the energy consumption for rural living is still dominated by the direct combustion of traditional biomass such as straw. Farmers use straw directly for cooking and heating, so the straw-to-fuel utilization ratio in this region is also relatively high.

Straw as fuel can effectively reduce fossil energy consumption and carbon emissions and is a renewable and sustainable zero-carbon or negative-carbon energy source [62]. Huo et al. applied the Life Cycle Assessment (LCA) method to investigate the GHG emissions of heating technologies such as straw bale burning, briquette fuel and pyrolysis charcoal gas co-production using corn stalks as raw materials in Beijing and found that their carbon emissions are only one-tenth to one-seventh of coal, providing a feasible technical path for China’s rural energy transformation [63]. Zhang et al. used the CDM methodology to estimate that 8,965,300 t of standard coal could be replaced with a 50% straw utilization rate in Jiangsu Province, reducing many pollutant emissions [64]. Chen et al. used the life cycle assessment method to evaluate China’s large and medium-sized biogas comprehensive utilization systems. The research showed that the biogas comprehensive utilization project can provide high-quality energy and organic fertilizers with excellent ecological and economic benefits [65]. In addition, some studies have shown that the production of biological natural gas by anaerobic fermentation using straw as raw material could achieve a maximum GHG emission reduction of 1.97 × 10⁸ t/a by replacing natural gas, which is close to 2% of China’s total greenhouse gas emissions [66]. This indicates that straw biogas technology has good potential to achieve carbon emission reduction. Yang et al. used the whole life cycle assessment method to study the environmental performance of processing straw into biochar through biomass pyrolysis polygeneration technology and co-generate combustible gas and electricity and found that the GHG emission reduction could reach 136.45g/MJ by replacing fossil fuels and biochar carbon sequestration, achieving negative carbon emission with good economic performance [67]. Therefore, straw-to-fuel utilization technologies such as straw anaerobic fermentation and straw pyrolysis co-production have good application prospects.

Table 5. Carbon emission reduction efficiency of different straw utilization methods.

Utilization Methods	Type of Straw	Calculation Baselines	Method	Calculation Boundaries	Carbon Reduction Efficiency (kg CO2e kg−1)	Data Sources
Straw to produce fuel ethanol	Corn straw	Burning in field	LCA model; GREET (Greenhouse Gases, Regulated Emissions and Energy in Transportation) model	From crop planting to ethanol fuel used in vehicles	0.71	[68]
	Crop straw	Gasoline production	IPCC guidelines	Ethanol production	0.84	[69]
Straw to produce bio-oil	Corn straw	Diesel production	LCA model; GREET model	From crop planting to bio-oil production.	0.51	[70]
	Crop straw	Field burning and diesel production	LCAmodel; GREET model	From crop straw collection to bio-oil production.	0.64	[71]
	Crop straw	Diesel production	LCA model; GREET model	From crop planting to bio-oil production.	0.49	[72]
Straw to produce aviation fuel	Corn straw	Fossil aviation fuel production	LCA model	From crop planting to aviation fuel used in airplanes (include engineering construction)	0.34	[73]
	Rice straw, corn straw and wheat straw	Fossil aviation fuel production	LCA model; Aspen plus model	From crop planting to aviation fuel used in airplanes (include engineering construction)	0.10–0.15	[74]
	Rice straw, corn straw and wheat straw	Field burning and fossil aviation fuel production	LCA model	From crop planting to aviation fuel used in airplanes	0.46	[71]
Straw to produce briquette fuel	Corn straw	Coal production	LCA model and SPREADSHEET	From crop straw collection to the utilization of briquette fuel	1.17	[75]
	Corn straw, peanut shell and saw dust	Field burning and coal production	CDM(clean development mechanism) methodology	From crop straw collection to the utilization of briquette fuel	1.14	[76]
	Corn straw	Coal production	Hybrid-LCA model	From crop planting to the utilization of briquette fuel	1.09	[77]
	Corn straw	Coal production	LCA model	From crop straw collection to the utilization of briquette fuel	1.10	[78]
	Crop straw	Field burning and coal production	IPCC guidelines	From crop straw collection to the utilization of briquette fuel	0.97	[8]
	Crop straw	Straw matural decomposition	CDM methodology and IPCC guidelines	From crop straw collection to the utilization of briquette fuel	1.56	[79]
Straw direct combustion power generation	Crop straw	Field burning and coal-fired power generation	LCA model	From crop plant-ing to electric power used	1.24	[80]
Straw baling direct-fired central heating	Crop straw	Straw matural decomposition	CDM methodology and IPCC guidelines	From crop straw collection to the utilization of briquette fuel	1.58–1.67	[81]
Straw gasification power generation	Wheat straw	Coal-fired power generation	LCA model	From crop straw collection to electric power used (include engineering construction)	0.71	[82]
	Crop straw	Field burning and coal-fired power generation	IPCC guidelines	From crop straw collection to electric power used	0.87	[8]
	Crop straw	Field burning and coal-fired power generation	LCA model	From crop straw collection to electric power used	0.84	[71]
Straw to biogas	Crop straw	Field burning and farmers cooking energy	CDM methodology and IPCC guidelines	From crop straw transportation to biogas used	3.56	[83]
	Crop straw	Straw natural decomposition	CDM methodology and IPCC guidelines	From crop straw transportation to biogas used	0.87	[84]
	Crop straw	Field burning and biogas production	LCA model	From crop straw transportation to biogas used	1.95	[71]
	Crop straw	Field burning and biogas production	IPCC guidelines	From crop straw collection to the utilization of biogas	1.05	[8]

summarizes the results of previous studies on the carbon emission reduction efficiency of straw-to-fuel utilization.

It has been found that the current average carbon emission reduction in straw-to-fuel utilization in China is 1.04 kg CO₂e kg⁻¹. Among these, large-scale straw to biogas and biological natural gas has the highest carbon emission reduction efficiency with an average value of 1.86 kg CO₂e kg⁻¹. Straw-to-aviation fuel has the lowest carbon emission reduction efficiency with an average value of 0.31 kg CO₂e kg⁻¹. However, due to the differences in the types of straws, system boundaries, calculation baselines and the selection of emission factors, the carbon emission reduction efficiencies of straw-to-fuel utilization presented by different research results are quite different and the reliability is relatively low. Therefore, to further explore the carbon emission reduction potential of straw-to-fuel utilization in China, it is necessary to establish a standard and unified carbon emission accounting system for straw utilization. In addition, the carbon emission reduction potential of straw utilization also depends on factors such as each utilization method’s technical and economic suitability and the amount of utilization, which still requires further investigation.

Studies have shown that straw-to-fuel has the greatest GHG emission reduction potential among the five major straw utilization methods. It is estimated that by 2030 and 2060, the emission reduction contribution of straw-to-fuel utilization will reach 55.2% and 62.8% of the total GHG emission reduction in straw utilization in China [8]. According to statistics, before 2009, the literature published in English paid relatively close attention to applying straw-to-fertilizer and straw-to-feed. After 2009, more research has shifted to straw utilization as fuel [85]. This shows that in the past ten years, straw-to-fuel utilization has become a research hotspot of academic research, which will undoubtedly promote the improvement of straw utilization and carbon emission reduction.

3.1.4. Straw as a Base Material

Using straw as a base material means that in the production of soilless cultivation, a culture medium is made according to a specific production formula where straw is used as raw material and combined with auxiliary materials. This medium can be used for edible mushroom cultivation and also for plant seedling and plant cultivation. It can provide nutrients to promote growth and prevent diseases and deworms [86,87]. Commonly used straw-to-base material technologies include straw used for edible fungus cultivation technology, straw preparation cultivation substrate and container technology.

Although the straw-to-base material technology has been well developed with high technical and economic suitability and thus has a high application potential, the demand for straw as a base material is nearly negligible compared with China’s huge total straw production. The volume of straw used as a base material is relatively low among the five major straw utilization methods. Regarding different regions, the proportion of straw-to-base material utilization rate in the middle and lower reaches of the Yangtze River, the southwest region and the South China region are 1.5%, 1.3% and 1.0%, respectively, all higher than the national average [1]. The straw-to-base material utilization rate in the North China, northwest and northeast regions are 0.3%, 0.3% and 0.2% [1].

In recent years, the annual output of edible fungi in China has been about 45mt, and the annual consumption of base material from straw has exceeded 10 mt [88]. Straw can totally or partially replace wood as a cultivation substrate and effectively reduce wood consumption. Studies have shown that rice straw and bean straw can be used to replace broad-leaved tree sawdust to cultivate black fungus partially. When the straw replacement ratio is 25% to 35%, the growth of mycelium and fruiting body is the same as using broad-leaved tree sawdust as a cultivation base [89]. Using corn cob, cottonseed husks, bagasse and rice straw powder as raw materials and supplemented by organic nitrogen sources and mineral elements for oyster mushroom production, the biotransformation efficiency could reach 100% to 150% [90]. Huo et al. used the emission factor method to estimate the carbon emission reduction effect of using straw as a base material. The authors found that replacing forest wood resources with straw-based cultivation material in the forest carbon sink can reach 0.462 kg CO₂e kg⁻¹ [8]. Straw as a cultivation base can reduce production costs and improve straw utilization and solve environmental pollution problems such as resource waste and carbon emissions.

3.1.5. Straw as a Raw Material

Straw has a similar composition to wood as it is rich in cellulose and lignin and has good biodegradability, making it possible to replace wood as an environmentally friendly raw material for industrial processing. Straw-to-raw material utilization usually adopts a series of production processes to prepare various industrial raw materials, including straw artificial board production technology, composite material production technology, clean pulping technology, woven mesh technology, polylactic acid production technology, wall material technology and membrane preparation technology. Currently, straw is mainly used as a raw material with artificial board production, clean pulping technology and woven mesh technology in China. In addition, some new technologies for straw utilization have also been applied to develop biodegradable polymer materials to replace wood, plastic, and nanocellulose crystals, using cellulose and lignin in straw as the basic raw materials. The new straw material has the advantages of diversification of raw materials, plasticized preparation process, ecological product, cost-efficient application, and low-carbon regeneration [91,92,93]. It plays an important role in comprehensive straw utilization and has potential in relation to energy-saving, emission reduction, convenience and recyclability.

The use of straw as a raw material in China is slightly higher than its use as a base material, but it is still far lower than the other three utilization methods. Due to the strong seasonal character of straw availability and some factors, such as the silicide wax layer on the surface of straw, the straw-based artificial board is used less in building constructions in China [94]. There are also practical problems when using straws for paper making. For example, the preparation cycle is rather long, and a large amount of chemicals are needed, the yield is low and the production cost is relatively high. In addition, the high cost of straw collection, storage and transportation, the under-developed straw collection, storage and transportation system and the inability to have a long-term stable straw supply all limit the use of straw as a raw material. Regarding different regional utilization, the proportion of straw-to-raw material utilization rates in the southwest region and the middle and lower reaches of the Yangtze River exceeds the national average (1.0%), which are 1.3% and 1.5%, respectively [1]. The utilization ratio of straw-to-raw material in the northeast region is 1.0%, the same as the national average. However, the proportions of straw-to-raw material utilization in South China (0.9%), North China (0.7%) and Northwest China (0.3%) are relatively low [1].

In China’s papermaking industries, wastepaper pulp and wood pulp are the mainly used raw materials, and the proportion of non-wood pulp raw materials is relatively low. In 2020, wood pulp consumption accounted for 40% of the total pulp consumption [95]. The high demand for raw wood pulp has led to the deforestation of many forest resources, which is not conducive to the green and sustainable development of the papermaking industry. To solve the increasingly tense connection between supply and demand of papermaking raw materials and forest resources, China has been devoted to the investigation of non-wood raw materials, especially agricultural straw pulping and papermaking technology and has carried out pilot production practices in some papermaking enterprises for many years. It has been proved that using straw as a raw material for papermaking has considerable potential to replace wood and reduce GHG emissions [96,97,98]. When using wood fiber as a raw material, 3 m³ of wood is needed to produce 1 m³ of the ordinary artificial board. Using straws to replace wood fiber can reduce deforestation and reduce carbon emissions. Wanhua Ecology estimated the GHG emission reduction through using waste crop straw to replace wood to produce artificial panel board and found that a straw board production line with an annual output of 250,000 m³ could reduce emissions of about 380,000 t CO₂e per year and reduce emissions of 1.52 t CO₂e when producing 1 m³ of straw-based artificial panel board [99]. Chen et al. further investigated the production process of straw-based panels of the Wanhua Ecological Board Industry with the material balance algorithm. The GHG emissions of straw burning or rotting, using wood and straw as a raw material to produce artificial panels were evaluated. The results showed that the carbon emission reduction in producing 1 m³ crop straw-based artificial board was about 1.42 t CO₂e [100]. Sun used the emission factor method to estimate the carbon emissions of a straw paper-making project with an annual output of 100,000 t of straw pulp and 220,000 t of household paper in Heilongjiang Province and found that the carbon emission reduction in using per one ton straw can reach 0.99 t CO₂e [101].

3.2. Discussions

Straw utilization in China initially formed a five-dimensional utilization model, which includes the dominant directions of straw-to-fertilizer, straw-to-feed, and straw-to-fuel, complemented with straw-to-base material and straw-to-raw material utilization. Straw utilization has also been developed with priorities in agricultural application and straw-to-fuel utilization and is characterized by intensification, industrialization and high value. Meanwhile, each region has formed its straw utilization structure according to its local conditions and factors such as the natural environment, resource allocation, industrial development, economic and social development [11] (Figure 3).

Under China’s five-dimensional straw utilization model, straw returning to the field as fertilizer by ploughing and deep ploughing, rotary tillage and mulching and no-tillage mulching is easy to promote due to its advantages of low cost, high production efficiency and convenient operation. On the other hand, its application should be accompanied by scientific straw decomposition laws and control methods to prevent soil compaction and the harm of diseases and pest infestation. For straw-to-feed utilization, the straw green (yellow) storage technology, straw cutting and kneading technology and straw extruding–bulking technology have a more promising application potential due to the advantages of simple technology, less nutrient loss, high feed conversion rate, good palatability and long storage time. In addition, adding a certain proportion of roughage during processing is an important measure to improve the quality of straw forage and reduce carbon emissions from straw utilization. The straw-to-fuel utilization technology represented by straw carbonization and straw solidification has attracted much attention because of its high thermal efficiency, good energy-saving potential and emission–reduction effect. Straw-to-fuel has been increasingly investigated in scientific research, especially in energy consumption reduction in the straw-to-fuel production process, development of high-intensity production equipment, and production cost reduction. Although using straw as a base material and raw material accounts for a relatively small proportion of the comprehensive straw utilization in China, it is an indispensable method and has good utilization potential in the future. In particular, straw-to-base material and straw-to-raw material have played an important role in replacing wood chips, increasing forest carbon sinks, reducing carbon emissions and protecting the environment.

This study found that the five-dimensional straw utilization model in China has a great contribution to agricultural carbon emission reduction, and the carbon emission reduction contributions of different straw utilization methods vary significantly. Comprehensive straw utilization accounts for 92.7% of China’s greenhouse gas emission reduction [8]. Straw-to-fuel utilization is of great significance in the future use of straw and its carbon emission reduction. According to the data in Table 5, it can be estimated with the emission factor method that the carbon emission reduction due to straw-to-fuel utilization in China in 2020 reached 63.43 × 10⁹ kg CO₂. Meanwhile, the carbon emission reduction efficiency varies significantly in different regions, with different crop straws and utilization methods. China has not yet formed a standard and unified carbon emission accounting method for straw utilization. However, carbon accounting is the basis for all work to achieve carbon peaking and carbon neutrality. The establishment and unification of China’s standard carbon emission accounting methods can integrate with the global carbon emission system, thereby stimulating the vitality of the national carbon trading market and promoting the improvement of carbon reduction potential in various industries.

At present, the carbon emission accounting methods applied for the case of straw utilization in China can be divided into two categories, namely the measurement-based and calculation-based methods. The calculation-based methods can be further divided into the material balance and the emission factor methods. Based on the above three types of methodologies, the IPCC inventory method, life cycle assessment (LCA) method, CDM methodology and modelling method have been developed and evolved in practical applications. The emission factor and the LCA methods are most used in straw utilization carbon emissions accounting in China. In contrast, the actual measurement method and material balance method are mostly used in theoretical or experimental research. In addition, among the CDM methodologies, only eight methodologies related to agriculture are approved by the Chinese Executive Council, accounting for only 3.2% of the total methodology, which is not yet well developed. The modelling method has also been applied in some studies. For instance, the DNDC model [102], Logistic model [103], Leap model [104] and CGE model [105] are relatively commonly used for carbon emission estimation. Therefore, at present, most of the existing research only estimated the carbon emission reduction potential predictions for a specific utilization method of a single type of straw (especially the straw-to-fuel utilization of corn, wheat, and rice straw). Due to the differences in research areas, system boundaries, research baselines and types of straws, it is yet possible to compare and analyze the existing estimated results.

4. Recommendations

The study has demonstrated that the FDSUM is of great significance in the utilization of straw resources in China. However, there is a need for sustainable interactions to advance the utilization of straw resources for improving its utilization and reducing carbon emissions. Given this, the study proposes the following recommendations:

• According to the planting structure, industrial structure and economic characteristics of various regions in China, the FDSUM should be further developed according to local conditions;
• Future development of straw utilization in China should prioritize straw-to-fertilizer, straw-to-fuel and straw-to-feed;
• After returning straw as fertilizer, the impact on soil structure and fertility and the decomposition and transformation mechanisms of straw need to be further studied;
• When straw is used as feed, it is necessary to use additives that can retain more nutrients and reduce the methane emission of livestock and poultry after overeating;
• Establishing an economic straw collection, storage and transportation system and developing straw-to-fuel utilization technology with a high production efficiency and good environmental protection is the foundation of straw power generation technology;
• Adding appropriate auxiliary agents or reducing production costs by developing new technologies can improve the utilization rate of straw-to-based and straw-to-raw materials and reduce the waste of forest resources;
• Strengthening the cooperation between the government and non-governmental organizations to establish a national unified carbon emission accounting system for straw utilization is of great significance for guiding agricultural emission reduction.

5. Conclusions

This study provided agricultural resource utilization and environmental sustainability perspectives on China’s latest development of straw utilization. In 2020, the total amount of straw available in China was 7.22 × 108 tons. Straw-to-fertilizer, straw-to-feed and straw-to-fuel have become the main utilization modes for economic and technical reasons. The research estimates that the carbon emission of straw-to-fuel will be reduced by 63.43 × 109 kg CO₂.

The efficient utilization of straw resources is one of the ways to alleviate the energy crisis and develop clean energy. Although China has abundant straw resources and has significantly reduced environmental pollution, the economic cost, technical constraints and environmental protection will become the three key factors for the utilization of straw resources in the future.

China is rich in biomass resources such as straw, which is beneficial when utilized but serves as waste when not used. In the future, research institutions, universities and enterprises need to cooperate in developing appropriate physical or chemical additives to adjust the composition of straw when it is returned to the field or used feed. Straws can better promote soil fertility, meet the nutritional needs of livestock and poultry and reduce greenhouse gas emissions such as carbon dioxide and methane. In addition, China’s rapid economic development and huge population require a lot of energy consumption. Bioenergy generation from various renewable energy resources, notably crop straw, is one of the sustainable approaches for waste management, particularly in China. In this regard, producing raw materials for power generation by carbonization and solidification of straw has a good prospect.

Thus, improving the utilization of straw is an effective way to turn waste into treasure, meet the needs of human society and reduce environmental pollution. It will not damage biodiversity or have adverse effects on other areas. The key feature is to establish a national overall strategy according to each region’s composition of straw resources, development needs, and economic characteristics. It helps to make efficient use of straw resources, support the development of straw utilization technology with a good economy and high environmental protection and promote inclusive and sustainable development. Meanwhile, the standard carbon emission accounting system is needed to evaluate the straw utilization mode, providing a scientific basis for the producers and decision-makers of the bioenergy system from the perspective of environmental sustainability. This study suggests further improving the utilization of the FDSUM to promote sustainable development.