Role of Crop Residue Management in Sustainable Agricultural ...

Role of Crop Residue Management in Sustainable Agricultural Development in the North China Plain

1540-7578 1044-0046 WJSA Journal of Sustainable Agriculture Agriculture, Vol. 32, No. 1, May 2008: pp. 1–24

Zhang, Yang, Journal of Sustainable and Wu Agriculture

Qingzhong Zhang Zhengli Yang Wenliang Wu

ABSTRACT. Crop residue, the largest product of agricultural harvests, contains large amounts of assimilated carbon (C) and nutrients such as nitrogen (N), phosphorus (P), and potassium (P); these elements must be recycled for the sustainable development of agriculture. Crop residue management should serve a double function, both confronting global warming and food security by increasing carbon sequestration in agriculture and increasing grain yields. Historically, the North China Plain has experienced different crop residue management practices. While direct burning in the field remains an environmental problem in the region, crop residue amendment triggers benign cycling of C and nutrients in agriculture. Data showed that soil organic carbon (SOC) in topsoil increased by 0.174 to 1.74 g kg−1, Qingzhong Zhang is affiliated with the Institute of Environment and Sustainable Development in Agriculture, CAAS, Beijing, 100081, China (E-mail: ecolog [email protected]). Zhengli Yang is senior researcher, Institute of Environment and Sustainable Development in Agriculture, CAAS, Beijing, 100081, China (E-mail: yangzl@ cjac.org.cn). Wenliang Wu is professor, College of Resources and Environmental Science, China Agricultural University, Beijing, 100094, China (E-mail: wuwenl@ cau.edu.cn). We thank the National Natural Science Foundation of China (Project 30270220) and the Project for Youth Teachers No. 1926 by the Ministry of Education of China for financial support. Address correspondence to: Wenliang Wu at the above address. Journal of Sustainable Agriculture, Vol. 32(1) 2008 Available online at http://jsa.haworthpress.com © 2008 by The Haworth Press. All rights reserved. doi:10.1080/10440040802121502

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with an average of 0.79 g kg−1 after wheat residue amendment collected from 35 sites in the North China Plain. The average increase in grain yield achieved by wheat residue amendment in the region is 260 kg ha−1 year−1 for wheat and 310 kg ha−1 year−1 for maize, for a total of 570 kg ha−1 year−1. At the same time, the available potassium (K2O), the available phosphorus (P2O5), and total nitrogen in soil increased, significantly or not. With the development of the economy, technology, and supporting policy, crop residue management can play an increasingly important role in sustainable agricultural development.

KEYWORDS. Crop residue, management, North China Plain, soil organic carbon (SOC), sustainable agriculture

INTRODUCTION Concerns regarding global warming and food security have led to a surge in interest in the management of crop residues to increase carbon sequestration and grain yield in agriculture. Soil organic carbon (SOC) content, which plays an important role in soil sustainability, is a key indicator of soil fertility. The main source of SOC in cropland is crop residue; therefore, crop residue amendment is considered one of the most important management practices in maintaining soil fertility. Soil is the second largest terrestrial carbon (C) reservoir, estimated at 1500 Pg C (1 Pg = 1015 g) to 1 m in depth, in which 168 Pg C is contained in agricultural soils (Paustian et al., 1997; Jacinthe et al., 2002). Sequestration strategies for C proposed by soil scientists generally involve management practices that increase crop residue input while minimizing C losses (Jacinthe et al., 2002). In contrast, some agricultural engineers suggest that crop residues may be used as a biofuel substitute for fossil fuel energy. Crop residue is the largest agricultural harvest. Over half of all dry matter in the global harvest consists of cereal and legume straws; tops, stalks, leaves, and shoots of tuber, oil, sugar, and vegetable crops; and pruning and litter from fruit and nut trees (Smil, 1999). In the mid 1990s, the estimated annual global production of crop residue was about 3.4 to 3.8 Pg (Lal, 1997; Smil, 1999), which was greater than the total dry matter of harvested crops (about 2.8 Pg). Assuming a mean carbon content of 45%, the total carbon assimilated annually in crop residue is about 1.5 to 1.7 Pg worldwide. Total nitrogen (N), total phosphorus (P), and total potassium (K) assimilated in crop residue are estimated at about 25, 4,

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and 40 Tg (1 Tg = 1012 g), equivalent to approximately 30%, 30%, and 200% of the amount of each nutrient, respectively, contained in available chemical fertilizers (Smil, 1999). Thus, careful consideration is required in the decision to return crop residue to the soil, burn it in the field, or remove it for other uses. In China, the annual production of crop residue is estimated at about 0.62 Pg, including rice (0.18 Pg), maize (0.17 Pg), wheat (0.11 Pg), cotton (0.013 Pg), and soybean (0.015 Pg) residues (Yang and Wang, 1999; Han et al., 2002). This amounts to about 60 times the annual harvest of pastures in the north of China. Total nitrogen, total potassium, and total phosphorus assimilated in crop residue are estimated at 3, 7, and 0.7 Tg, respectively (Yang and Wang, 1999). In some high-yielding regions, such as the North China Plain, the middle and lower reaches of the Changjiang River, and South China, the annual production of crop residue per hectare is >18 Mg (1 Mg = 106 g).

Main Uses of Crop Residue Currently, crop residue has many uses. Crop residue amendment, household fuel, and silage are the dominant uses in China, although other uses, including building materials, paper making, mushroom cultivation, and bedding for animals, are also well known. Most crop residue in China is used for household fuel. About 45% of crop residue is used in cooking and heating in winter. According to the China Department of Agriculture, from 1995 to 2000, 30% to 35% of household energy consumption in rural areas was from crop residue, which is equivalent to 115 to 125 Tg of standard coal each year (Han et al., 2002). About 31% of crop residue in China is used as feed (Han et al., 2002). From 1990 to 2000, 0.85 Pg of silage and 0.28 Pg of aminated straw were produced for feed. The amount of crop residue used for feed is the equivalent of approximately 20 Tg of grain each year (Han et al., 2002). Crop amendment is another important use of crop residue in China. According to a summary of crop residue amendment research in the principal grain regions of China by Zeng et al. (2002), crop residue amendment has a positive effect on grain yield and SOC content: in Northeast China, grain yield increased by 11.7% on average and SOC increased by 0.4 to 1.5 g kg−1; in North China, grain yield increased by 26.9% and SOC increased by 0.3 to 3 g kg−1; in Northwest China, grain yield increased by 15% and SOC increased by 0.2 to 2.89 g kg−1; in Southwest China, grain yield increased by 11.14% and SOC increased by 1 to 6 g kg−1;

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along the middle and lower reaches of the Changjiang River, grain yield increased by 12.21% and SOC increased by 0.1 to 5.4 g kg−1; in South China, grain yield increased by 12.68% and SOC increased by 0.5 to 2.67 g kg−1. These increases in SOC were focused in the topsoil. Bearing in mind that crop residue is an important resource, and using the North China Plain as a case study, the aim of this study was to understand the role of crop residue management in the sustainable development of agriculture, based on its effects on various agricultural factors, including SOC content, grain yield, soil nutrient content, and carbon sequestration.

CASE STUDY: CROP RESIDUE AMENDMENT IN THE NORTH CHINA PLAIN The North China Plain is an alluvial plain formed by the Yellow, Haihe, and Huaihe Rivers. The geographical area of the North China Plain is about 3.5 × 105 km2 and accounts for about one-third of the total area of plains in China. The climate is warm-temperate and semi-humid, with a mean annual temperature of 10 to 15°C, with a ≥10°C accumulated temperature of 3800 to 4900 °C; the mean annual precipitation is 500 to 1000 mm. The main soil types include Hapli-Udic Agrosols, Hapli-Ustic Argosols, Ochri-Aquic Cambosols, Shajiang Calci-Aquic Vertosols, and Crustic Aqui-Orthic Halosols. The North China Plain is one of the most important agricultural production areas of China, with a cropland area of 1.867 × 105 km2 or 18.8% of China's croplands. A winter wheat–maize rotation is the dominant cultivation system in this region; other cultivation systems include winter wheat–cotton, winter wheat–soybean, winter wheat–peanut, and winter wheat–rice rotations.

Crop Residue Management in the Region Before 1970, agriculture in the North China Plain was traditional; crop residue, including stubble, was removed from fields for use as household fuel, fodder, and bedding, and was partly returned to the soil as yard manure or compost. Because the total crop biomass was low during this period, C and nutrient cycling was also low. In the mid 1970s, special field trials for crop residue amendment emerged in the region. In the early 1980s, as a crop residue management practice, the government recommended leaving stubble about 20 cm high in the field. Following

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economic development and the wide use of chemical fertilizers, the use of organic manure by farmers declined. Fossil fuels were used for household energy consumption, and chemical fertilizers were used to increase grain yields in some developed areas. The practice of burning crop residue directly in the field then developed, because crop residue had come to be considered a waste product and this burning became a regional environmental problem. Later, with the promotion of sustainable development in agriculture, wheat residue amendment combined with mechanisation was encouraged by the government. Today, wheat residue amendment is well developed in the North China Plain; techniques and problems in manipulation have been resolved, and some positive effects have been reported.

Effect of Crop Residue Amendment on Biomass Some research has shown that crop residue amendment can cause significant increases in grain yield, especially when combined with NP or NPK fertilizer (Table 1). In an 18-year experiment in Shandong, Sun et al. (2004) found that for either wheat or maize, the yield from treatment with NP fertilizer plus crop residue amendment was significantly higher than that from treatment with NP fertilizer alone, and the average increase in grain yield was 163 kg ha−1 year−1 for wheat and 367 kg ha−1 year−1 for maize. A crop residue amendment experiment in Henan showed that the yield from treatment with NP fertilizer plus crop residue amendment increased by 261 to 390 kg ha−1 for wheat, 341 to 432 kg ha−1 for maize, and 171 kg ha−1 for soybean compared to that with NP fertilizer treatment alone; the yield from treatment with NPK fertilizer plus crop residue amendment increased by 156 to 325 kg ha−1 for wheat, 218–339 kg ha−1 for maize, and 30 kg ha−1 for soybean compared with that with NPK fertilizer (Wang et al., 1996). These data indicate that the average increase in grain yield achieved by wheat residue amendment in the North China Plain is 260 kg ha−1 year−1 for wheat and 310 kg ha−1 year−1 for maize, for a total of 570 kg ha−1 year−1, consistent with the estimation of 503 to 845 kg ha−1 year−1 by Zeng and Zhang (1997) based on a 5-year crop residue amendment experiment in the region. Assuming that crop residue amendment does not change the ratio of grain to residue, and applying the grain to residue ratios of 1:1.33 for wheat and 1:1.25 for maize obtained from one site (Zhao, 2004) to the whole region, the increase in biomass derived from crop residue amendment can be approximated at about 1300 kg ha−1 year−1.

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Taian, Shandong Luoyang, Henan Luohe, Henan Zhumadian, Henan Wuqiao, Hebei

Site

Ochri-Aquic Cambosols Hapli-Ustic Argosols Ochri-Aquic Cambosols Shajiang Calci-Aquic Vertosols —

Soil type

Wheat–maize Wheat–maize Wheat–maize Wheat–soybean Wheat–maize

Crop

530 448 664 187 503–845

Increase (kg ha−1 year−1)

4500 Entire wheat residue Entire wheat residue Entire wheat residue Entire wheat residue

Amount of amended crop residue (kg ha−1 year−1)

Reference

Sun et al., 2004 Wang et al., 1996 Wang et al., 1996 Wang et al., 1996 Zeng and Zhang, 1997

TABLE 1. Effects of crop residue amendment on grain yield in the North China Plain

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Effect of Crop Residue Amendment on SOC The gain in SOC has been shown to be 87 kg with 1000 kg of crop residue amendment (Yang and Wang, 1999). Field experiments have shown that the SOC content in topsoil increased by 0.116 to 0.696 g kg−1, with an average of 0.389 g kg−1, after 2 years of wheat residue amendment at three levels of 3000, 4500, and 6000 kg ha−1 year−1 (Zeng et al., 1995; Zeng and Zhang, 1997). Zeng et al. (2002) summarized 35 field experiments of the effect of wheat residue amendment in the North China Plain and determined that SOC increased by 0.174–1.74 g kg−1, with an average of 0.79 g kg−1. A 14-year experiment of crop residue amendment in Shandong showed that SOC increased by 0.43–0.45 g kg−1 when 6750 kg ha−1 of crushed maize residue was amended each year (Lao et al., 2002). Another experiment in Hengshui, Hebei Province, showed that SOC increased by 0.06 to 0.07 g kg−1 after 6 years of wheat residue amendment at 6750 kg ha−1 (Fan and Liu, 2005). Crop residue amendment can increase the biomass yield and SOC content, and the increase in SOC improves soil fertility. Thus, the practice of crop residue amendment forms a positive relation between SOC content and biomass yield. Data collected in Huantai County, Shandong Province, show such a positive relation (Figure 1; Zhang, 2006), and analyses of SOC content, the amount of residual C returned, and the annual grain

FIGURE 1. Soil organic carbon (SOC) content (g kg−1), amount of residual C returned (kg C ha−1), wheat grain yield (kg ha−1), and maize grain yield (kg ha−1) in Huantai County (adapted from Zhang, 2006).

Grain Yield or Amount of Residual C Returned (kg ha–1)

9000 8000 7000 6000 5000

10.0 wheat maize residual C returned SOC

9.0 8.0 7.0

4000 3000

6.0

2000 1000 5.0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 Year

SOC Content (g kg–1)

10000

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yield (wheat grain yield plus maize grain yield) indicate that these factors are well correlated, with r = 0.84 for SOC content and annual grain yield, r = 0.96 for amount of residual C returned and annual grain yield, and r = 0.87 for amount of residual C returned and SOC content. The regression equation between grain yield and SOC content is established as follows:

y = 4.3133 x − 21.002

(1)

where y is the sum of grain yield (Mg ha−1) in one year, and x is the SOC content (g kg−1) at the same year. The soil bulk density of Huantai soil at 0 to 20 cm is 1.41 g cm−3 measured in 1982 (Liu, 2004). If changes in soil bulk density caused by increases in SOC were not considered, estimation of carbon sequestration in topsoil is about 3369 kg C ha−1 over the 22 years.

Effect of Crop Residue Amendment on Soil N, P, and K Following crop residue amendment, the contained nutrients are gradually released, with P and K being released faster than N. He et al. (1995) found that 96.2, 87.3, and 1.9% of the K, P, and N, respectively, contained in mulched rice residue was lost and 86.3, 84.0, and 18.8%, of the K, P, and N, respectively, contained in deeply buried rice residue was lost by 10 days after amendment. By the time of crop harvest, 97%, 86%, and 45% of K, P, and N, respectively, contained in deeply buried rice residue was lost. Yuan (1996) found that 85% of K from mulched rice residue applied to wheat was lost after 60 days. Another experiment showed that 95% of K contained in rice residue was lost 4 months after the residue was applied to a wheat field (Zeng and Zhang, 1997). These experiments indicate that crop residue amendment can rapidly relieve shortages of K and P in soil for crop growth. In the last 4 years of an 18-year experiment (Sun et al., 2004), the average available potassium (K2O) content in soil was 67.7 mg kg−1 for the crop residue amendment treatment and 56.0 mg kg−1 for the no amendment control, not a significant difference. In contrast, the average available potassium (K2O) content in soil was 51.3 mg kg−1 for the crop residue amendment plus NP fertiliser treatment, significantly more than the 41.7 mg kg−1 for the NP fertiliser treatment without residue amendment (p < 0.05; Sun et al., 2004). Zeng et al. (1995) showed that 2 years of application of 6000 kg ha−1 year−1 of wheat residue amendment increased the available potassium (K2O), available phosphorus (P2O5),

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and total nitrogen in soil by 37.9 mg kg−1, 2.2 mg kg−1, and 0.05 g kg−1 for the residue treatment, and 105.1 mg kg−1, 5.6 mg kg−1, and 0.02 g kg−1 for the residue plus fertilizer treatment, respectively. The N, P, and K contents in wheat residue are about 0.64%, 0.29%, and 1.07%, respectively (Shen et al., 1998; Yang and Wang, 1999). Assuming that the wheat residue yield is 7200 kg ha−1 year−1 and that this entire amount is retuned to the soil, wheat residue amendment could contribute 46.8 kg N ha−1, 20.9 kg P ha−1, and 77.0 kg K ha−1 to the soil each year, partly replacing the use of chemical fertilizers.

IMPROVED MANAGEMENT PRACTICE The direct amendment of crop residue in the North China Plain is a good and feasible practice for improving agro-ecosystem sustainability, and the practice has been widely extended in some developed areas. However, in poorer areas, two factors have limited the use of direct amendment of crop residue: crop residue is needed as a household fuel, and the direct amendment of crop residue requires special machinery that small farms cannot afford. Moreover, the C use efficiency of direct amendment is low because most C from crop residue is emitted to the atmosphere as CO2 during decomposition. Thus, improved crop residue management practices should be developed. Using crop residue to generate methane and returning the sullage and residue from methane generation to cropland soil is one improved crop residue management practice. Methane generated from crop residues can supply the household energy needed for cooking, heating, and lighting. Most C contained in crop residue is transferred to CH4 by microorganisms under anaerobic conditions. The capacity for methane generation using wheat residue and maize residue is 0.45 and 0.50 m3 kg−1 TS (∼55% CH4 in concentration), respectively, and a methane generation container of 6 to 8 m3 is large enough to supply small households in the region. Sullage and residue make excellent manure and fodder, and the residual solution can also be used as a biopesticide to kill some crop or seed pathogens. Compared with direct amendment, methane generation improves the C use efficiency of crop residue and supplies clean energy for household consumption (Figure 2). Relative to the direct burning of crop residue for cooking and heating, methane generation can improve heat energy availability from 10% to >60%.

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FIGURE 2. Model framework for (a) direct amendment of crop residue and (b) methane generation using crop residue. b

a

CO2

Crop Crop

Cooking, heating, and

CO2 Crop Crop

Methane

Soil

Methane generation Sullage & residue

Decompositio

Nutrients

SOC

Soil

Nutrients

SOC

After decades of rapid development, China now has the economic capacity to provide monetary allowances to help householders build methane generation containers. Attention should be paid to agricultural energy and environmental problems, and policies should be made to support Chinese ecological agriculture (CAE). Economic incentives should be provided to householders who adopt practices in favour of sustainable development, whereas practices harmful to sustainable development, for example, the direct burning of crop residue in the field, should be banned. The widespread use of methane generation in the North China Plain will improve agricultural production in the region to new levels.

CONCLUSION The practice of crop residue amendment triggers the benign cycling of C and nutrients such as N, P, and K in agriculture. In the past, crop residue was, and remains, in areas of poverty, used for household fuel (cooking and heating), fodder, and bedding for animals, and SOC levels were low compared to those in natural soils. Poor soil fertility resulted in poor grain yields with high annual fluctuations prior to 1980. Intensive burning of crop residue in the field is currently an environmental problem in China, especially in the primary agricultural regions. Sustainable devel-

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opment requires the reconsideration of the management of crop residue, the highest yielding agricultural product. Crop residue amendment to fields is in accord with the principles of ecology and helps to improve the stability of agro-ecosystems. The practice not only improves soil fertility by increasing SOC and nutrient contents, but also decreases environmental pollution by reducing direct burning in the field, and leads to increased carbon sequestration in agriculture. Therefore, crop residue amendment is a key factor in agricultural sustainability. However, the direct amendment of crop residue is just the first step in improving agro-ecosystem conditions that have deteriorated as a result of past management practices. There are various uses for crop residue. Extending energy and nutrient cycling and improving use efficiency is the main goal of crop residue management. Methane generation from crop residue may be beneficial for this region. Economic and technological developments make this high-efficiency application of crop residue possible, but policies are required to promote such beneficial uses. Crop residue is a valuable resource and can provide irreplaceable services; thus, crop residue management will play an important role in future sustainable agricultural development.

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