Assessment of Crop Residues and Corresponding Nutrients Return to Fields via Root, Stubble, and Straw in Southwest China

Abstract

China stands as one of the world’s largest agricultural powerhouses, boasting abundant crop resources. Nonetheless, there remains a lack of clarity regarding the extensive scale of crop residue return in the fields. Drawing from direct field measurements and comprehensive survey data, this paper pioneers the reporting of residues from the five primary crops, shedding light on the associated nutrient components, including carbon (C), nitrogen (N), and phosphorus (P) replenishment via crop roots, stubble, and straw in the Southwest China region for the year 2012. The results showed that the total amount of the main crop residue resources was 97.4 Mt, which was composed of 17.8 Mt, 12.6 Mt, and 67 Mt for crop root, stubble, and straw, respectively. After crops harvested, there were 7165.8 kilotonne nutrient C, 132.2 kilotonne nutrient N, and 9.8 kilotonne nutrient P of crop residues returned to the fields through crop root, respectively, accounting for 44.6%, 48.2%, and 43.4% of the total nutrient returned, which was the main part of crop nutrients return to fields. The amount of nutrient C, N, and P returned through stubbles were 5017.3 kilotonne, 75.9 kilotonne, and 6.8 kilotonne, respectively, accounting for 31.3%, 27.6%, and 30.6% of the total return of crops. From the composition proportion of residues nutrients return to field, the orders were all expressed as follows: root > stubble > straw. According to the optimum fertilization amount of the main crops in Southwest China, the returned of crop residues nutrient N in maize, rice, rapeseed, and wheat can replace approximately 5.6%, 18.4%, 11.2%, and 14.8% of nitrogen fertilizer, and 2.4%, 8.3%, 3%, and 9.2% of phosphate fertilizer, respectively. This conclusion is beneficial for regulating the practice of returning crop residues to the fields and the use of agricultural fertilizers, aiming to achieve sustainable development in agricultural production.

1. Introduction

China is one of the largest agricultural countries in the world. In 2015, China’s total crop output was 621.44-million tons, and the sown area was 1133.405-million hectares [1]. At the same time, China is currently the largest producer and consumer of synthetic fertilizers, accounting for about 35% of total global consumption [2]. Excessive fertilizer use causes the loss of biodiversity in farmland ecosystems [3] and increased pollution from agricultural non-point sources [4]. As an important measure of sustainable agricultural practice, direct straw returning to the field is considered to be the most economical and ecological straw utilization method [5]. China is an agricultural giant, and it generates a tremendous amount of crop straw annually. Since 2010, the average annual output of field crop straw is about 750-million tons, accounting for about 1/4 of the total crop straw output in the world [6]. No matter where, due to the lack of practical treatment and utilization technology, the annual amount of straw incineration exceeds 200 Mt, and the loss of nutrients such as nitrogen and phosphorus is equivalent to about 60% of the total national fertilizer production [7]. The improper treatment of crop straw will cause many impacts such as air pollution and the deterioration of soil structure, as well as a great impact on ecological environment [8,9,10]. At the same time, excessive straw returning to the field will also cause the denitrification potential of soil and the production of nitrous oxide (N₂O) [11,12,13]. Therefore, it is important to study the effective nutrient resources of straw and the amount of fertilizer that can be replaced by straw returning to the field to maintain the sustainable development of agriculture [14,15,16,17].

Act as an agricultural resource to replace traditional organic fertilizer, the direct or indirect return of crop residue to soil can alleviate the shortage of fertilizer needed for agriculture and reduce the pollution of fertilizer on farmland environment [10,18,19,20,21]. After crop harvest, crop residues can be removed from the field (to be used for livestock feed, for fuel, or for other off-field purposes), burned in the field (for pest control or for simple residue removal management), covered directly, or plowed to cover the field surface [22,23]. In addition, crop residues were considered an important source of soil carbon, nitrogen, and other nutrients to agricultural soils [9,10,22,24,25,26]. Therefore, it is crucial to accurately estimate crop residue resources and their direct or indirect return to fields in the researches on soil organic carbon and nitrogen cycling in farmland [26,27,28,29]. At present, most of the research on crop residue resources at the national or regional scale were usually estimated by the conversion of economic yield obtained from agricultural statistic yearbooks and harvest index or the proportion of straw to grain [7,30,31,32]. In these studies, the following problems exist: First, the same crops in different regions usually adopt particular grain to straw proportion and moisture content of the economic yield, which tends to result in lower accuracy [33]. Second, due to a lack of data on the straw returning proportion of crop residues at a regional scale, particular parameters or estimated values were also used. For example, Li et al. [24] used the DNDC model to compare the dynamics of soil organic carbon in farmland in the United States and China on a national scale. The model simulation results showed that the United States agricultural soil has a net increase of 72 Mt of carbon per year, while China’s farmland soil loses 74 Mt of carbon per year, which was in a negative equilibrium state. In addition, the author believes the treatment of crop straws in China and the United States is one of the main reasons for the difference of soil organic carbon dynamic in farmland. However, the latest research results of Zhao et al. on soil organic carbon in Chinese farmlands show that since the Second National Soil Survey (completed in the late 1970s and early 1980s), the rate of soil organic carbon sequestration continuously increased [26]. The main reason for the difference between the two research results may be that the research of Li et al. underestimated the crop residues returning proportion of China’s main crops in the application of DNDC model simulation, which was set by the study less than 20% [24]. In addition, as an important part of crop residue return, the stubble was often returned to the field together with underground roots of crops. However, in the calculation of the amount of straw returning, because of the lack of data on the height of stubble, the amount of stubble return to the field was not calculated separately. This part was often estimated together with the straw, resulting in the underestimation of the amount of crop residues returned.

Unfortunately, although the studies on estimations of crop straw resources are abundant in China, there is very little research on the amount of crop residues and corresponding nutrient returned to fields at the regional or national scale. Due to the influence of farmers’ farming habits, agricultural mechanization, national policies, and economic factors, the proportion of straw returning to the field and the height of stubble in main crops varies greatly [22,34]. Moreover, in situ investigations of these data are quite lacking. Most of the studies have to use empirical values or fixed parameters, resulting in the low credibility of the results [24]. Therefore, this study is based on field measurement and field survey combined with farmer household visit to estimate the quantity of the five main crops residue resources and corresponding residue nutrient C, N, and P return to fields in different regions of the southwest of China. The aim of these research findings is to explore the varying contributions of different crop straw incorporation to various nutrients and their potential impact on soil quality. This research aims to guide the rational allocation of agricultural fertilizers and crop straw incorporation, providing valuable insights for the sustainable development of agriculture.

2. Materials and Methods

2.1. Measurement of Crop Biomass Density

Based on the characteristics of the cropping system in Southwest China, eight typical agricultural counties were selected (Table 1; Figure 1). More information about the county selection may be found in Zhao et al. [26]. In each typical county, three sample points for each main crop were established for crop biomass determination about 1–2 weeks before harvesting. The sample size of high-stalk crops such as maize and rapeseed was 100 cm × 50 cm × 30 cm (length × width × depth). In addition, the sample size of low-stalk crops such as rice, wheat, and barley was 50 cm × 50 cm × 30 cm (length × width × depth). In each sample plot, the whole plant body was collected. The number of crops and planting density in the sample was recorded. After the sample was marked, the crop was cut along the surface with a knife. Plant samples were wrapped in plastic paper and taken to the laboratory. All the clods mixed with the root system (underground part) were dug out and placed in a woven bag for marking. Then, transport the rooted soil woven bag to the nearest local pond or river, soak it in water, and then gently shake the woven bag until the soil was thoroughly rinsed. The rooted woven bag was labeled and delivered to the laboratory for the measurement of drying weight, the total carbon, total nitrogen, and total phosphorus content of each crop straw and root. The organic carbon content was determined using the potassium dichromate oxidation method, while the total nitrogen content was measured using the Kjeldahl method. The C/N ratio was calculated by dividing the organic carbon content by the total nitrogen content.

Table 1. Typical county and main crops investigated in Southwest China in 2012.

Province	Typical County	Main Crop
Sichuan	Yanting County, Guanghan City	Maize, rapeseed, wheat, and rice
Chongqing	Dianjiang County	Rapeseed, maize, and rice
Guizhou	Zunyi City, Puding County	Maize, rape, and rice
Yunnan	Luliang County, Luxi City	Wheat, maize, and rice
Tibet	Dazi County	Barley

Table 1. Typical county and main crops investigated in Southwest China in 2012.

Province	Typical County	Main Crop
Sichuan	Yanting County, Guanghan City	Maize, rapeseed, wheat, and rice
Chongqing	Dianjiang County	Rapeseed, maize, and rice
Guizhou	Zunyi City, Puding County	Maize, rape, and rice
Yunnan	Luliang County, Luxi City	Wheat, maize, and rice
Tibet	Dazi County	Barley

Figure 1. The agricultural crops and farmland distribution in the southwestern region of China.

Figure 1. The agricultural crops and farmland distribution in the southwestern region of China.

2.2. Survey of Residue Return

According to the distribution of county-level highways in the southwestern region, survey points were set along the county road routes, and each survey site was separated by about 10 km. The survey site selects the location with good visual field conditions and tries to ensure that the survey points are evenly distributed in the main farming area. According to the planting situation of the main crops in each province, the straw return was investigated after the crops harvested along the design route.

A total of 2349 survey points was set up in the southwest. Among them, Sichuan Province surveyed 521 points; Chongqing City surveyed 454 points; Guizhou Province surveyed 459 points; Yunnan Province surveyed 519 points; and Tibet surveyed 396 points. The survey contents of each survey site include location information of the surveyed farmlands, crop stubble height, crop rotation, planting and harvesting method, and situation of residue return (including proportion of burning, proportion of total return, proportion of partial return and proportion of partial return amount, etc.).

2.3. Calculation of Crop Residue and Corresponding Nutrient Resources and Their Return Amount

Crop residue resources, the amount of residue return, and corresponding nutrient C, N, and P of each crop were calculated by the following formula:

$${\mathrm{T}}_i=\sum {\mathrm{B}}_{1i}·{\mathrm{A}}_i+\sum {\mathrm{B}}_{2i}·{\mathrm{A}}_i$$

(1)

$${\mathrm{R}}_{\mathrm{i}}=\left(\sum {\mathrm{B}}_{2i}·{\mathrm{A}}_i\right)·\left[{\mathrm{r}}_1+\left({\mathrm{r}}_2·{\mathrm{r}}_3\right)+\left(\frac{{\mathrm{h}}_i}{{\mathrm{H}}_i}\right)\right]$$

(2)

$${\mathrm{R}}_{\mathrm{C}}=\sum {\mathrm{R}}_i·{\mathrm{P}}_{\mathrm{C}i}$$

(3)

$${\mathrm{R}}_{\mathrm{N}}=\sum {\mathrm{R}}_i·{\mathrm{P}}_{\mathrm{N}i}$$

(4)

$${\mathrm{R}}_{\mathrm{P}}=\sum {\mathrm{R}}_i·{\mathrm{P}}_{\mathrm{P}i}·2.29$$

(5)

Within this context, the variable “i” represents different crop types, while “T”, “B₁”, “B₂”, “A”, and “R” correspond to the crop residue resource, root biomass density, crop straw biomass density, crop planting area, and residue return to fields, respectively. “H” and “h” denote the height of the crop and stubble during the harvest stage. Furthermore, “r₁”, “r₂”, and “r₃” refer to the proportions of total return, partial return, and the corresponding quantities of partial return, respectively. “R_C”, “R_N”, and “R_P” signify the quantities of nutrient carbon (C), nitrogen (N), and phosphorus (P) returned to the fields, while “P_C”, “P_N”, and “P_P” represent the concentrations of total organic carbon, total nitrogen, and total phosphorus. Lastly, a conversion coefficient of 2.29 is applied for the transformation of elemental phosphorus into phosphorus pentoxide.

3. Results

3.1. Biomass Density and Residue Resource

The planting area, as well as the biomass density of straw and root, across five main crops was shown in

Table 2. Biomass density (t/hm²) and residue resource (Mt) of main crops at the harvest stage in Southwest China in 2012.

Province	Crop Type	Sown Area (hm2)	(Straw + Stubble) Biomass Density (t/hm2)	Root Biomass Density (t/hm2)	(Straw + Stubble) Resources (Mt)	Root Resources (Mt)
Sichuan	Maize	1,371,100	5.68 ± 0.08	0.56 ± 0.06	7.8	0.8
	Rice	1,997,800	5.84 ± 0.88	1.29 ± 0.31	11.7	2.6
	Rapeseed	9,801,400	9.70 ± 0.80	1.54 ± 0.37	9.5	1.5
	Wheat	1,234,100	7.67 ± 1.43	1.90 ± 0.17	9.5	2.3
Chongqing	Maize	468,400	3.00 ± 0.10	0.26 ± 0.03	1.4	0.1
	Rice	687,000	6.45 ± 0.90	2.05 ± 0.57	4.4	1.4
	Rapeseed	204,600	8.64 ± 2.03	1.30 ± 0.45	1.8	0.3
Guizhou	Maize	778,400	9.35 ± 0.63	0.66 ± 0.12	7.3	0.5
	Rice	684,500	8.08 ± 0.70	3.14 ± 0.54	5.5	2.2
	Rapeseed	560,800	6.62 ± 1.41	1.61 ± 0.71	3.7	0.9
Yunnan	Maize	1,505,100	4.13 ± 0.10	0.31 ± 0.04	6.2	0.5
	Rice	1,152,700	6.94 ± 0.47	3.74 ± 0.20	8.0	4.3
	Wheat	437,300	6.19 ± 1.32	1.05 ± 0.03	2.7	0.5
Tibet	Barley	37,800	3.06 ± 0.39	0.56 ± 0.07	0.1	0.02

. The straw biomass density of maize ranged from 3.00 to 9.35 t hm⁻², and the biomass density of roots ranged from 0.26 to 0.66 t hm⁻². The range of straw plus stubble and root biomass density of rice was from 5.84–8.08 t hm⁻² and 1.29–3.74 t hm⁻², respectively. The highest straw biomass density in the straw plus stubble biomass was the rapeseed planted in Sichuan Province, and the smallest is the barley planted in Tibet. The highest root biomass density is rice planted in Yunnan Province, and the smallest is maize in Chongqing.

According to the Formula (1), the total residue resource of five main crops in Southwest China was 97.4 Mt, of which the stubble and straw resource was 12.6 Mt and 67 Mt, and the root resource was 17.8 Mt. Rice is the largest crop of straw resources in Southwest China, accounting for 41.1% of total straw resources, followed by maize and rapeseed (25.2% and 18.1%), and the smallest is Tibet’s barley, accounting for only 0.1%. From the perspective of the main crop straw resources in various provinces, the amount of rice straw in Sichuan Province was the largest (11.7 Mt), and the smallest was barley planted in Tibet (only 0.1 Mt).

An accurate amount of residue resource is an important basic data for the study of nutrient cycling and biomass energy resource utilization in farmland ecosystems [22,32,35,36]. At present, there are usually two main methods for estimating the amount of straw resources: one is estimated by biomass sample, the other is estimated based on the economic yield of crops and grain-to-straw proportion [7,37]. Both methods have their advantages and disadvantages. The first method has a large amount of work in the field because of the large number of crop biomass samples required, and there are few studies using this method. The basic data of the second method is mainly derived from statistical yearbook, but the results are greatly affected by the proportion of grain to straw. Although there are few studies using the sample method, our results are within the reported range [25,30].

3.2. The Situation of Crop Straw Return

The situation of crop straw return after harvesting, including total straw return, straw burning, partial return, and the proportion of partial return amount, were investigated (

Table 3. The situation of residues return of main crops in Southwest China in 2012.

Crop Type	Province	Stubble Height (cm)	Proportion of Burning (%)	Proportion of Total Return (%)	Proportion of Partial Return (%)	Proportion of Partial Return Amount (%)
Maize	Sichuan	20.0	33.9	8.8	57.3	6.1
	Chongqing	10.5	7.4	0.3	92.3	3.3
	Guizhou	13.2	11.9	0.0	88.1	2.3
	Yunnan	15.2	6.9	12.2	81.0	7.4
Rice	Sichuan	26.1	26.0	23.3	50.8	4.5
	Chongqing	28.9	12.3	10.5	77.3	9.7
	Guizhou	15.0	11.2	3.6	85.2	4.4
	Yunnan	24.6	6.0	16.2	77.8	17.9
Rapeseed	Sichuan	25.3	69.3	2.6	28.0	17.5
	Chongqing	9.9	3.6	0.0	96.4	2.4
	Guizhou	8.2	30.6	0.0	69.4	1.0
Wheat	Sichuan	28.6	55.9	13.2	30.8	23.7
	Yunnan	18.5	29.1	8.4	62.4	16.1
Barley	Tibet	14.8	1.2	0.0	98.8	19.4

). As shown in Table 3, the average stubble height of the five main crops in Southwest China was 14.7 cm for maize, 23.7 cm for rice, 14.5 cm for rapeseed, 23.6 cm for wheat, and 14.8 cm for barley, respectively. The average stubble height of wheat and rice is relatively high, and the lowest is rapeseed. From the regional point of view, the height of rice stubble in Sichuan and Chongqing was relatively high, close to 30 cm, and the height of rapeseed planted in Guizhou Province and Chongqing City was relatively low (8.2 cm and 9.9 cm, respectively). The stubble height of crops is related to farmers’ harvest habits, harvest methods (manual or mechanical), and whether crop straw are used as feed [22,38]. For example, if the paddy field is ploughed with cattle, when harvesting rice, the height of stubble will be lower to facilitate next planting season, and more straw can be used as feed for cattle. If it is ploughed by machine, the height of the stubble will be higher, and even in many places, only the rice ears will be harvested.

Overall, the proportion of total return was the lowest (less than 7%), and the partial return proportion was relatively high. Burning phenomena existed for all major crop residues in each province. In comparison, the average proportion of burning of rice or maize straw was relatively low, and that of wheat and rapeseed straw was relatively high. Among them, the burning proportion of wheat straw in Sichuan Province was more than 65%, which was the highest in all crops, and the lowest was barley straw in Tibet, which was only 1.2%. Among the proportion of the total return, rice straw in Sichuan Province was the highest (18.3%). It was worth noting that there was no total return of straws in three main crop straws (rice straw, maize straw, and rapeseed straw) in Guizhou Province, and there was barley straw in Tibet. The proportion of partial return of barley, rice, and maize straws was relatively higher, and the highest was barley straw in Tibet (98.8%), indicating that there was a widespread partial return of straw.

China has a long history of agriculture, and there are records of the use of crop residues very early [30,39]. The return of crop residue to the field is related to the habits of farmers harvesting, labor situation, the degree of agricultural mechanization, and the availability of crop residues, etc. [6,25,32]. About 30 years ago, crop residues in China were mainly used for rural life fuel, livestock feed, compost, etc., so the proportion of crop residues directly to the fields was not high [22,25]. With the development of society, especially with the rapid expansion of agricultural mechanization, crop residue management has fundamental changes [22]. For example, with the development of the rural economy and the popularity of natural gas fuels during the last two decades, fuel availability has improved, and rural households are becoming more dependent on fossil fuels and less dependent on crop residues as a fuel for life. As agricultural machinery replaces traditional cattle farming, the proportion of residues as animal fodder is also decreasing [25]. Therefore, the main problem at this stage is how to handle and utilize such a large amount of crop residues in China. Due to the scattered distribution of residue resources in China, the high cost of collection and transportation, and the low degree of industrialization of comprehensive utilization of residues, the phenomenon of randomly throwing and burning residue in the field can be seen everywhere [32,40,41]. The behavior of residue burning not only causes urban and rural air pollution but also threatens the lives and property of farmers and normal transportation [8,9,10,40]. In the past decade or so, the state and local governments have formulated a series of regulations and policies that prohibit crop residues burning [40,42]. The phenomenon of residues burning in various places has been significantly reduced, but it still exists in many areas, especially in mountainous areas and remote areas. Most of crop residue burning is burnt in the field simply for the convenience of planting the next seasonal crop [23]. In the plains, mechanically harvested crop residues can be easily returned to the field [40,42]. Yet in most rural areas, especially in mountainous areas with poor traffic conditions, farmers lack machinery to adequately chop crop residues for incorporation into the soil, so they have to burn crop residues in their fields or thrown away [22,25].

3.3. The Amount of Crop Residue Return

Based on the height of stubble and crop plants at the harvesting stage, the amounts of stubble return of main crops were estimated (

Table 4. The number of residues of the main crops return to field in Southwest China in 2012.

Province	Crop Type	Amount of Root Return (Kilotonne)	Amount of Stubble Return (Kilotonne)	Amount of Straw Return (Kilotonne)
Sichuan	Maize	769.5	533.6	886.9
	Rice	2581.2	2571.4	2325.6
	Rapeseed	1510.0	1252.6	623.4
	Wheat	2349.6	3130.7	1299.1
Chongqing	Maize	121.1	50.9	44.1
	Rice	1405.1	1106.8	596.4
	Rapeseed	265.9	103.2	38.5
Guizhou	Maize	510.2	354.2	141.0
	Rice	2152.1	897.6	341.5
	Rapeseed	902.8	174.0	23.6
Yunnan	Maize	465.8	328.5	1067.8
	Rice	4308.2	1491.9	1959.0
	Wheat	458.6	542.3	400.5
Tibet	Barley	21.3	21.4	18.1

). The total amounts of residue return of the five main crops were 40,145.7 kilotonnes. Among them, the total amounts of root, stubble, and straw return were 17,821.4 kilotonnes, 12,559.1 kilotonnes, and 9765.3 kilotonnes, respectively, accounting for 44.4%, 31.3%, and 24.3% of the total. It showed that the total amount of straw return of five main crops was lower than the amount of return by roots and stubble. The average amount of stubble return in maize, rice, rapeseed, and wheat was 316.8 kilotonnes, 1516.9 kilotonnes, 509.9 kilotonnes, and 1836.5 kilotonnes, respectively, and was 466.6 kilotonnes, 2611.7 kilotonnes, 892.9 kilotonnes, and 1404.1 kilotonnes for root return, respectively.

Judging from the composition of crop residues returned in various provinces (Figure 2), Guizhou province has the lowest proportion of straw return to total residue return, which was only 9.2%, and the highest was Yunnan (31.1%). The range of the proportion of stubble return to total residue return was 21.4% (in Yunnan province) to 37.8% (in Sichuan province), with an average of 30.8%. The average proportion of root return to total residue return was 46.3%, the highest was 64.9% (in Guizhou), and the lowest was 35% (in Tibet).

The contribution of the stubble and root return reached 77.2%, which showed that these two parts were the mainly important part of crop residue return to fields. However, due to the difficulty in obtaining data on the amount of stubble at the regional scale, there are few related reports on the amount of crop stubble. In the studies of crop residue return to fields at the national or regional scales, most research did not consider the amount of crop stubble, and the stubble was directly calculated together with the straw. In the case of straw partial returned to the field, such a calculation method may lead to the underestimation of the amount of crop residue returned to the fields.

3.4. Amount of Crop Residue Nutrient Resources and Their Return

According to Formulas (3)–(5), the amounts of root and straw nutrient resources in main crops were estimated. As shown in

Table 5. Crop residue nutrient (C, N, P and C/N) resources (103 t) in root, stubble, and straw of main crops in Southwest China in 2012.

Province	Crop Type	Root				Stubble				Straw
		C	N	P	C/N	C	N	P	C/N	C	N	P	C/N
Sichuan	Maize	329.2	5.7	0.3	57.75	215.2	6.2	0.5	34.71	2922.3	83.6	6.2	34.96
	Rice	998.5	19.7	1.3	50.69	955.3	14.2	1	67.27	3376.4	50.2	3.6	67.26
	Rapeseed	615.8	11.3	0.7	54.5	517.8	7.8	0.4	66.38	3417.1	51.2	2.9	66.74
	Wheat	960.6	13.3	1.2	72.23	1312.3	13.7	2.2	95.79	2653.6	27.7	4.5	95.8
Chongqing	Maize	52.3	1	0.1	52.3	21	0.5	0.03	42.00	558.8	13.4	0.9	41.7
	Rice	555.7	10.7	0.7	51.93	444.5	6.7	0.5	66.34	1335.8	20.1	1.5	66.46
	Rapeseed	111	2.1	0.1	52.86	43.5	0.6	0.04	72.50	700.9	9.8	0.7	71.52
Guizhou	Maize	214.4	3.6	0.3	59.56	145.1	3.6	0.2	40.31	2836.9	69.5	4.4	40.82
	Rice	840.6	17.3	1	48.59	345	5.5	0.3	62.73	1782	28.2	1.8	63.19
	Rapeseed	374.9	7.1	0.5	52.8	73.1	1.1	0.1	66.45	1485.9	22.6	1.1	65.75
Yunnan	Maize	193.4	3.3	0.3	58.61	130.9	3.2	0.2	40.91	2343.8	58	4.5	40.41
	Rice	1728.6	33.9	2.9	50.99	582.2	10.2	0.9	57.08	2541.3	44.6	3.7	56.98
	Wheat	181.7	3.2	0.5	56.78	221.5	2.6	0.3	85.19	883.7	10.3	1.1	85.8
Tibet	Barley	9.2	0.2	0.01	46	9.9	0.1	0.01	99	43.7	0.5	0.01	87.4
Total		7165.8	132.2	9.8	54.2	5017.3	75.9	6.8	66.10	26,882.1	489.6	37.1	54.91

, the total nutrient resources of C, N, and P of five crops in Southwest China were 39,065.2 kilotonnes, 697.7 kilotonnes, and 53.7 kilotonnes, respectively. Among them, the total nutrient resources of C, N, and P in root and stubble were 12,183.1 kilotonnes, 208.1 kilotonnes, and 16.6 kilotonnes, accounting for 31.2%, 29.8%, and 30.9% of the total nutrient resources, respectively. The crop with the largest amount of stubble and straw nutrient C was rice, which was 11,362.5 kilotonnes. Maize was the crop with the largest nutrient N and P resources of stubble and straw, which was 237.9 kilotonnes and 17.1 kilotonnes, respectively. The nutrient C, N, and P resources of crop roots were 7165.8 kilotonnes, 132.2 kilotonnes, and 9.8 kilotonnes, respectively, which were all lower than the rest. Therefore, straw and stubble return to the field were conducive to adding more nutrients into the fields.

On the basis of the amount of residue return, the total amount of nutrients C, N, and P return of the main crop types was calculated. The total amount of nutrients C, N, and P return by crop residue of five main crops were 16,041.7 kilotonnes, 274.6 kilotonnes, and 22.1 kilotonnes, respectively. Among them, the total amounts of nutrients C, N, and P return by straw were 3858.6 kilotonnes, 66.5 kilotonnes, and 5.5 kilotonnes, respectively. In fact, the total amount of nutrient return by root and stubble was equal to the amount of their nutrient resource because root and stubble were all returned to the field. Rice was the crop with the largest amount of nutrient return in Southwest China, and the proportion of C, N, and P return to total nutrient return was 52.8%, 54.7%, and 50.4%, respectively. The barley was the crop with the smallest amount of nutrient return.

Upon examining the C/N ratios of different parts of crop residues, it is evident that the C/N ratios of straw from each crop in the five regions consistently exceed the ideal range of 25–28. Furthermore, the Stubble component exhibits the highest average C/N ratio, reaching 66.1:1, aligning with the findings of other researchers [43]. In the case of wheat straw from Sichuan, the C/N ratio across three different parts is at its lowest, measuring 72.23:1. This value surpasses the threshold for optimal crop fertilization. The efficient microbial decomposition of straw typically occurs when the C/N ratio is around 28 [43,44]. Elevated C/N ratios in straw during microbial decomposition result in the fixation of soil-available nitrogen, reducing the uptake of nitrogen by crop roots. Nevertheless, this study bears significant implications as it allows us to calculate the necessary nitrogen fertilizer ratios based on the varying C/N ratios of straw observed in different regions. This calculation enables us to determine the optimal quantities of both straw and nitrogen fertilizer, effectively reducing excessive fertilizer use and promoting soil health.

From the composition proportion of nutrient return, the orders were all expressed as follows: root > stubble > straw (Figure 3). Among them, the total amount of nutrient C, N, and P returned to fields through the roots were 7165.8 kilotonnes, 132.2 kilotonnes, and 9.8 kilotonnes, respectively, accounting for 44.6%, 48.2%, and 43.4% of the total nutrients returned, respectively, which was the main part of crop nutrients return to fields. The amount of nutrient C, N, and P returned to fields through crop stubbles were 5017.3 kilotonnes, 75.9 kilotonnes, and 6.8 kilotonnes, respectively, accounting for 31.3%, 27.6%, and 30.6% of the total return of crops. Although the nutrient resources of the straw were higher than the stubble part, the contributions of the stubble to the nutrients return were all higher than that of the straw return, which was 24.1%, 24.2%, and 25% for nutrient C, N, and P, respectively.

According to the optimum fertilization amount of the main crops in Southwest China [7], the returned of crop residues nutrient N in maize, rice, rapeseed, and wheat can replace approximately only 5.6%, 18.4%, 11.2%, and 14.8% of nitrogen fertilizer, and 2.4%, 8.3%, 3%, and 9.2% of phosphate fertilizer, respectively, under the current level of residue return to the fields (

Table 6. The amounts of residue nutrient (kg/hm²), residue nutrient returned (kg/hm²), and the optimum fertilizer rates (kg/hm²) of main crops in Southwest China in 2012, and the percentage of crop residue nutrient substituted for chemical fertilizer in next crop season under total return and actual return to fields.

Crop	Residue Nutrient Amount (kg/hm2)		Residue Nutrient Return Amount (kg/hm2)		Optimum Fertilization Rate * (kg/hm2)		Percentage of Chemical Fertilizer Substituted by the Residue Nutrient (%)
							Total Return to Field		Actual Return to Field
	N	P2O5	N	P2O5	N	P2O5	N	P2O5	N	P2O5
Maize	61.0	10.0	12.1	2.0	213.7	83.0	28.6	12.0	5.6	2.4
Rice	57.7	9.8	33.2	5.6	180.2	67.9	32.0	14.4	18.4	8.3
Rapeseed	65.0	8.5	19.6	2.7	175.4	88.7	37.1	9.6	11.2	3.0
Wheat	42.3	13.5	24.1	7.3	162.9	79.4	26.0	17.0	14.8	9.2

* The optimum fertilization rates were the average values of different regions, and the data were from Song et al. (2018) [7].

). However, if all the residues are all returned to the field, the proportion of maize, rice, rape, and wheat straw that can replace nitrogen fertilizer application is 28.6%, 32.0%, 37.1%, and 26%, and 12%, 14.4%, 9.6%, and 17% for the proportion of replacement phosphate fertilizer, respectively. It is shown that increasing the proportion of crop residue return to the field can effectively reduce the application of chemical fertilizer. Crop residue return is an important measurement in modern agricultural practice. Studies have shown that about 2/3 of straw returning can effectively improve soil quality, alleviate soil nutrient loss, increase soil fertility level, and soil microbial activity [8,24,45,46]. It can increase grain production by 5% to 30% and reduce the amount of nitrogen, phosphorus, and potassium fertilizer by 10% to 20% [7].

It is worth noting that due to the low content of nitrogen and phosphorus in the straw, the release rate is relatively slow, and the phenomenon that the microbes compete with the crops for nutrient elements is easy to occur in the early stage of straw decomposing. Therefore, it is necessary to apply a certain amount of nitrogen and phosphate fertilizers while returning the straw to the field. In short, in the straw returning field, factors such as straw type, soil, climate, and vegetation must be comprehensively considered to understand the release law and effectiveness of straw nutrient so as to improve the straw returning technology.

4. Conclusions

Based on the first-hand field measurement and the survey data, this paper report for the first time the amount of five main crops residues and corresponding nutrient carbon (C), nitrogen (N), and phosphorus (P) returned through crop root, stubble, and straw in Southwest China in 2012. The results showed that the total amount of straw return of five main crops was lower than the amount of return by roots and stubble. In addition, the contribution of the root and stubble to the total residues return reached 77.2%, which was showed that these two parts were the mainly important part of crop residue return to fields. From the composition proportion of nutrient return, the orders were as follows: root > stubble > straw. The total amount of nutrient C, N, and P returned to fields through the roots were the main part of crop nutrients return to fields. Although the nutrient resources of the straw were higher than the stubble part, the contributions of the stubble to the nutrients return were all higher than that of the straw return. Under the current level of residue return to the fields, the returned of crop residues nutrient N in maize, rice, rapeseed, and wheat can replace approximately only 5.6%, 18.4%, 11.2%, and 14.8% of nitrogen fertilizer, and 2.4%, 8.3%, 3%, and 9.2% of phosphate fertilizer, respectively.