A Comprehensive Assessment of Rice Straw Returning in China Based on Life Cycle Assessment Method: Implications on Soil, Crops, and Environment

Abstract

Straw returning has been shown to improve farmland soil, increase crop yield, and reduce global warming. This study investigated the models of six rice straw-returning techniques based on the life cycle assessment method. Compared to the direct modes, the indirect ones showed a greater environmental impact; up to 20.56 times in acidification and emission potentials. There was no significant difference in climate change among the six types. Except for the burning effect, all other modes showed improvement in soil fertility; phosphorus and organic matter content increased by 66.66% and 30.85%, respectively, microbial biomass carbon content doubled (105%), the four organic carbon components increased by >50%, crop morbidity was reduced, and diversity of soil fungus was improved. Feeding, as an indirect method for enhancing soil fertility and economic benefits, is set to emerge as a leading practice in China’s straw management. However, straw-returning technology is limited by cost, greenhouse gases, and increased risk of diseases and pests. There is an urgent need for further improvement and development of carbon sequestration and emission reduction in China’s agriculture.

1. Introduction

China is one of the most abundant crop straw resources. According to the Food and Agriculture Organization statistics, its annual rice production ranked first globally in 2021 [1], accounting for 27% of the global production value, close to 222 million tons of rice straw [2]. If not effectively utilized and treated, these straw resources would be a great resource of waste and environmental pressure [3]. The Government of China has recently announced its goal to achieve “carbon neutrality and carbon peaking” [4]. To accomplish this goal, returning straws could be the most economical and sustainable way of utilizing straw resources to improve soil fertility and promote agriculture while effectively solving the problem of agricultural waste treatment [5,6].

Straw-returning modes are mainly divided into two categories: direct and indirect. The first includes mulching and ploughing, while the second includes composting, feeding, and carbonization [7]. Currently, direct straw-returning modes are mainly used, because the indirect modes have limited application due to their high cost and immature technology [8]. The results in Table S1 show that the current status of straw treatment in representative countries around the world, including the U.S., as a developed country, has reduced straw burning to less than 0.6% and uses 20% of it as feed for livestock, leading to indirect mode [9]. Returning straw can effectively replace the burning straw technique. It also reduces negative environmental impact by improving soil structure, fertility, and carbon pool through straw recycling [6,10,11,12]. According to the data from the Ministry of Agriculture and Rural Affairs of the People’s Republic of China, returning straw can increase the organic matter content of the soil by 5.0–7.0%, the crop yield by 2.0–4.5% [13], and as an organic fertilizer, it can replace about 40% of the used amounts of chemical fertilizers [14]. Overall, returning straw can promote agricultural carbon sequestration, crop yield, and emission reduction [15,16]. There are different methods of straw returning, but some can increase the risk of crop diseases, affect seed emergence and seedling growth, reduce crop yield, and increase greenhouse gas emissions from farmlands [17]. Therefore, the assessment of a straw-returning method is of great significance for improving soil and promoting agricultural carbon sequestration and emission reduction.

Life cycle assessment (LCA), applied to the entire ecological industry chain, helps in the evaluation of resources, energy consumption, and environmental impact on crop production processes [18]. Therefore, crop straw-returning LCA has become an important reference for choosing practical returning methods [19]. Current research predominantly examines single or relatively straightforward methods of straw incorporation. For instance, Li et al. [20] conducted a focused study on direct straw incorporation into the fields. Similarly, Liu et al. [21] exclusively investigated partial indirect straw incorporation. Xu et al. [22] broadened the scope by examining both direct and indirect methods, though their study was not exhaustive. These studies aim to address the issue of establishing a clear understanding of the environmental emission baselines and the environmental impact of various models under diverse scenarios.

To scientifically evaluate carbon sequestration and emission reduction base, crop disease risk, soil fertility, and crop yield improvement potentials of different straw-returning modes, an LCA of six rice straw-returning modes was conducted based on the LCA standard method [23]. The present study established an analysis model for the resource, energy consumption, and environmental emissions of the returning modes, by assessing the impact on farmland environment (greenhouse gases), soil (nutrients and carbon pools), crops (morbidity and yield), and cost-effectiveness. The outcome of the study will prove beneficial in providing technical support and a theoretical basis for selecting reasonable and scientific returning methods of rice straw and promoting agricultural carbon sequestration and emission reduction.

2. Material and Methods

2.1. Research Objective

Crop straw utilization in China reached 647 million tons in 2021, with a comprehensive rate of 88.1%, and the amount of returning straw was about 400 million tons [24]. The average annual rice yield without straw returning is 500 kg/acre. The distribution and utilization rate of rice straw resources in China is shown in Figure 1. To provide a theoretical basis and decision support for the ecological and low-carbon utilization of rice straw in field management, we comprehensively evaluated the effects of different returning modes on soil nutrients and carbon storage, crop diseases, and greenhouse gas emissions in agricultural fields. This study focuses on Chinese rice straw, with a functional unit of 100 kg and a grain-to-straw ratio of 1:1.

2.2. Assessment Strategy

With reference to ISO 14040:2006, we compared the energy consumption, greenhouse gas emissions, soil impact (fertility and carbon pool), and crop impact (disease and yield) of six rice straw-returning modes (burning, mulching, ploughing, feeding, composting, carbonizing), using the LCA method. Next, we evaluated the cost-effectiveness of LCA using the cost analysis method. In addition, a non-straw-returned control group was set up for comparison purposes. When conducting LCA, the main considerations are harvesting, transportation, processing, and returning. The system boundary of the six modes of rice straw returning is shown in Figure 2.

2.3. Evaluation Method

(1) Characterization: Based on the three environmental impacts—energy depletion, global warming, and environmental acidification [19]—we used the equivalent factor method to evaluate the greenhouse effect through CO₂ equivalent [26] and the environmental acidification potentials [27] through SO₂ equivalent calculation (Table 1). The latter refers to the sum of all the environmental impacts throughout the entire life cycle, as referred to in the following formula;

$$E_{p\left(x\right)}=\sum E_{p\left(x\right)i}=\sum \left[Q_{\left(x\right)i}{E}_{F\left(x\right)i}\right]$$

(1)

where E_p(x) is the impact potential of the x-th environment during the process of returning straw; E_p(x)i is the potential impact of the i-th emission substance on the x-th environment; Q_(x)i is the emission of the i-th substance; E_F(x)i is the equivalent factor of the i-th substance’s potential environmental impact on the x-th substance.

(2) Standardization and Weighted Evaluation: The 2000 world per capita environmental impact potential was used as a benchmark for standardization [28], and the weight coefficient set was selected through expert evaluation in the study according to Wang’s research [29]. Knowing that, there are three common methods for determining weights [30]; the standardized benchmark values and weight coefficients are shown in Table 2, and the formula for calculating the environmental impact value after standardization and weighting is as follows:

$$S=\sum W_x\left[E_{p\left(x\right)}/E_{p\left(2000\right)}\right]$$

(2)

where S is the environmental impact value during the process of returning straw; W_x is the weight value of the x-th potential environmental impact; E_p(x) is the potential environmental impact of the returning straw process on the x-th type of potential environment; E_p(2000) is the benchmark value for the world’s per capita environmental impact in 2000.

Table 1. Equivalence factor of various environmental impacts [26,31].

Type	Global Warming				Environmental Acidification
Influential substances	CO2	CO	CH4	N2O	SO2	NOx
Equivalent coefficient	1	2	21	310	1	0.7

Table 1. Equivalence factor of various environmental impacts [26,31].

Type	Global Warming				Environmental Acidification
Influential substances	CO2	CO	CH4	N2O	SO2	NOx
Equivalent coefficient	1	2	21	310	1	0.7

Table 2. Benchmark value and weight of environmental impact index ([32,33]; Supplementary Table S3).

Type	Unit	Standard Value	Weight Coefficient
Energy depletion	MJ·per−1·a−1	2,590,457	0.15
Global warming	kg·per−1·a−1	6869	0.12
Environmental acidification	kg·per−1·a−1	52.26	0.14

Table 2. Benchmark value and weight of environmental impact index ([32,33]; Supplementary Table S3).

Type	Unit	Standard Value	Weight Coefficient
Energy depletion	MJ·per−1·a−1	2,590,457	0.15
Global warming	kg·per−1·a−1	6869	0.12
Environmental acidification	kg·per−1·a−1	52.26	0.14

2.4. Inventory Analysis

Based on the inventory data collected using the above method, a complete LCA inventory was conducted on the energy consumption input and environmental emission output of the six straw-returning modes, as shown in

Table 3. Life cycle list of different straw-returning modes.

Modes		Burning	Mulching	Ploughing	Feeding	Composting	Carbonizing
Input	Standard coal (kg)	0.00	0.00	0.00	0.00	0.00	6.51
	Gasoline (L)	0.00	0.00	0.00	0.49	0.49	2.45
	Diesel oil (L)	0.14	0.26	0.58	0.84	0.84	1.19
	Energy consumption (MJ)	4.65	8.42	19.17	50.67	50.67	283.10
Output	CO2 (kg)	240.80	150.17	130.08	183.20	179.46	128.26
	N2O (g)	226.56	162.80	221.08	471.22	561.34	816.60
	CH4 (g)	1264.01	3766.03	1981.26	7896.10	1962.77	577.05
	CO (g)	6833.89	1.62	3.68	21.60	21.60	57.13
	NOx (g)	337.78	1.42	3.23	6.33	6.33	15.94
	SO2 (g)	200.36	0.65	1.28	2.49	1.72	59.23
	PM2.5/10 (g)	2053.34	26.52	27.86	31.24	31.24	48.07

. In addition, the energy consumption list is shown in Tables S4 and S5, and the emission data are shown in Tables S6 and S7. The specific operating procedures and limiting factors are shown in Table S2.

(1) Harvesting: Following the Guiding Opinions on the Scientific Return of Crop Straw for Autumn Harvest in 2022 issued by the Ministry of Agriculture and Rural Affairs of the People’s Republic of China [34], the direct returning method of rice straw used for harvesting is a combination of the harvest with a straw shredding and a scattering device. While in mulching mode, harvesting is a crushing function. For the ploughing mode, in addition to harvesting and crushing, a deep loosening and leveling machine is also required for soil compaction. For the other straw-returning modes, regular agricultural harvesters are used. The mulching method involves cutting straw into 5–10 cm lengths by the harvester. In contrast, the ploughing ring method buries the straw 8–10 cm deep within the soil. The emission factors of straw collection, according to the standards GB/T 24675.6-2021 and IPCC guidelines (2006), [35,36] define the straw collection method to obtain the energy consumption and environmental emissions data during this stage.

(2) Transportation: According to the provisions of GB/T 42118-2022 [37] and the Road Traffic Safety Law revised in 2021, when straw transportation is within a short distance of 10 km, agricultural vehicles can be used for transportation, otherwise, specialized vehicles should be used. Based on the emission factors of the IPCC guidelines (2006) and Jia’s research, energy consumption and environmental emission data of transportation were obtained [38,39,40].

(3) Baling: According to the Guiding Opinions on the Scientific Return of Crop Straw for Autumn Harvest in 2022 issued by the Ministry of Agriculture and Rural Affairs of the People’s Republic of China, GB/T 42118-2022, and the literature data [41], the baling process for the feeding and composting modes requires small square bundling machines, while the carbonization mode requires additional mechanical baling for large bundle shape. According to the emission factors of the IPCC guidelines (2006) and the energy consumption power of the baler, the energy input and environmental emission data for this process are calculated.

(4) Processing: The straw processing of the six different straw-returning modes is shown in Figure 2. The silage method of the feeding mode [27] requires the addition of 0.45 kg of urea, approximately 1 kg of quicklime, and water to achieve a water content of about 50%. After 10 days, processing emissions data are obtained from the digestion and fermentation processes of straw in the stomach of cows. The environmental emissions are then determined based on the literature data. As for the composting mode [42], 0.1 kg of straw composting agent is added to 0.5 kg of urea. The carbonization mode requires fossil energy input, where straws are directly carbonized (400 °C high-temperature pyrolysis), and the energy consumption for pyrolysis of 100 kg of straw is equivalent to 6.51 kg of standard coal [43]. Noting that the incineration process of the burning mode refers to the environmental emissions data involved in the direct burning process in the field [44,45].

(5) Scattering: The feeding and carbonization modes require a step of scattering organic fertilizer and biomass charcoal into the field. Selecting the crawler type is based on the emission factors according to the IPCC guidelines (2006) and Wang et al. [46]. The corresponding energy consumption input and environmental emissions data are then calculated.

(6) Returning: Based on the current research, the following year’s environmental emission data of the six straw-returning modes (100 kg straw) were compared and analyzed, using the non-straw-returning mode as a control [47,48,49,50,51,52,53,54].

3. Results and Analysis

3.1. Environmental Impact

(1) Environmental impact assessment: energy consumption, global warming, and environmental acidification potentials, as well as the PM₂._5/10 emission potential of the six returning modes are shown in Figure 3.

The energy consumption of the indirect returning modes was found to be higher than the direct modes due to the high energy input required for harvesting and transportation, especially in the carbonization process (283.10 MJ) [43], whereas in the burning mode, energy was only required during the harvesting process (4.65 MJ). As for climate warming, greenhouse gases were the key factors, and the source of their emissions was mainly from energy consumption, processing, and straw returning. The feeding mode had the highest potential (495.13 gCO₂e; higher than the burning straw mode), mainly due to the large amount of greenhouse gases, such as CH₄, produced by the straw passing through the stomach of cows during overfeeding [55]. Interestingly, the potential values of the composting and burning modes were similar (351.24 gCO₂e), possibly due to the large amount of greenhouse gases generated during the straw composting process [56]. In environmental acidification, the main influencing factors were energy consumption during processing and emissions from burning or carbonization. Although the burning mode had the lowest energy consumption potential, its burning process generated a large amount of SO₂ and NO_x [57], resulting in its high acidification potential (436.81 gSO₂e) compared to the other modes. On the other hand, the carbonization mode had an acidification potential of 70.39 gSO₂e, mainly derived from the consumption of a large amount of coal and acidic gas emissions from the carbonization process of straw. The acidification potential of other modes was relatively low and originated from the emissions during the energy consumption process. As for PM₂._5/10 emission potential, the content of 95.20 g/100 kg straw in the literature was used as the reference value for evaluating PM₂._5/10 in this study [32,58]. The results indicated that the burning mode had the highest emission potential, with an increase of 2056.87% compared to the baseline value, mainly due to the large amount of PM generated by the burning straw [59]. PM₂._5/10 emission potential of the other modes was mainly due to the emissions of harvesting and energy consumption, which significantly decreased by 50–75% compared to the benchmark value.

(2) Environmental Impact Index: To compare the different types of environmental impacts and their degree, a benchmark for World Per Capita Environmental Impact in 2000 was used for standardization and weighted average. The environmental impact and comprehensive indices of the six returning modes were calculated, as shown in

Table 4. Standardization and Weighted Analysis of Environmental Impact Potential of Different Straw-Returning Modes.

Modes	Standardized Index			Weighted Index
	Energy Consumption(kg·per−1)	Global Warming(kg CO2e·a−1·per−1)	Environmental Acidification(kg SO2e·a−1·per−1)	Energy Consumption	Global Warming	Environmental Acidification
Burning	1.80 × 10−6	5.11 × 10−2	8.36	2.69 × 10−7	6.14 × 10−3	1.17
Mulching	3.25 × 10−6	4.07 × 10−2	3.15 × 10−2	4.87 × 10−7	4.89 × 10−3	4.40 × 10−3
Ploughing	7.40 × 10−6	3.50 × 10−2	6.78 × 10−2	1.11 × 10−6	4.20 × 10−3	9.49 × 10−3
Feeding	1.96 × 10−5	7.21 × 10−2	1.33 × 10−1	2.93 × 10−6	8.65 × 10−3	1.86 × 10−2
Composting	1.96 × 10−5	5.75 × 10−2	1.18 × 10−1	2.93 × 10−6	6.90 × 10−3	1.65 × 10−2
Carbonizing	1.09 × 10−4	5.70 × 10−2	1.37	1.64 × 10−5	6.84 × 10−3	1.89 × 10−1

.

The results showed that the carbonization, feeding, and burning modes had the highest standardized value of energy consumption (1.09 × 10⁻⁴ kg·per⁻¹), global warming (7.21 × 10⁻² kg CO₂e·a⁻¹·per⁻¹), and environmental acidification (8.36 kg SO₂e·a⁻¹·per⁻¹), respectively, generated after returning 100 kg of straw to farmland, and equivalent to 0.01%, 7.21%, and 836% of the benchmark. Overall, the indirect modes showed a higher environmental impact compared to the direct modes (carbonization mode, with the highest energy consumption index, is 61 times and 33.68 times higher than the burning and mulching returning modes, respectively). Additionally, there are significant differences in the environmental impacts of returning straw under different modes. The variance analysis results show significant differences among the treatment modes in terms of energy consumption (F = 1033.2, p < 0.001), global warming potential (F = 47.8, p < 0.001), and environmental acidification (F = 1187.3, p < 0.001). However, the six modes were not significantly different for the global warming index. The environmental acidification index for the burning mode was 265.9 and 123.3 times higher than the mulching and ploughing modes, respectively, indicating that many acidic gases such as SO₂ are generated during the straw-burning process [60]. Using the principle of Monte Carlo simulation, the uncertainty results of the environmental impact weighted index for different straw-returning modes were obtained (Figure 3d). Because the environmental impact of returning straw mainly comes from the pollution gases emitted from incineration and carbonization steps, the uncertainty range was 0.59–1.76 and 0.10–0.29, respectively. Therefore, burning straw and fossil fuels in the field is prone to incomplete combustion, which can lead to uncertainty in the amount of pollution gas emissions.

3.2. Evaluation of Soil Fertility

Nitrogen, phosphorus, potassium, magnesium, calcium, sulfur, and other elements contained in rice straw are abundant fertilizer resources and essential nutrients for crop growth [61]. Therefore, straw returning can effectively improve farmland soil and enhance soil fertility. The calculated impact of different straw-returning modes on soil physical properties (porosity, bulk density, moisture) and soil nutrients (total nitrogen, exchangeable potassium, available phosphorus) are shown in Figure 4. In addition, the impact on soil fertility and physicochemical properties is shown in Tables S8 and S9 [11,33,50,62,63,64,65,66,67,68,69,70].

(1) Soil physical properties: As shown in Figure 4a, all returning modes improved the porosity of the soil, which significantly increased by 13.71% in the feeding mode. Similarly, the soil bulk density showed a decrease which renders the soil loose and porous [71,72], especially in ploughing mode (12.06%). Both the burning and conventional straw-returning modes can significantly preserve moisture and water content [73]; the straw feeding mode increased the soil moisture by 79.34%, while other returning modes showed an increase of 20–30%. However, the burning mode resulted in a 15–20% decrease in water content due to the high temperature from combustion.

(2) Soil nutrients: As shown in Figure 4b, the conventional straw-returning modes, except for the burning mode, improved soil nutrients [5,61], while the promoting nutrient effect of indirect modes was higher than the direct modes; carbonization mode showed the highest increase in total nitrogen content (14.37%), which is 3.28 times higher than the mulching mode. The available potassium content increased by 11–23%, most significantly in the ploughing, feeding, and carbonization returning modes (all >20%). In addition, the available phosphorus in indirect returning modes increased more than in indirect modes, particularly in the composting mode (66.66%); which was 11.28 times higher than in the burning mode. The main reason for this result is that the composting process has a phosphorus-dissolving effect on bacteria, which improves the phosphorus content in straws [74,75]. The organic matter content significantly increased mostly in carbonization (30.85%) and feeding (26.35%) modes. The former was mainly carbonized biochar, while the latter was organic matter formed by composting and fermentation. Except for some increase in potassium and phosphorus (14.29% and 5.91%), the content of other total nitrogen and organic matter has significantly decreased (9.85% and 15.90%), because, in the burning mode, organic matter and nitrogen-containing substances in the straw and soil generate volatilization and loss, while potassium and phosphorus are retained with the ash content [76].

3.3. Evaluation of Soil Carbon Pool

The carbon pool of soil was calculated and evaluated based on the carbon pool content of the non-straw-returning mode [65,77,78]. Accordingly, the total organic carbon (TOC), labile organic carbon (LOC), microbial biomass carbon (MBC), and dissolved organic carbon (DOC) of the soil are shown in Figure 4c. The conventional returning modes increased the soil carbon content, while the straw-burning mode showed a negative effect for all the carbon content (MBC decreased by 18.13%), except for the 4.36% increase in LOC.

The feeding mode showed the highest increase in TOC (57.14%); 10.13 times higher than the lowest increase in the composting mode. As for DOC, which plays an important role in maintaining soil nutrients and biological fertility [79], all modes showed an increase of 10–60% except for the straw-burning mode; the highest increase was in the feeding mode (60%), followed by the mulching mode (40%). LOC is an active chemical component in soil, which can significantly affect the dissolution, adsorption, desorption, absorption, and migration of soil chemicals [80]. Compared to the non-straw-returning mode, the LOC content of the feeding mode increased by 77.72%; which was 27.17 times higher than the lowest increase (carbonization mode). The main reason for this result is that during the feeding process, a large amount of organic carbon is produced by the decomposition of microorganisms in the cow’s stomach [77]. However, the LOC content of the ploughing mode decreased by 13.26%, mainly due to the longer time required for the growth and reproduction of microorganisms that decompose straw. Within one year, straw decomposition is reduced significantly and mostly exists in the form of steady-state carbon [77]. MBC, an important link in soil carbon conversion, significantly increased (16–105%) in the returning modes, except for the burning mode. It doubled in the composting mode (105%), followed by the feeding mode (70.83%). Both types have a large amount of microbial degradation and conversion of straw, which increases the organic carbon content of soil microorganisms. The burning mode would increase soil carbon emissions from farmland, while other modes would increase the input of soil carbon, thereby increasing the content of the soil carbon pool in rice fields, especially the feeding and composting modes.

3.4. Evaluation of Crop Diseases

The use of inappropriate straw-returning modes can increase the risk of crop diseases, which is one of the main reasons why farmers are reluctant to use straw returning to the field [17]. The effects of different straw-returning modes on soil fungi (OTUs abundance, Shannon diversity) and the impact of crop morbidity (corn/rice stem root, cabbage root swelling) are shown in Figure 4d and calculated according to the literature [81,82,83,84,85,86,87,88].

(1) Soil Fungus: As shown in Figure 4d. Compared to non-straw-returning modes, the fungal abundance in direct modes had significantly increased; the ploughing mode having the highest (5.26%), followed by the mulching mode (4%), due to the large amount of inert carbon brought to the soil, which helps the growth and abundance of soil fungi [55]. However, the abundance in other modes decreased significantly. Except for the burning mode, the decrease in carbonization mode was at most (−30%); 3–10 times compared to the composting and feeding modes, due to the high temperatures that killed a large number of fungi. Therefore, direct returning modes increased the risk of soilborne diseases compared to the indirect modes. Except for the burning mode, the others affected fungal diversity. The highest increase was observed in the ploughing mode (2.94%), followed by the feeding (2.48%), and carbonization modes (2.17%), indicating that these treatment methods have more types of fungi originating from carbon [85]. Accordingly, these four modes increase the carbon content in the soil, provide nutrients and energy for soil microorganisms, regulate the metabolic functions and structure of soil microbial communities, and improve soil quality, especially the mulching mode, which reduces the overall fungal community diversity by increasing the relative abundance of some fungal groups [89].

(2) Crop morbidity: The impact of returning straw on crop morbidity is shown in Figure 4d, where straw burning can kill a large number of pathogenic bacteria [81], which reduces the morbidity of crops (10.39%). All the other direct modes can increase the risk of crop diseases, because the straw will be scattered in the soil, and its slow decomposition process is suitable for the overwintering of disease pathogens, reproduction, and accumulation of primary infection sources, which greatly increase the morbidity [83]. The ploughing returning mode has the largest crop morbidity (68.77%), followed by mulching (48.20%). On the contrary, the porous structure of straw biochar is more conducive to the retention of nutrients and the growth of beneficial bacterial communities. The regulation of soil acidity, mineral nutrition, and interaction between microorganisms assisted in preventing and controlling diseases [87]; with disease rate reduction in the carbonization mode being the largest (60%). The significant decrease in the composting (41.66%) and feeding modes (13.20%) may be due to the significant increase in soil nutrient content, allowing plants to absorb nutrients and grow more robust, which results in disease resistance [17].

3.5. Benefit Evaluation of Straw-Returning Modes

Based on research surveys [54,68,90,91,92] and the annual yield of 7500 kg/hm of rice under non-straw-returning modes, the life cycle costs of the six straw-returning modes and the estimated yield of rice for the following year were calculated (Figure 5).

Life cycle costs are mainly derived from resource and mechanical equipment investments, labor, coal, diesel, and other expenses during the straw-returning process (Figure 5a). The cost of indirect returning modes is much higher than the direct modes, and it increases significantly with the complexity of the land-returning process. In addition, the costs of several returning modes are mainly due to equipment and labor, accounting for over 80% of the total cost, which to some extent limits farmers from adopting straw-returning technology. The rice yield and cost data for the next year are shown in Tables S10 and S11. Compared to the non-straw-returning mode, all modes showed a significant increase in rice yield (Figure 5b), except for the burning mode which caused a decrease by 15% (6375.0 kg/hm). The feeding mode had the highest increase of 44.44% (10,833.0 kg/hm), followed by the composting mode with an increased yield of 25.06%, consistent with the positive effects of the above two modes on soil fertility and carbon storage. An appropriate straw-returning mode can effectively enhance soil fertility, improve soil quality in rice fields, and have a certain promoting effect on increasing rice yield [93]. Overall, the economic benefits of the carbonization and burning modes were observed to be the lowest as compared to the feeding and composting modes, due to their higher cost and their limited effect on soil fertility and carbon storage, resulting in a minor increase in rice yield. However, the straw-burning mode has mainly a negative effect on soil fertility and carbon storage, leading to a significant reduction in rice yield; The other modes, especially feeding and composting, have a positive promoting effect on soil fertility and carbon pool, which leads to increased yield and rice income.

A comprehensive evaluation and analysis of the effects of different straw-returning modes on soil fertility, carbon pool, crop diseases, and rice yield was conducted (Figure 6). The overall effect of the burning mode is negative, especially in reducing the abundance and diversity of soil microorganisms, while the other modes present a positive effect. Direct returning modes significantly increase the risk of crop diseases, with limited effect on soil fertility and carbon storage, which lowers farmers’ interest, whereas indirect returning modes can effectively reduce the risk of crop diseases, improve soil structure, and increase soil fertility, especially in the feeding mode, which effectively increased soil fertility and organic carbon. However, its long cycle, high cost, and high greenhouse gas emission limit its promotion and application. In summary, the conventional mode of returning straw faces the situation of “low cost, high risk” and “low risk, high cost”.

4. Conclusions

Straw return is a widely adopted agricultural practice, especially in Europe and North America, where it has become one of the primary farming methods. In the United States, using straw as feed is the predominant approach for indirectly incorporating it back into the fields. Based on this study, the indirect return of straw to the field has a greater environmental impact, with emissions 20.56 times higher than those of direct straw return. Notably, the feeding method results in 449.74 g of carbon dioxide emissions, which is even higher than the 351.24 g emitted by burning. Feeding increases rice yield by 44.44% and moisture by 79.34%, while composting raises phosphorus by 66.66%, and carbonization peaks nitrogen at 14.37%. These methods cut disease by up to 60% but are costlier than direct methods, which, though cheaper, pose higher disease and soil risks. Feeding is more effective and cost-efficient than composting or carbonizing. It is poised to become a leading practice in China’s straw management. Yet, the high greenhouse gas emissions need optimization, possibly through selective animal breeding or innovative bio-fermentation to reduce emissions.

To conclude, straw returning is the main method for straw utilization and a reasonable means to improve soil, increase crop yield, and reduce greenhouse gas emissions. Although straw-returning technology has become increasingly developed, there are still some problems that need to be solved urgently; (a) lack of systematic research and analysis on the greenhouse gas emissions of conventional rice straw returning and burning returning modes, while neglecting the large amount of greenhouse gas emissions and their impact on crop diseases caused by the incorrect returning mode; (b) The research on straw returning needs to be deepened, especially in context to the carbonization mode. Currently, there are many problems such as high costs, high greenhouse gas emissions, high potential for environmental acidification, and insufficient improvement of soil fertility and organic carbon storage, which results in low participation of farmers; (c) The technology of returning straw needs further development and innovation. Coordinating the environmental impact of straw returning with factors such as soil fertility and carbon storage according to local conditions is the key to promoting green, low-carbon, and efficient utilization of straw returning. Therefore, the development and promotion of ecologically efficient straw-returning technologies, such as environmental insect ecological overbelly returning and biochar-based microbial inoculants, are important directions for future straw-returning modes, to further improve the utilization level of straw returning and assist in the development of agricultural carbon sequestration and emission reduction.