Spatial and temporal dynamics of agricultural residue resources in the last 30 years in China

Abstract

The availability and distribution of biomass resources are important for the development of the bioenergy industry in a region. Biomass resources are abundant in China; however, the raw material is severely deficient, which makes the Chinese bioenergy industry an embarrassment and a contradiction. Unclear reserves and distribution and changing trends of biomass resources are the reason for this situation. A collection coefficient model of Chinese agricultural residue resources was established and the spatial and temporal pattern dynamics of agricultural residue resources in the last 30 years were analyzed. The results show that agricultural residue resources increased in stages from 1978 to 2011, including a rapid increase from 1978 to 1999, a significant fall from 2000 to 2004, and a slow increase from 2004 to 2011. Crops straw and livestock manure are the main ingredients of agricultural residue resources with proportions of 53–59% and 31–38%, respectively. However, the former has gradually decreased, while the latter is increasing. This mainly resulted from the strategic reorganization of the Chinese agriculture structure and the rapid development of large-scale livestock breeding and agricultural mechanization. Large regional differences existed in Chinese agricultural residue resources, and three distribution types formed, including resource-rich areas in North China, Northeast and Inner Mongolia, resource-limited areas in Central and Southwest China, and resource-poor areas along Northwest and Southeast coasts. This pattern is a reverse of the distributions of climatic conditions, water resources, economic development, human resources, and technological levels. Finally, it can be predicted that livestock manure and biomass conversion technology at low temperature will play increasingly significant roles in bioenergy industry development.

Keywords

Agricultural residue crops straw livestock manure temporal pattern spatial pattern changing characteristic bioenergy industry

Introduction

Under the dual drivers of fossil energy shortage and global warming, bioenergy has been developed rapidly in China since the early twenty-first century (Baeyens et al., 2015; Zhang et al., 2009; Zhao and Liu, 2014). However, compared with wind energy, solar energy, and nuclear energy, bioenergy does not at present have a significant role in substituting fossil energy and mitigating global warming (IEA, 2014). Nevertheless, biomass can be converted into liquid fuel and used to reduce rural environmental pollution (Cui and Cheng, 2015); it is in this that other renewable energies cannot compare. Therefore, there are two trends of bioenergy industry development in China. One is to convert bioenergy into liquid fuels by cultivating energy plants (Liu et al., 2013; Su et al., 2015; Zhang et al., 2012), and the other is to make the best use of agricultural residues according to local conditions (Long et al., 2013; Ping et al., 2012). Currently, the former method is still being tested in the laboratory (Liu et al., 2013). As for the latter, agricultural residue as an existing pollutant has caused serious rural nonpoint source pollution. Therefore, it is urgent to control the rural nonpoint source pollution and improve the rural environment in the process of urbanization (Yang et al., 2014). As a result, developing bioenergy with agricultural residue is the main trend for Chinese bioenergy industry development at present.

As the largest agricultural country, China is abundant in agricultural biomass resources and theoretically great potential of bioenergy industry development (Jia et al., 2014a; Shi et al., 2014). However, the shortage of feedstock is the greatest obstacle to agricultural residue resource utilization in practice (Castillo, 2014; Seay and Fazleena, 2014). It is widely accepted that the collection of crops straw is difficult and that the price is continuously increasing in the biomass power generation industry (Jia et al., 2014b; Singh, 2016). In addition, because of feedstock shortage, it is difficult to keep household biogas digesters working continuously (Chen et al., 2013; Zhang et al., 2014). Contradictions existed between apparent sufficiency and actual shortage of agricultural residue resources, and the reasons are chiefly as follows. The first is that agricultural biomass is characterized by its light weight, scattered distribution, and difficulty in collection, transportation, and storage (Castillo, 2014; Jia et al., 2014a). The second is that researchers usually neglected the changes in agricultural planting structure, and explained resource dynamics in terms of static data (Cui et al., 2008; Guo et al., 2012; Singh, 2016; Zhu et al., 2012). In this work, a collection coefficient model of agricultural residues was established, and the spatial and temporal dynamics of agricultural residues in the last 30 years in China were analyzed. The results of this work are expected to be beneficial for the utilization of agricultural residue resources effectively, thus enabling the bioenergy industry to develop scientifically.

Appraise indexes and research methods

Appraise indexes

In order to estimate the collectable agricultural residue resources, several parameters are necessary: crops yields, amount of livestock, resource production coefficient, and availability. The collectable resource (P_CR) is defined as the resource production of collectable agricultural residues in terms of weight, and it can be estimated as follows

P_{CR} = \sum_{i = 1}^{n} P_{i} λ_{i} η_{i} δ_{i}

(1)

where P_i is the yield of crop i or the amount of livestock i, $λ_{i}$ is the generation coefficient of residue I, $η_{i}$ is the collection coefficient of residue, and is the standard coal coefficient.

Parameter selection

Resource generation coefficient

The resource generation coefficient includes crops straw generation coefficient, crops processing residue generation coefficient, and livestock manure generation coefficient. More authoritative data from the latest research are needed, because the crops straw generation coefficient is declining with the development of modern planting technology, as shown in Table 1. The livestock manure generation coefficient is determined by animal species, animal daily excretion and breeding cycle, which are listed in Table 2 according to the recent survey by Ministry of Agriculture and the Ministry of Environmental Protection of the People’s Republic of China.

Table 1.

Production coefficients of crops straw and crops process residue.

Name	Coefficient	Name	Coefficient	Name	Coefficient
Wheat stalk	1.17	Cotton stalk	3.0	Beets stem	0.43
Corn stalk	1.04	Peanut stalk	1.14	Rice shell	0.18
Straw	1	Sesame stalk	2.0	Corn cob	0.16
Sorghum stalk	1.6	Rapeseed stalk	2.87	Peanut shell	0.27
Soybean stalk	1.6	Bast fiber crop stalk	1.9	Bagasse	0.16
Potato stem	0.57	Sugarcane stems	0.43	Beet pulp	0.05

Note: data sourced from Zhu et al. (2012) and Guo et al. (2012).

Table 2.

Livestock manure production coefficient.

Animal	Livestock manure production coefficient (kg/d)
Animal	North Region	Northeast Region	Northwest Region	South Central Region	Southwest Region
Pigs	3.4	4.1	3.54	3.74	3.57
Milk cow	46.05	48.49	31.39	50.99	46.84
Beef cattle	22.1	22.67	20.42	23.02	20.42
Draft cow	23.02	22.90	17.0	27.63	21.90
Egg-laying poultry	0.17	0.1	0.1	0.12	0.12
Dorking poultry	0.12	0.18	0.18	0.06	0.06
Rabbit	0.15	0.15	0.15	0.15	0.15
Horse	5.9	2.9	5.9	5.9	5.9
Donkey/mule	5.0	5.0	5.0	5.0	5.0
Sheep	0.87	0.87	0.87	0.87	0.87
Animal	Livestock manure production coefficient (kg/d)			Dry matter content(%)	Breeding days(d)
Animal	Eastern Region		Average value	Dry matter content(%)	Breeding days(d)
Pigs	2.97		3.55	20	199
Milk cow	46.84		45.10	20	>365
Beef cattle	23.71		22.06	18	>365
Draft cow	21.90		22.39	18	>365
Egg-laying poultry	0.15		0.13	80	>365
Dorking poultry	0.22		0.14	80	210
Rabbit	0.15		0.15	75	90
Horse	5.9		5.9	25	>365
Donkey/ Mule	5.0		5.0	25	>365
Sheep	0.87		0.87	75	365

Note: data sourced from Ministry of Environmental Protection of the People’s Republic of China (2004), Institute of Environment and Sustainable Development in Agriculture and Nanjing Institute of Environment Sciences (2009), and Wei et al. (2013).

Resource collection coefficient

Resource collection coefficient refers to the proportion of the collectable resources to the total theoretical resources. The residues from processed crops are usually highly centralized and easy for collection, so the collection coefficient of crops processing residue can be assumed as 100%. As a matter of fact, the scattered crops straw and livestock manure are distributed varying with the industrial development of agriculture and stockbreeding.

The crops straw collection coefficient is determined by crops stubble height and plant height. Moreover, the stubble heights vary with different ways of harvesting. Moreover, the harvesting ways to estimate the crops straw resource should be taken into account. The calculation formula of crops straw collection coefficient can be expressed as follows

η_{i, s} = \sum α_{j} β_{j}

(2)

where $η_{i, s}$ is the crops straw collection coefficient, $α_{j}$ is harvest coefficient of harvesting way j, and $β_{j}$ is the proportion of harvesting way j.

Since mechanical harvesting is widely used in wheat, rice, and corn, and less in other crops including cotton, soybean, peanut, sesame, and so on with proportion less than 10%, harvesting ways are only used to estimate the coefficients of wheat, rice, and corn, which are listed in Tables 3 and 4 according to equation (2). Other crops straw collection coefficients can be referred to in recent studies (Cui et al., 2008; Wang et al. 2010).

Table 3.

Stubble height of main crops under different harvesting ways.

Crops	Harvesting ways	Plant height /cm	Stubble height /cm	Harvesting coefficient
Wheat	Artificial harvesting	85	6	0.93
Wheat	Mechanical harvesting	85	25	0.71
Rice	Artificial harvesting	100	7	0.93
Rice	Mechanical harvesting	100	16	0.84
Corn	Artificial harvesting	250	6	0.98
Corn	Mechanical harvesting	250	15	0.94

Table 4.

Mechanical harvesting rate and straw harvested efficiency of main crops from 1978 to 2011.

Year	Mechanical harvesting rate			Straw harvested efficiency
	Wheat	Rice	Corn	Wheat	Rice	Corn
1978				93.00%	90.44%	98.00%
1980	11.62%	–	–	93.00%	90.44%	98.00%
1985	13.42%	–	–	93.00%	90.05%	98.00%
1990	28.70%	–	–	93.00%	89.00%	98.00%
1991	31.80%	–	–	93.00%	88.50%	98.00%
1992	36.38%	–	–	93.00%	88.00%	98.00%
1993	40.9%	–	–	93.00%	87.00%	98.00%
1994	45.45%	–	–	93.00%	86.69%	98.00%
1995	47.20%	3.80%	–	93.00%	86.00%	98.00%
1996	48.70%	2.42%	–	93.00%	85.00%	98.00%
1997	54.78%	7.25%	–	93.00%	84.00%	98.00%
1998	58.72%	10.09%	–	93.00%	83.00%	98.00%
1999	63.81%	12.16%	–	92.66%	82.62%	98.00%
2000	66.89%	15.44%	1.69%	92.78%	82.29%	98.00%
2001	69.72%	18.02%	1.63%	92.35%	80.95%	98.00%
2002	69.89%	20.60%	1.74%	92.09%	80.08%	98.00%
2003	72.79%	23.40%	1.89%	91.91%	78.96%	98.00%
2004	76.20%	27.30%	2.50%	91.61%	78.28%	97.93%
2005	76.14%	33.50%	3.11%	91.38%	77.66%	97.93%
2006	78.32%	38.80%	4.73%	91.15%	77.62%	97.93%
2007	79.17%	46.20%	7.23%	90.89%	76.99%	97.92%
2008	83.84%	51.16%	10.61%	90.54%	76.24%	97.90%
2009	86.07%	56.69%	16.91%	89.99%	76.25%	97.88%
2010	88.45%	64.50%	25.78%	89.51%	75.77%	97.81%
2011	91.05%	69.32%	33.59%	88.84%	75.58%	97.71%

Note: data sourced from Huang and Chen (2012) and Ministry of Agriculture of the People’s Republic of China (2000–2012b).

The livestock manure collection coefficient mainly depends on the breeding method and the industrial level of the livestock farm. Due to the diversified land use types and breeding methods in China, the animal breeding conditions can be divided into three categories.

Completely captive breeding. Generally speaking, pigs and rabbits are completely captive animals and the manure collection rates can be up to 100% (Yuanchun, 2011). Meanwhile, all livestock and poultry in large and middle-scale farms are also dominated in captivity (except pastoral and semi-pastoral areas); the manure collection rates can also be assumed as 100%. The breeding scale is the main factor affecting the manure collection coefficient. In this study, the large and middle-scale farm was defined as the annual production of pig for slaughter over 50, the poultry for stock over 2000, the cattle for stock over 10, and the sheep for slaughter over 30.

Semi-captive breeding. This refers to livestock breeding in pastoral areas and family breeding in non-pastoral rural areas. In the former areas, the livestock are usually captive at the stage of plant returning green with captive time from about 45–60 days. At other times of grazing, the manure collection rates are about 100% and 40%, respectively, according to Huang and Chen (2012). As for the latter, sheep, hinny, horse, cattle and other livestock are free in the summer and autumn with grazing time of 5 months, and they are captive in the rest of time. Under these circumstances, the manure collection rates can be up to 60% (Huang and Chen, 2012).

Complete free range breeding. Chicken, duck, and goose are entirely free in family breeding; the manure collection rate is about 40% (Huang and Chen, 2012).

Based on the above assumptions, the Chinese livestock manure collection coefficient can be estimated, except for livestock with completely captive breeding, which include pigs and rabbits, using

η_{i, m (l)} = α_{i} \times 100 % + β_{i} \times (5 / 12 \times 60 % + 7 / 12 \times 100 %) + χ_{i} \times (2 / 12 \times 100 % + 10 / 12 \times 40 %)

(3)

η_{i, m (p)} = α_{i} \times 100 % + β_{i} \times 40 %

(4)

where $η_{i, m (l)}$ is the manure collection coefficient of livestock except pigs, rabbits, and poultry, $α_{i}$ is the proportion of large and middle scale breeding, $β_{i}$ represents the proportion of family breeding in non-pastoral areas, $χ_{i}$ is the proportion of livestock in pastoral and semi-pastoral area, and $η_{i, m (p)}$ is the manure collection coefficient of poultry.

The amounts of livestock with large and middle scale breeding, livestock in pastoral and semi-pastoral areas, and the estimated manure collection coefficient are listed in Table 5.

Table 5.

The amounts of the large-scale breeding of cattle, chickens, and sheep and the livestock manure collection coefficient.

Year	Total amount of livestock *			Livestock with large-scale breeding*
	Cattle for stock/10 kg	Chickens for slaughter/10 kg	Sheep for stock/10 kg	Cattle/10 kg		Chickens/10 kg		Sheep/10 kg
1978	7072.4	47,626.8	16,994	—		—		—
1980	7167.6	52,359.6	18,731.1	—		—		—
1985	8682	130,000	15,588.4	—		—		—
1990	10,288.4	157,881.8	21,002.1	—		—		—
1991	10,459.2	179,171.5	20,621	—		—		—
1992	10,784	214,572.3	20,732.9	—		—		—
1993	11,315.7	218,053.2	21,731.4	—		—		—
1994	12,331.8	282,357.7	24,052.8	—		—		—
1995	13,206	282,357.5	27,685.6	—		—		—
1996	11,031.8	319,254.3	23,728.3	—		—		—
1997	12,182.2	397,760.3	25,575.7	—		—		—
1998	12,441.9	512,823.2	26,903.5	—		—		—
1999	12,698.34	488,392.2	27,925.8	—		30.37		—
2000	12,866.30	557,127.2	27,948.2	—		33.36		—
2001	12,824.20	638,853.2	27,625.0	—		34.00		—
2002	13,084.80	684,378.7	28,240.9	1612.93		42.61		11,109.98
2003	13,467.20	743,165.1	29,307.4	1774.83		46.01		12,759.82
2004	13,781.82	809,857.1	30,426.0	2012.28		48.76		12,495.86
2005	14,043.00	808,834.8	29,792.7	2345.30		54.33		14,313.27
2006	13,944.20	832,858.9	28,369.8	2720.72		60.67		14,069.76
2007	10,594.79	888,587.8	28,564.7	2663.65		70.01		14,701.10
2008	10,576.00	907,021.8	28,084.9	2861.47		78.38		14,809.97
2009	10,726.53	943,091.4	28,452.2	3231.79		89.60		16,136.86
2010	10,626.43	930,548.3	28,087.9	3699.64		99.26		16,888.50
2011	10,360.46	957,867	28,235.8	3902.02		101.04		17,444.84
Year	Livestock in pastoral and semi-pastoral areas*		The manure collection coefficient
	Cattle/10 kg	Sheep /10 kg	Cattle /%		Chickens /%		Sheep /%
1978	—	—	77.00		40.00		70.00
1980	—	—	77.00		40.00		70.00
1985	—	—	77.00		40.00		70.00
1990	—	—	77.00		40.00		70.00
1991	—	—	77.00		40.00		70.00
1992	—	—	77.00		40.00		70.00
1993	—	—	77.00		40.00		70.00
1994	—	—	77.00		40.00		70.00
1995	—	—	77.00		42.40		78.33
1996	—	—	77.00		44.80		78.83
1997	—	—	77.00		47.20		79.33
1998	—	—	77.00		49.60		79.83
1999	2869.15	3576.47	77.00		52.00		80.17
2000	2137.10	3744.59	77.00		54.40		80.67
2001	2109.46	4180.84	77.00		56.80		81.67
2002	2436.46	4283.24	77.00		59.20		82.17
2003	2322.18	4932.99	77.97		61.60		82.67
2004	2422.30	5693.82	75.80		64.52		83.17
2005	2445.47	6629.25	77.80		64.71		83.67
2006	2581.19	7086.93	77.85		65.22		84.17
2007	2747.06	7058.59	79.18		70.70		85.15
2008	2523.90	7197.85	79.78		71.07		85.19
2009	2350.12	7287.89	79.91		72.26		83.99
2010	2875.02	7915.02	80.31		73.04		83.90
2011	2938.70	8063.71	80.41		75.77		83.26

Note: Pig and rabbit are completely captive animals and manure collection rates can be up to 100%. Due to lack of statistics, the manure of horse, ass, and mule are estimated by cattle dung.

Data from China Livestock Statistical Yearbook 2000–2012 (Ministry of Agriculture of the People’s Republic of China, 2000–2012b).

Standard coal coefficient

The standard coal coefficients adopted in this work, which are according to Liu and Shen (2007), are listed in Table 6.

Table 6.

Standard coal coefficients of agricultural residue (tce t⁻¹).

Residue	Coefficient	Residue	Coefficient	Residue	Coefficient
Straw	0.429	Potato stem	0.486	Cattle manure	0.471
Wheat stalk	0.500	Oil crops stalk	0.529	Chickens manure	0.643
Corn stalk	0.529	Bast fiber crops stalk	0.5	Rabbits manure	0.643
Sorghum stalk	0.529	Carbohydrate plant stalk	0.441	Horse / mules manure	0.529
Bean/ Cotton stalk	0.543	Pigs manure	0.429	Sheep manure	0.529

Data sources

Crops yields were sourced from the China Statistical Yearbook (National Bureau of Statistics of the People’s Republic of China, 2000–2012) and the Chinese Agricultural Report (Ministry of Agriculture of the People’s Republic of China (2000–2012a). The amount of animal breeding is cited from China Statistical Yearbook and China Livestock Statistical Yearbook (Ministry of Agriculture of the People’s Republic of China (2000–2012b), including annual data of all kinds of crops and livestock from 1978 to 2011. Besides, the amount of livestock is determined by growth cycle. If the breeding time is above 365 days, the amount of livestock for stock was chosen; otherwise, the amount of livestock for slaughter was chosen.

Results and discussion

Changes in the total amount of agricultural residue

The amount of agricultural residue in China shows an increasing trend from 1978 to 2011, as seen from Figure 1. The theoretical agricultural residue resource was about 260 million tons of standard coal equivalents (tce) in 1978, and increased to 568 million tce in 1999 with an annual growth rate of 5.01%. Suddenly, it decreased to 548 million tce in 2000, and kept at 550 million tce in 2001, 2002, and 2003. This was primarily caused by the strategy of Agriculture and Rural Economics Structure Strategically Adjustment proposed by the State Council in 1998, which suggested changing agricultural production goals from providing food and raw materials for citizens to increasing agricultural planting efficiency and farmers’ income. Since 2004, the amount of agricultural residue resource began to rise again, and then entered a new stage of slowly increasing with an annual growth rate of 2.84%.

The Chinese collectable agricultural residue resource from 1978 to 2011 generally shows trends similar to the theoretical resource. The main difference between them is that the growth rate from 1978 to 1999. The collectable agricultural residue was about 184 million tce in 1978, and increased to 484 million tce in 1999 with an annual growth rate of 6.23%, which is slightly higher than theoretical resource (5.01%). This is mainly due to the rapid developments of large-scale livestock breeding and agricultural mechanization, in addition, the speed of the former being higher than the latter.

Changes in the structure of agricultural residue

From the overview given in Figure 2, crops straw and livestock manure are the main ingredients of agricultural residue with a cumulative proportion of up to 90%. Meanwhile, the proportion of crops processing residue was only about 8.05–9.52%. Moreover, it also indicates that the proportion of crops straw gradually decreased, but that livestock manure increased. The proportion of crops straw was about 59.12% in 1978, and fell to 53.04% in 2011. Meanwhile, the proportion of livestock manure increased from 31.71% in 1978 to 38.03% in 2011, with an annual increase rate of 0.65%. The proportion of crops processing residue was low and always kept below 10% without significant changes.

On the whole, three characteristic stages of the Chinese agricultural residue resource development can be drawn from the changes of crops straw, livestock manure, and crops processing residue.

Steady development stage from 1978 to 1992. Crops straw accounted for a high proportion of total agricultural residue resource, and the proportion in collectable agricultural residue resource was higher than that in theoretical resources in this period. This was mainly caused by low mechanization levels of crops harvesting. For example, the wheat harvested by machine was less than 30%, and all the rice and corn were harvested manually. Meanwhile, the proportion of livestock manure was relatively low, and the proportion in collectable agricultural residue resource was less than that in theoretical resources. This can be ascribed to the facts that the livestock are mainly bred by families, and the manure is difficult to collect.

Rapid change stage from 1993 to 2003. The proportion of crops straw resource exhibited a rapid declining trend while livestock manure resources increased quickly in this period. One reason may be that the collection coefficient of crops straw lowered and the livestock manure increased, which was caused by the rapid expansion of agricultural mechanization and the large and middle scale livestock farming, respectively. For example, the mechanical harvesting rate of wheat was about 36.38% in 1993 and increased to 72.79% in 2003, with the annual growth rate of 7.18%. With the crops mechanical harvesting rate increasing, the straw collection rate declined. Therefore, the collection coefficient of wheat straw has decreased from 90% in 1978 to 76% in 2003. The second reason is that the amounts of livestock increased rapidly driven by the enormous demand for meat, eggs, and milk and the grain acreage shrank with the falling food prices. The amounts of pig and poultry for slaughter and cattle in stock were about 3.42 million, 378.24 million, and 3977.6 million in 1993, and increased to 8.93 million, 557.02 million, and 8885.88 million in 2003, with annual growth rates of 10.07%, 3.95%, and 8.36%, respectively.

Steady development stage from 2004 to 2011. More and more people are inclined to harvest rice and corn by machine, so the mechanical harvesting rate of rice and corn rapidly increased from 23.4% and 1.89% in 2003 to 69.32% and 33.39% in 2011. However, this did not result in a significant reduction of the collection coefficients of rice straw and corn stalk owing to insignificant differences between the artificial harvesting coefficient and mechanical harvesting coefficient. The collection coefficients of rice straw and corn stalk were about 90.89% and 97.92% in 2003, and declined to 86.76% and 86.75% in 2011, respectively. The annual decline rates were only 0.58% and 1.5%. Therefore, crops straw resources did not decline with agricultural mechanization development, but increased slowly from the low point of 2003, and the proportion remained at around 53%. As for livestock manure, the same trend with crops straw is observed and it returned to normal growth trend after the high point in 2003; the proportion remained at around 39%. However, the turning point has not appeared in livestock manure resource from 1978 to 2011.

Changes in spatial and temporal pattern of agricultural residue

There are marked regional differences in the distribution of agricultural residue resources in China, and this is becoming more and more obvious with time.

(1) There is a shift from south to north for the distribution of total collectable agricultural residue resources, as shown in Figure 3. The total collectable agricultural residue resource in South China was 83.4 million tce in 1980, and accounted for 58.4% of national total. However, it increased to 141.17 million tce in 2011, and the proportion decreased to 45.75%. This is mainly ascribed to the gradual spread of grain production from the South to the North, and to Northeast and Southwest regions since 1970s for food security. In addition, livestock farming has also gradually flourished in North China with crops production spreading. According to statistics, the annual growth rates of cattle and sheep for stock and pigs and poultry for slaughter in the North China were 2.12%, 4.6%, 1.49%, and 10.69% from 1980 to 2011, respectively, and they are all higher than that of South China. With the development of grain production and livestock breeding in North China, the dominant position of agricultural residue in South China was gradually lowered, and eventually the proportion dropped to below 50% in 2005. Overall, it has shifted from “South more than North” to “North more than South” in Chinese agricultural residue resources distribution. Currently, the total agricultural residue resources are mainly concentrated in areas including the Northeast Plain, North-China Plain, Huang-Huai Plain, Yangtze River Plain, and Sichuan Basin.

(2) As demonstrated in Figure 4, the resource density of Chinese agricultural residue was increasing and the area with high-density was gradual expanding. The areas with resource density of over 100t/km² only were three provinces (cities, districts) including Jiangsu, Zhejiang, and Shanghai in 1980, and increased to 17 provinces (cities, districts) in 2011. The resource densities of Henan and Shandong rank the top two, with values of over 300 t/km². Insignificant changes occurred in the distribution pattern of Chinese agricultural residue resource density by comparison between the data of 1980, 1990, 2000, 2005, and 2011. The areas with high resource density are Northern China, Northeast, Southern China, and Central China. There are superior natural soil conditions and wide arable land with a proportion of more than 30% for simple landform types and many river alluvial plains in Northern, Northeast, and Central China. Therefore, the yield of crop per unit area is high and agricultural planting is booming. As well, these zones are in warm climatic conditions and the cropping index is high in Southern China, including Guangxi and Guangdong and part of Central China (including Hubei and Hunan), and the agricultural residue resource density is correspondingly high. However, it owned only limited arable land with a proportion of less than 20%, so the total agricultural residue resource is not the highest.

(3) The regional distribution pattern of annual growth rate of agricultural residue resource showed insignificant changes in the last 30 years, and formed three types of region, which is fast-growing regions, slow-growing regions, first increasing and then decreasing regions.The annual growth rates of agricultural residue resource are growing rapidly in Heilongjiang, Jilin, Liaoning, and Inner Mongolia with the annual growth rates of over 3.5%, which were much higher than national average level with a value of 2.79%, that is fast growing in Northeast regions. Meanwhile, the annual growth rates of agricultural residue resource increased rapidly from 1980 to 1990 and then decreased from 2000 to 2011 in the areas including Shanghai, Zhejiang, Fujian, and Guangdong, i.e. they first increased and then decreased in the Southern coastal region. In the other areas, including Western China, Central China, and Northern coastal regions, the annual growth rate showed slow growth from 1980 to 2011, and there was slow growth in Midwest regions. This distribution pattern formed is mainly caused by the large regional differences of agricultural planting due to the different regional soil texture, economic development, and industrial structure in the process of Chinese major grain producing areas expanding from south to north. The crops acreage had continued to increase in the Northeast and Inner Mongolia regions since 1985 with a growth rate of over 1.5%, which were much higher than the national average level with the value of 0.06%. Meanwhile, the crops acreage had continued to decrease in these areas including Zhejiang, Fujian, Guangdong, and Shanghai since 1978, with an annual decline rate of 2.5%.

(4) According to the resources amount, resource density, and annual growth rate of agricultural residue in 31 provinces (cities, districts) of China, these could be divided into three types by cluster analysis. The first is the resource-rich areas in North China, Northeast, and Inner Mongolia; the total amount of agricultural residue resources in these areas accounts for over 50% of the national total. The resource density per unit area is more than 100 t/km², which is nearly 1.7 times the national level. Moreover, the regional annual growth rate is higher in these regions, with an annual growth rate up to 5% in Heilongjiang, Inner Mongolia, and Liaoning. The second is the resources-limited areas in Central China and Southwest, where the total amount of resources accounts for 35% of the national total and the resource density is 54 t/km², which is slightly lower than the national level. Additionally, the annual growth rate is low, and it even exhibited a gradual declining trend in Guangdong and Guizhou from 2005 to 2011. The third is the resource-poor areas in Northwest–Southeast coasts, where the total amount of resources only accounts for 13% of the national total and the resource density is only 27.4 t/km², which is far lower than the national level.

Overall, the distribution pattern of agricultural residue resources (Figure 6) is completely inconsistent with the distributions of climatic conditions, water resources, economic development, human resources, and technological levels. This means that the agricultural residue resources utilization will be faced with some problems in the resource allocated of energy, water, human, and other aspects. These all need to be fully considered in the planning of agricultural residue resources industrial development.

Conclusions and recommendations

The spatial and temporal dynamics of agricultural residue in the last 30 years in China were analyzed. Through the data analysis in this study, some conclusions can be drawn.

Owing to national food security guarantees, the total amount of agricultural residue will be stepped up into a stable stage with a slow growth rate in the future. Livestock manure will grow more rapidly and be more easy to collect; it will be an important feedstock with large potential for bioenergy industrial development and also attract more funds for its utilization.

The distribution of agricultural residue resources may present an obvious pattern of North more than South in the future, which is the reverse to the climatic conditions needed for biomass resource utilization. Therefore, it is necessary to overcome the technological limits of biomass conversion at low temperature for comprehensive utilization of agricultural residue.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by the National Natural Science Foundation of China (grant number 41201578) and the National Social Science Foundation of China (grant number 14BJY064).

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