Crop Residue Management for Lowland Rice-Based ...

C H A P T E R

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Crop Residue Management for Lowland Rice-Based Cropping Systems in Asia Bijay-Singh,* Y. H. Shan,† S. E. Johnson-Beebout,‡ Yadvinder-Singh,* and R. J. Buresh‡ Contents 1. Introduction 2. Criteria for Evaluating Crop Residue Management Options 2.1. Productivity and profitability 2.2. Environmental impact and sustainability 3. Type and Abundance of Crop Residues 4. Existing and Emerging Residue Management Options 4.1. Rice following rice or a non-flooded crop 4.2. Non-flooded crop following rice 5. Evaluation of Options with Residues Managed During a Rice Crop 5.1. Productivity 5.2. Profitability 5.3. Environmental impact 5.4. Sustainability 6. Evaluation of Options with Residues Managed During a Non-Flooded Crop 6.1. Productivity 6.2. Profitability 6.3. Environmental impact 6.4. Sustainability 7. Crop Residue and Bioenergy Options 8. Summary Acknowledgment References

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Department of Soils, Punjab Agricultural University, Ludhiana 141 004, Punjab, India College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225009, China International Rice Research Institute, Los Banõs, Philippines

Advances in Agronomy, Volume 98 ISSN 0065-2113, DOI: 10.1016/S0065-2113(08)00203-4

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2008 Elsevier Inc. All rights reserved.

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Intensification of rice-based cropping systems in Asia has substantially increased production of food and associated crop residues. The interval between crops in these systems is often brief, making it attractive for farmers to burn residues in the field to hasten and facilitate tillage for the next crop. Open-air burning causes serious air quality problems affecting human health and safety, and it has been banned by many Asian governments. In this chapter, we evaluate for rice-based cropping systems existing and emerging in-field alternatives to burning residues based on criteria of productivity, profitability, environmental impact, and sustainability. In intensive rice monocropping systems, residue is typically managed under conditions of soil flooding and anaerobic decomposition during the rice crop. In systems, where rice is rotated with an upland (non-flooded) crop, there are two major categories: residue of upland crop managed during flooded rice and rice residue managed during the upland crop. One option during the flooded rice crop is incorporation of residues from the previous rice or upland crop into the soil. Many studies have examined incorporation of crop residue during land preparation for flooded rice. In the vast majority of cases there was no significant increase in yield or profit. Residue decomposition in anaerobic flooded soil substantially increases methane (CH4) emission relative to residue removal. Surface retention of residue during rice cropping is challenging to implement because residue must be removed from the field during conventional tillage with soil flooding (i.e., puddling) and then returned. Alternatively, rice must be established without the traditional puddling that has helped sustain its productivity. Mulch is a good option for rice residue management during the upland crop, especially with reduced and no tillage. Mulch can increase yield, water use efficiency, and profitability, while decreasing weed pressure. It can slightly increase nitrous oxide (N2O) emission compared with residue incorporation or removal, but N fertilization and water management are typically more important factors controlling N2O emission than residue management. Long-term studies of residue removal have indicated that removing residue from continuous rice systems with near continuous soil flooding does not adversely affect soil organic matter (SOM). The use of crop residue as a mulch with reduced or no tillage for upland crops should be promoted in rice-based cropping systems. On the contrary, residues from the crop preceding rice on puddled and flooded soil can be considered for removal for off-field uses.

1. Introduction Rice (Oryza sativa L.) is the lifeline of Asia. More than 90% of the world’s total rice crop—or ~570 million tons of the estimated 630 million tons of global rice production in 2006/2007—is produced in Asia (FAO, 2007; USDA, 2007). Modern cultivars of rice with growth duration of

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90–125 days can be cultivated in rotation with one or two additional crops in a year. Intensification and diversification are two main trends of rice-based cropping systems as they have evolved in different agroecological regions in Asia. The most prevalent cropping systems are rice–rice, rice– rice–rice, rice–rice–pulse, rice–wheat (Triticum aestivum L.), rice–oilseed crop, and rice–maize (Zea mays L.). Intensive irrigated rice systems, with two and sometimes three rice crops produced each year in the same field, are a dominant agricultural land use in the lowland tropics and subtropics of Asia (Cassman and Pingali, 1995). Intensive rice-based systems also show great diversity across Asia, where wheat, maize or one of many other secondary crops are grown during the part of the year when rice is not in the field. The rotation of rice and wheat, for example, is a major agricultural production system, which accounts for ~30% of the area of both rice and wheat grown in South Asia (Timsina and Connor, 2001; Ladha et al., 2003). Rice-based cropping systems are the most productive agroecosystems in Asia and produce the most food for the most people. Along with grain yield, these systems generate large amounts of crop residue. Historically, crop residues were often removed from fields for livestock bedding and feed, fuel for cooking, and other off-field purposes. More recently, the off-field uses of crop residues have tended to decrease in parts of Asia even as increasing quantities of crop residues have been produced as crop yields and cropping intensity increase. The intensification of land use results in less time between crops for managing these residues, which can interfere with tillage and seeding operations for the next crop. The lack of alternative uses for crop residues and lack of appropriate mechanization to handle increasing quantities of residue have driven Asian farmers increasingly to burn crop residues as a method of disposal (Flinn and Marciano, 1984; Yadvinder-Singh et al., 2005). Open-field burning of crop residues is recognized as a major contributor to reduced air quality and human respiratory ailments, particularly in China and northwestern India, which represent major irrigated rice ecosystems in Asia. Streets et al. (2003) estimated that 730 Tg of biomass are burned in a typical year from both anthropogenic and natural causes, excluding biofuel. Crop residue burning accounted for 34% of that total. Of the total crop residues burned, China contributed 44%, India 33.6%, Bangladesh 4.4%, Pakistan 4%, Thailand 3.1%, and Philippines 2.8%. The problems of openfield burning straw include atmospheric pollution and nutrient loss. One ton of crop residue on burning releases 1,515 kg CO2, 92 kg CO, 3.83 kg NOx, 0.4 kg SO2, 2.7 kg CH4, and 15.7 kg nonmethane volatile organic compounds (Andreae and Merlet, 2001). These gases and aerosols consisting of carbonaceous matter lead to adverse impacts on human health in addition to contributing to global climate change. Following the IPCC methodology (IPCC, 1996) for estimation of emission from open-field burning of crop residue and assuming 25% of the available residue is burned in the field, the

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estimated emissions in 2000 from open-field burning of rice and wheat straw in India were 110 Gg CH4, 2306 Gg CO, 2.3 Gg N2O, and 84 Gg NOx (Gupta et al., 2004). For every ton of wheat residue burned, an estimated 2.4 kg of N is lost (Kumar et al., 2001), and up to 60% of the S content is lost (Lefroy et al., 1994). Many governments in Asia have made it illegal to burn crop residues, but these laws have been difficult to enforce. There has been increased realization that crop residues are a resource constituting a readily available source of nutrients and organic material for rice farmers. About 40% of the N, 30–35% of the P, 80–85% of the K, and 40–50% of the S absorbed by rice remain in the vegetative parts at maturity (Dobermann and Fairhurst, 2000). Typical amounts of nutrients in rice straw at harvest are 5–8 kg N, 0.7–1.2 kg P, 12–17 kg K, 0.5–1 kg S, 3–4 kg Ca, 1–3 kg Mg, and 40–70 kg Si per ton of straw on a dry weight basis (Dobermann and Witt, 2000). Residue removal can therefore have a significant effect on soil nutrient depletion. Residue management also influences availability of micronutrients such as zinc and iron, and it is an important factor in maintaining the cumulative Si balance in rice (Dobermann and Fairhurst, 2000, 2002). Residues must be carefully managed for obtaining positive effects on soil and crop production and avoiding negative effects such as interference with the planting of crops, N immobilization, and emission of greenhouse gases. The return of crop residues to flooded soils, which are typical in ricebased cropping systems, influences the chemical, physical, and biological soil environment in different ways than the return of residues to aerobic soils prevalent when crops are grown under non-flooded soil conditions. The desired objectives of adopting a particular crop residue management option can be achieved only if the management option is feasible under a given set of soil, climate, and crop management conditions; is compatible with available machinery; and is socially and economically acceptable. This chapter considers existing and emerging in-field management practices. In addition, there are a variety of potentially attractive and competing offfield uses for crop residues such as animal feed, roof thatch, manufacture of paper or cardboard, and biofuel feedstock. Several reviews on crop residue management of rice systems have appeared in recent years. Kumar and Goh (2000) reviewed crop residues in terms of soil quality, soil N dynamics, crop yields, and N recovery. Their chapter primarily deals with the decomposition and turnover rates of residues in relation to nutrient cycling. Yadvinder-Singh et al. (2005) reviewed work on crop residue management in rice-based cropping systems in the tropics, dealing with short- and long-term effects on cycling of C, N, and other nutrients in order to provide necessary understanding for developing suitable new crop residue management options. They also explained the need for evaluating the relative costs of different residue management options for rice-based cropping systems in terms of environmental impact,


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C sequestration, and long-term soil fertility. In this chapter, we describe important in-field residue management options for rice-based cropping systems including China, which did not receive much attention in the earlier reviews, and we evaluate management options based on criteria of productivity, profitability, environmental impact, and sustainability. We include residue removal in our evaluation of in-field options, but we have not detailed off-field options. We exclude high-N residues, such as from green manures, because they are not currently common in rice-based cropping systems in Asia. This chapter focuses on lowland rice-based cropping systems in Asia, in which fields are typically surrounded by earthen levees or bunds to impound water during the period of rice cropping. ‘‘Lowland’’ indicates a cultivation practice of growing rice under either irrigated or rainfed conditions with impoundment of water to flood the soil—typically during land preparation for rice production and during at least part of the rice growing season. The soil is largely anaerobic during the periods of flooding. ‘‘Upland,’’ in contrast, refers to the cropping period or crop in rice-based cropping systems when the soil is not flooded and aerobic. In this chapter, the terms ‘‘upland crop’’ and ‘‘non-flooded crop’’ synonymously refer to the crop (typically not rice) in the cropping system grown without soil flooding. In this chapter, crop residue is defined as the above-ground part of the plant remaining after the grain is harvested. It includes both the stubble left standing during the harvest process and the leaves and stems left over after threshing. Because harvest practices and nomenclature vary across Asia, ‘‘residue removal’’ and ‘‘straw removal’’ can mean different things in different locations and literature. Sometimes it means removal of all biomass from the soil surface upwards; but often it means removal of biomass except the standing stubble, which can represent an important quantity of biomass depending on the height of crop harvest. Roots are also a source of organic material that crops contribute to soil every season, but they are not included in our definition of crop residue. Roots are almost always retained in the soil, and there are few other management options for them in rice-based cropping systems. Composts, animal wastes, and manures produced from residue removed from the field are outside the scope of this chapter.

2. Criteria for Evaluating Crop Residue Management Options In order to make and implement sound decisions about residue management, it is necessary to scientifically understand the short- and long-term effects of different crop residue management practices and to develop residue management technologies that provide agronomic benefit

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in a cost-effective and environmentally acceptable fashion. Crop residue management options are evaluated in this chapter using criteria of productivity, profitability, environmental impact, and sustainability for the cropping system. These criteria coincide with those used in the approach of ecological intensification for intensive crop production systems, which aims to satisfy the increasing demand for food, feed, fiber, and fuel while meeting acceptable standards of environmental quality (Cassman, 1999; Witt, 2003). Success in achieving ecological intensification depends greatly on sustaining yield increases in major irrigated and favorable rainfed cereal systems such as the rice-based cropping systems covered in this chapter. This chapter focuses on evaluating potential large-scale effects of residue management options rather than on the effects of residue management on specific processes, which have already been reviewed for soil fertility (Wilhelm et al., 2004; Yadvinder-Singh et al., 2005), C cycling (Martens, 2000), pesticide interactions with soil microorganisms (Moorman, 1989), soil-borne diseases (Chung et al., 1988), and root health (Allmaras et al., 1988).

2.1. Productivity and profitability Productivity and profitability are criteria directly relevant to farmer’s decision making. The quantifiable indicators of short-term productivity (i.e., 1–3 seasons of a given management option) include grain yield, fertilizer use efficiency (grain yield per unit fertilizer applied), water use efficiency (grain yield per unit water applied), and yield loss due to disease, insect, or weed pressure. Profitability indicators include income from yield less inputs (i.e., labor, fertilizer, seed, machinery, irrigation water, and pesticide). Residue management options can differ in their effects on these indicators of productivity and profitability. We have therefore assessed the residue-associated changes for each indicator for different in-field residue management options as compared to either removing or burning residues.

2.2. Environmental impact and sustainability Environmental impact and sustainability are criteria that are not typically important determinants for farmers in their selection of a particular residue management option, but these criteria can be important for policy making such as with the banning of open-field burning of crop residues. The main short-term (i.e., measurable within 1–3 seasons) environmental impacts associated with residue management include changes in air quality and greenhouse gas emission. Sustainability refers to the medium- and longterm (i.e., 5 years or more) ability of a residue management option to maintain or increase the productivity and profitability of the cropping system. Indicators include trends through time in yield, input use efficiency,


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soil parameters (such as N supply capacity, organic matter, K, S, and bulk density), and profitability.

3. Type and Abundance of Crop Residues The geographical distribution of crop residues in Asia is skewed by the large crop production in India and China. These two countries in 2006/2007 accounted for ~51% of global rice production and ~57% of Asian rice production (FAO, 2007; USDA, 2007). Frolking et al. (2002) by combining remote sensing and ground census data to develop maps for the distribution of rice in China showed 25% of the lowland rice cropland was planted as single rice, 15% was a double-crop rotation with two rice plantings per year (rice–rice), 19% was a double-crop rotation with a single rice planting (rice– other crop), and 41% was a triple-crop rotation with two rice plantings per year (rice–rice–other crop). Rice was rotated on an estimated 4.7 million ha with wheat, 4.5 million ha with rapeseed (Brassica napus L.), and 2.2 million ha with oat (Avena sativa L.). More recently, the area of rice–wheat in China was estimated at 3.4 million ha (Dawe et al., 2004). Yadav and Subba Rao (2001) estimated an area in India of 9.2 million ha for rice–wheat, 2.4 million ha for rice–oilseed, 3.5 million ha for rice–pulse. More recently, the area of rice–wheat was estimated at 10 million ha in India and 13.5 million ha for the Indo-Gangetic Plain, which includes Bangladesh, India, Nepal, and Pakistan (Ladha et al., 2003). The cropping patterns in rice-based cropping systems remain dynamic in response to markets, policies, and labor availability. We estimated total production area, grain yield, and production of residues for rice-based cropping systems in Asia from available 2004 data (Table 1). Rice-based cropping systems are defined as those with at least one rice crop per year grown either as a sole crop or in rotation with rice or a non-flooded crop. In these systems the rice would typically be grown on flooded soil, which markedly influences the management options for residues. Available data for area of a given crop in Asia typically does not specify the rotational system in which the crop is grown. Therefore, the areas for nonrice crops in rice-based cropping systems shown in Table 1 are based on estimations on a country basis. Economic yield data (grain or cane yield) were computed from the online databases (FAO, 2007). The residue-toeconomic yield ratios as listed in Table 1 are based on a range of reported data (Barnard and Kristoferson, 1985; Beri and Sidhu, 1996; Koopmans and Koppejan, 1997; Muehlbauer and Tullu, 1997; Yevich and Logan, 2002). According to these estimations for 2004, rice accounted for ~84% of total residue from rice-based cropping systems while wheat and maize accounted for 9% and all other crops accounted for 7%. The estimations highlight the large quantity of residues produced in rice-based cropping systems in Asia.

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Table 1 Residue production and area for rice and different crops grown in rotation with rice in Asia in 2004

Crop

Rice Wheat Maize Sugarcane Rapeseed Soybean Oats Lentil/Pulses Total

Harvested area (106 ha)

Economic yield (106 t)

Residue-toeconomic yield ratio

134 19 1.4 1.7 7.1 0.3 0.3 2.2

546 57 4.5 107 11 2.7 0.7 1.7

1.4 1.3 2.0 0.17 3.5 2.5 1.5 1.2

Residues (106 t)a

764 74 9 18 38 7 1 2 913

a

Data pertaining to production of residues was computed by multiplying economic yield of different crops with residue-to-economic yield ratios. Source: Data and conversion factors were obtained from FAO (2007), Frolking et al. (2002), Yadav and Subba Rao (2001), Koopmans and Koppejan (1997), Barnard and Kristoferson (1985), Yevich and Logan (2002), Beri and Sidhu (1996), and Muehlbauer and Tullu (1997).

This quantity will increase as yields and intensity of cropping continue to increase. The residues from nonrice crops as a proportion of the total would be expected to increase as rice-based cropping systems diversify. Little data are available on the use of these residues, but there have been several attempts in China and India to estimate the amount returned to soil. Gao et al. (2002) estimated 37% of ~600 million tons of crop residue produced in China are returned to soil, often in the form of rice stubble. In 15 provinces of China with most of the country’s cereal production, crop residues were returned to fields on an estimated 18.2 million ha, comprising of 37% of the total cultivated land in 2000 (Han et al., 2002). In India, an estimated 250 million tons of residues is produced annually in rice–wheat cropping systems in the Indo-Gangetic Plain (Pal et al., 2002). Huge amounts of residues are available either for retaining in fields to enhance productivity and fertility of the soil or for removing from the field for alternative uses, but in many areas of Asia the crop residues produced in rice-based cropping systems have been considered a nuisance by farmers and disposed through burning in fields.


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4. Existing and Emerging Residue Management Options Most lowland rice ecosystems in Asia have a rainy season when climatic conditions favor production of rice rather than other crops. Nonflooded crops are often grown in rotation with rice during the drier season. Soil in irrigated and rainfed lowland ecosystems with sufficient water typically remains flooded for most of the rice-cropping season. The choices for managing crop residue can consequently differ between periods with rice cropping, when anaerobic decomposition of residues predominates, and periods with other crops, when the aerobic decomposition of residues predominates. In intensive nonrice cropping systems with reduced or no tillage such as in Europe, America, and Australia, crop residues are often left in the field after combine harvesting, and seed for the next crop is then sown directly into the residue without plowing. In a rice production system in California, where legislation banned traditional open-field burning of residue, many farmers have adopted a system of retaining residues as a habitat for migratory, foraging waterfowl that hasten the decomposition of the residue during a winter-flood fallow (Bird et al., 2000). Tropical rice production systems in Asia, which are characterized by intensive cropping, do not have such long fallow periods with ample water between crops. Rice production systems in Asia also do not lend themselves to reduced or no tillage options because they are characterized by soil puddling— the plowing and harrowing of soil when flooded. Puddling destroys soil structure, restricting downward movement of water to maintain flooding (Sharma and De Datta, 1986), and soil flooding controls weeds and helps sustain the productivity of rice-based cropping systems. Soil puddling, however, restricts options for surface application of crop residues. Mulching in a puddled field typically necessitates removal of the residue before puddling and then returning it afterwards. Management practices for retention of crop residues are listed for different regions of China in Table 2 and for South Asia in Table 3. The recommended management for crop residues during cropping with flooded rice has typically been incorporation into the soil during land preparation. Yet farmers in Asia have not often in recent years followed this recommendation, electing instead the open-field burning of residues. But with increasingly strict legislation against open-field residue burning, a trend of increasing residue return can be anticipated, either through incorporation or as mulch. The retention of residues on the soil surface as a mulch is often an option during the nonrice crop in rice-based cropping systems, which can be established with

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Table 2 On-field residue management practices in rice cropping systems in different regions of China

Region

Cropping system

Existing residue management practice

Northeast

Rice

Yangtze Valley

Rice–wheat Rice–rapeseed

Southwest

Rice–rice Rice–wheat Rice–rapeseed

South China

Rice–rice Rice–rice– rapeseed

Stubble remaining + shallow plowing Mulching with rice residues in wheat (or rapeseed) season Incorporation in rice season Mulching with rice residues in wheat (or rapeseed) season Incorporation in rice season Mulching with rice residues in rapeseed season Incorporation in rice season

Amount of residue returned per year (t ha–1)

2.3 4.0

4.2

4.9

Sources: Zeng et al. (2001, 2002).

conventional, reduced, or no tillage. A challenge for mulching with reduced and no tillage is to ensure sufficient soil–seed contact after sowing. Methods of harvesting and threshing are critical in determining what happens to crop residue, and are often chosen because of the intended use of the straw. Some of the common harvest methods for rice in Asia include hand-cutting, use of small harvesting machinery, and use of combined harvester-threshers (Gummert and Aldas, 1993; Saunders et al., 1980). When hand-cutting, workers sometimes cut at the soil surface if they want long straw for animal bedding or roof thatch, but they might choose to cut part way up (20–60 cm above soil) to reduce the weight to be carried. The harvested part is carried to a centralized location on- or off-field for threshing to remove the grain from the straw. It can be threshed either manually or mechanically. Manual threshing involves hand flailing, swinging the straw over the head and beating it against a firm object in front that allows easy collection of the grain. If the straw is valuable to the farmer, it can then be stored for future use. With mechanical threshing, the chopped straw accumulates on a pile at the location of threshing. Combined harvesting–threshing machines separate the grain from the straw as they cut the plant and move through the field, retaining the grain and leaving the

Rice–wheat

Rice–wheat Rice–oilseed Rice–pulses Rice–jute–rice Rice–vegetable Rice–vegetable– rice Rice–wheat Rice–wheat– pulses Rice–rice Rice–rice– rice Rice–pulses

Trans- and Upper IndoGangetic Plain

Middle- and Lower IndoGangetic Plain

South India

Source: Based on Gajri et al. (2002) and Pal et al. (2002)

Non-Indo-Gangetic Plain (Terai of Nepal, Bihar, and Uttranchal)

Cropping system

Incorporation of rice stubble (~1)

Incorporation of rice and wheat stubble (~1)

Incorporation of stubble remaining in the field (~1) Rice straw mulch in vegetable production (1–2)

Incorporation of rice and wheat stubble remaining in the field (~1)

Existing residue management practices and (amount of residue returned per year, t ha–1)

Incorporation of rice straw and stubbles (~4)

Mulching of rice residues in wheat (~4)

Mulching with rice straw in no-till wheat (5–7) Incorporation of straw and stubble of combine harvested rice in wheat (5–7) Incorporation of combine harvested wheat straw and stubble in rice (1–2) Mulching with rice straw in no-till wheat (~5) Incorporation of manually harvested or combine harvested wheat straw and stubble in rice (~1)

Emerging residue management options and (amount of residue that could be returned per year, t ha–1)

Existing and emerging in-field residue management practices in rice cropping systems in different regions of South Asia

Region

Table 3

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chopped straw spread on the field. It is usually advantageous to leave high stubble and only move the upper portion of the plant through the machine. Combine-harvested fields consequently tend to have relatively tall standing stubble and short pieces of chopped straw laying on the surface. Methods of harvesting and threshing determine the percentage of total biomass left in the field as standing stubble, and the condition and location of the threshed straw. If standing stubble is not burned in the field, it is typically incorporated during land preparation for the subsequent crop. The primary management options, therefore, usually relate to the threshed straw rather than the stubble, although one management decision could be to adjust the harvesting procedure to increase or decrease the proportion of biomass that is threshed. Interventions for management of residues in rice cropping systems can be categorized based on the type of residue and crop in the system as follows: rice residue for a subsequent rice crop, rice residue for a nonflooded crop, and residue of a non-flooded crop for a subsequent flooded rice crop. These three situations can be further simplified into the following two cropping system categories based on the critical distinction of whether soil is flooded or non-flooded during the crop receiving the residues. 1. Rice following rice (common in South China, Southeast Asia, and southern India) and rice following an upland or non-flooded crop (common in Central and North China and parts of South Asia). In both cases, crop residue is managed during a rice crop, typically established on puddled soil. 2. Upland or non-flooded crop following rice (common in the IndoGangetic Plain in South Asia and many parts of China). In this case, rice residue is managed during a crop grown on non-flooded soil. Intensive rice monocropping systems are often the most challenging for managing crop residues because of the short time interval between rice crops. In rotations with rice and a non-flooded crop, the two crops often differ in the magnitude of the challenges for residue management. Management options are affected by the time of year when residue becomes available and the time before the next crop is planted. In the rice–wheat system in the Indo-Gangetic Plain, there is a relatively short time between harvesting rice and sowing wheat, hence the management of rice straw during wheat cropping (i.e., non-flooded crop following rice) is a critical issue. There is a relatively longer fallow after wheat, and wheat straw is valuable for off-field uses, especially as animal feed (Samra et al., 2003). Hence, the management of wheat straw during rice cropping (i.e., rice following a non-flooded crop) is not as critical. In China, however, there is a very short fallow between wheat and rice, often less than 1 week, because of the typically long growth duration of rice, and wheat straw does not have off-field value as in India. Hence, the management of wheat residue during


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rice (i.e., rice following a non-flooded crop) presents the bigger challenge. In each category of cropping patterns, there are a series of management options differing in relative feasibility and attractiveness.

4.1. Rice following rice or a non-flooded crop Residue management in this category is strongly affected by soil flooding and anaerobic soil conditions during the rice crop. Residues from the previous crop, whether rice or a non-flooded crop, typically must be managed in an environment of historical soil puddling and at least partial anaerobic decomposition. Factors that influence decisions about residue management are harvest method of the previous crop, turnaround time between crops, land preparation practices, availability of implements and labor, and establishment method and water management of the new rice crop. Residue management scenarios vary greatly depending upon whether the previous crop has been harvested manually or by combine harvesters, threshed at locations in field resulting in straw piles, or threshed outside the field. The method of harvesting also determines the extent to which crop residues remain anchored or loose. 4.1.1. Incorporation Residue incorporation into flooded soil has continued to be promoted as an alternative to open-field burning in rice-based cropping systems across Asia. It is a potentially attractive option because rice residues can typically be plowed into the soil as part of the normal tillage operations for preparing the rice field, and residue incorporation therefore does not require an extra step in land preparation. Once incorporated the residue typically decomposes relatively fast thereby potentially providing benefits to the next rice crop. Incorporation also avoids loose residue on the soil surface that could interfere with the preparation of the seedbed or planting of the next crop. Incorporation can reduce the risk of pests and diseases as compared with mulching, and it can potentially increase soil organic C fractions and total organic C. Specific management decisions include tillage method and timing of incorporation relative to rice establishment by transplanting or direct seeding. During land preparation for lowland rice in Asia, the topsoil is typically inverted thereby incorporating crop residues remaining on the soil surface as standing stubble or loose straw. A moldboard plow or disk plow is commonly used for incorporating residues often with a shallow layer of floodwater (Ponnamperuma, 1984). The degree of incorporation varies among tillage systems depending on implement, intensity, and mechanization level (i.e., manual, animal traction, or mechanized) (Sharma and De Datta, 1986). Incorporation of large amounts of fresh residue is labor intensive if suitable machinery is not available (Dobermann and Fairhurst, 2000). When rice is

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harvested using a combine harvester that leaves straw spread in the field, residue can be incorporated into the soil by disking or plowing. If only stubble is retained, the amount of residue incorporated into the soil is determined by the manner and the height of harvesting. Incorporation of residues might not be feasible when long straw clogs field implements. In rice–rice systems in Hunan and Hubei Province of China with only a few days between early and late season rice, a practice for rapid incorporation of residue before immediate establishment of the next crop is cutting the rice residue into ~20–25 cm lengths followed by shallow mechanical incorporation (Zeng et al., 2001). In rice–rice cropping systems with 2–3 months between rice crops, the shallow incorporation of rice residue into aerobic soil soon after harvest rather than delaying the incorporation of residues until preparation of flooded soil immediately before establishment of the next rice crop has been proposed as a practice to accelerate residue decomposition and release of plant-available nutrients (Dobermann and Witt, 2000). Witt et al. (1998) reported rapid decomposition of rice residue following shallow incorporation into aerobic soil, leading to ~50% loss of residue-C within 30–40 days and increased supply of plant-available N. Thuy (2004) in three cropping seasons in the Philippines consistently found comparable or significantly higher KCl-extractable soil ammonium at rice transplanting and uptake of soil N by rice at panicle initiation when residue from the previous rice crop was incorporated into aerobic soil immediately after harvest rather than incorporated later by the traditional practice during puddling ~3 weeks before transplanting. Shallow incorporation of rice residue into aerobic soil after harvest can also help reduce weed growth, save irrigation water by reducing soil cracking, and allow additional time for phenol degradation under aerobic conditions (Dobermann and Witt, 2000). The time interval between incorporation of crop residue and land preparation, flooding, and transplanting the next rice crop is a crucial factor affecting residue management. This time interval influences the extent of residue decomposition before transplanting, depending upon soil and climatic conditions, thereby affecting the beneficial or adverse effects of residue incorporation on young rice seedlings. In intensive irrigated production systems, two or three short-duration crops are typically grown per year. In the Red River Delta of northern Vietnam, for example, rice–rice–maize is a common cropping system. In the Mekong Delta of southern Vietnam where rice is grown continuously, the intensity of rice cropping can reach 6–7 crops in 2 years (Dobermann et al., 2004). In such intensive triple cropping systems, the fallow between two crops can be only a few days. Whereas a relatively long fallow period of 2–3 months provides opportunities to manage residues for hastened decomposition and nutrient release, a fallow of only a few days does not enable appreciable residue decomposition and release of nutrients before establishment of the next rice crop. In such


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cases the incorporation of large quantities of crop residues poses challenges and potential detrimental effects on the subsequent rice crop. 4.1.2. Mulching Adding mulch to flooded rice is usually not feasible because the traditional practice of puddling will incorporate retained crop residue. Mulching typically requires transfer of biomass off of the field before soil puddling and then return of the biomass after land preparation. In rice–rice systems in South China, some farmers do not drain their fields at harvest of the early rice crop and keep the field flooded without tillage during the brief transition period to transplanting of the late rice crop. Straw from the early rice is placed as mulch in rows along the direction of transplanting for late rice, and late rice is transplanted between the rows of straw covered soil (Li, 1991). The continuous flooding helps ensure the soil is sufficiently moist for easy transplanting of late rice, and the mulch helps control weed growth and prevent ratooning of rice. Other rice farmers and researchers in China have been trying reduced and no tillage flooded rice systems that enable surface retention of crop residue. In Anhui, Guangdong, Hunan, Hubei, Jiangsu, and Zhejiang Provinces, farmers have used either the throwing of rice seedlings or the direct sowing of germinated rice seeds as methods for establishing rice in reduced or no tillage fields (Li, 2005). In such systems crop residue is retained on the soil surface. The soil is saturated or flooded during crop establishment, and herbicides are used to control weeds. In Anhui, Jiangsu, and Zhejiang Provinces, some farmers practice relay cropping whereby rice is sown in wheat fields before combine harvesting. The standing stubble of wheat then slowly decomposes during rice cropping. A ground covering rice production system (GCRPS) to save water and increase N efficiency has been developed in South China. It involves growing a lowland rice variety under non-flooded conditions (70–90% of water holding capacity) with the ground covered by rice straw mulch during growth (Fan et al., 2002; Huang et al., 1997; Lin et al., 2002; Luo, 1997; Zhao et al., 1999). Mulching increased soil organic C and total N (Fan et al., 2002; Liu et al., 2003), but grain yield was sometimes lower than when rice was grown under flooded conditions. 4.1.3. Composting Most composting techniques are off-field residue management options in which the produced compost is not returned to the main production field, and as such are not included in this chapter. Some composting can, however, occur in fields (in situ composting), and a small portion of the compost produced off-field can be returned to the main field. One example of in situ composting is where rice straw is piled in the field at threshing sites (Ponnamperuma, 1984). The straw decomposes slowly, largely aerobically,

132

Bijay-Singh et al.

and it can then be spread and incorporated into the soil at the beginning of the next season. Constraints of this practice include providing a favorable habitat for rodent pests and promoting excessive immobilization of N under the residue piles. Another type of in situ composting, in which the residue of the previous crop is buried in ditches paralleling rows of transplanted rice, has been examined in China for residues of non-flooded crops, such as wheat and barley, grown immediately before rice (Zhong et al., 2003). Crop residue can be removed from the field, composted alone or with other organic materials originating at the farm such as animal wastes, and then returned to soil as manure for the rice crop. The potential of composting to turn on-farm waste materials into a farm resource makes it an attractive proposition. Traditional methods based on a passive composting approach involve simply stacking crop residues in piles or pits to decompose over a long period with little agitation and management (Misra et al., 2003). The time requirement can be reduced through a few turnings, which slightly enhance passive aeration. Chinese rural composting methods also use a passive aeration approach based on turnings and aeration holes, and they provide output in 2–3 months (Ma, 2004). Low turnover and long time span are major bottlenecks. Traditional passive methods require several months from the time of crop harvest until the compost is ready for use. Composting involves labor input, but it is not capital intensive and does not require sophisticated infrastructure and machinery. Small farmers without manual labor constraints are most likely to benefit from composting technology.

4.2. Non-flooded crop following rice Managing rice residue during a non-flooded crop is somewhat easier than managing it during flooded rice because options for reduced or no tillage and mulching are more feasible. In addition, incorporated residue usually decomposes faster in aerobic than in flooded soil. There are, however, challenges. Major factors affecting residue management decisions include method of rice harvest, the time interval between crops, water management, and method of tillage for the subsequent non-flooded crop. The extent to which rice residue remains anchored after harvest also influences the way it can be managed in the following non-flooded crop. The most common options include incorporation with conventional tillage or surface mulching usually with reduced or no tillage. 4.2.1. Incorporation Incorporation of rice residue into the soil before planting a non-flooded crop has been frequently studied, particularly where rice residue does not have off-field economic use such as for animal fodder, fuel, or industrial purposes. Various options are available for farmers to incorporate crop


133

residues into the soil depending upon availability of machines, financial resources, and amount of straw (Ball and Robertson, 1990). For example, residues can be directly incorporated using a moldboard plow, or they can be chopped using a straw chopper after harvesting with a combine, and then the chopped residue can be easily incorporated into the soil using a disc plow (Sidhu and Beri, 2005). One reason for the popularity of residue incorporation among scientists is that residue can be incorporated during the preparatory tillage for the non-flooded crop, and hence it does not entail extra cost for implementation. Another option is incorporation of residue with a separate field operation several weeks before land preparation for the following crop. This allows more time for the residue to decompose and helps to control weeds. Residue can be incorporated partially or completely into the soil depending on method of cultivation used. The time interval between residue incorporation and planting of the next crop is determined by the cropping calendars and the time needed for residue decomposition. Yadvinder-Singh et al. (2004b), for example, in a rice–wheat cropping system in the northwestern India, observed that rice residue decomposition of ~25% during the pre-wheat fallow period was sufficient to avoid any detrimental effects on wheat yields. Rice and wheat productivity in a 7-year study was not adversely affected when rice residues were incorporated at least 10 days and preferably 20 days before the establishment of the succeeding crop. 4.2.2. Mulching with reduced or no tillage A reduced or no tillage system makes it relatively easy to retain residue on the surface as mulch simply by leaving it on the field during harvest. It is not necessary to remove the residue before tillage and then return it. However, if residue is threshed off-field, it must be transported to and spread on the field, resulting in no saving in labor for handling residues as compared to mulching with conventional tillage. Direct drilling is a method of sowing the crop after rice harvest without cultivation or incorporation of residue. According to Li (1991), no-till sowing of winter crops including wheat, barley (Hordeum murinum L.), and rapeseed is commonly practiced by farmers in rice-based cropping systems in eastern China. Surface seeding of wheat by making small holes in the soil (dibbling) followed by mulching with rice residue (4–6 t ha–1) is practiced on ~60% of the rice–wheat system in the Sichuan Basin (Humphreys et al., 2004). Because the time between rice harvest and wheat sowing is relatively long (60–85 days) in middle and southern China, the spreading of rice residue as mulch immediately after harvest has been examined as a practice for controlling weeds and reducing evaporation during the fallow before the next non-flooded crop (Zeng et al., 2001, 2002). This technique is attractive for farmers growing rapeseed instead of wheat after rice because they can broadcast the seeds, which are

134

Bijay-Singh et al.

small enough to fall through the rice mulch to the soil surface. A double zero-tillage system of no-till, direct seeded rice and no-till rapeseed with rice residue as mulch for rapeseed has been developed in Hunan Province (Zou et al., 2004b). Reduced and no tillage for wheat has been increasingly adopted by farmers in the Indo-Gangetic Plain in northwestern India since the late 1990s because it leads to large cost savings through reduced use of fuel and labor (Erenstein et al., 2007). In the eastern part of the Indo-Gangetic Plain, it also facilitates early sowing leading to potential yield benefits, especially after late harvested rice. Rice residue mulching in fields seeded with wheat with reduced and no tillage is also practiced by a small number of farmers in the Terai of Nepal and in eastern Uttar Pradesh and Bihar states of India (Humphreys et al., 2004). The area of reduced and no-till wheat in the Indo-Gangetic Plain has expanded at an exponential rate since the late 1990s, increasing to an estimated 20–30% of the rice–wheat area or 2–3 million ha in 2006 (RWC, 2006). No-till sowing of wheat after combine-harvested rice, however, involves some difficulties including residue accumulation in the furrow openers, traction problems with the drive wheel of the seed drill, difficulty with fertilizer metering systems in the loose straw, and nonuniform sowing depth due to frequent lifting of the drill to clear blockages. A number of approaches are currently being tested for direct drilling into rice residue to solve the problem of machinery clogging and ‘‘hair-pinning’’ when the straw bends but is not cut or buried, resulting in seed remaining on the surface. These include double and triple disc systems (Gupta and Rickman, 2002), the straw thrower (Shukla et al., 2002), and the stubble chopper (Garg, 2002); although none of these approaches has been particularly successful to date. A promising new approach is the ‘‘Happy Seeder,’’ which combines the stubble mulching and seed drilling functions into one machine (Blackwell et al., 2004). The stubble is cut and picked up in front of the sowing tines, which therefore engage bare soil, and deposited behind the seed drill as mulch. The evolution of the technology, leading to a machine called the Comboþ Happy Seeder, is described by Humphreys et al. (2006). Results to date from India suggest that wheat can emerge through 8 t ha–1 of evenly spread rice residue mulch with no detrimental effect (Humphreys et al., 2004), although 4–6 t ha–1 is considered optimum in Sichuan, China. 4.2.3. Mulching with conventional tillage Rice residue can be used as mulch for the following non-flooded crop established after conventional tillage. Not many farmers follow this option because it involves temporarily removing the residue from the field and then returning it after the crop has been planted. This option is more feasible for farmers with small land holdings and sufficient labor (Tang


135

et al., 2004). There are only sporadic reports of rice residue mulching practiced in wheat planted in conventionally tilled fields in the rice–wheat system in South Asia (Zaman and Choudhuri, 1995). 4.2.4. Transfer of biomass as mulch Rice residue can be removed from the field where it is grown and used as mulch for improved production of vegetable crops (Vos et al., 1995), chickpea (Cicer arietinum L.) and mustard (Brassica rapa L.) (Rathore et al., 1999), and crops like bamboo ( Jiang et al., 2002). In China, it is also being used for mulching horticultural crops and tea. The cost of transportation and labor compared with the profits earned by farmers from increased production and saving in inputs like water are important factors in determining the feasibility of this management option. Rice residue can also be removed from the field for a number of useful purposes such as livestock bedding, composting for mushroom cultivation, bedding for vegetables such as cucumber and melon, and conversion to biofuel and biopower.

5. Evaluation of Options with Residues Managed During a Rice Crop We now evaluate residue management options for the category of cropping described in Section 4.1 (Rice following rice or a non-flooded crop) in which residues are managed during a period of rice cropping, usually on puddled and flooded soil. Our evaluation is based on criteria for productivity, profitability, environmental impact, and sustainability as described in Section 2.

5.1. Productivity As reviewed in Section 4.1.1, the incorporation of residue into soil during rice cropping is one of the most studied and promoted alternatives to residue burning across Asia. Much data are therefore available for evaluating its productivity and profitability. When interpreting these data, it is important to remember that large variations among seasons at a given location and among locations can exist in amount of residue incorporated, the time period for residue decomposition before establishment of the rice crop, and the soil aeration status (i.e., primarily aerobic or anaerobic) during residue decomposition. Soil and plant parameters can be affected by whether residues from only one or all crops in a year are incorporated. Mulching of lowland rice with residue of a preceding rice or upland crop is not a very practical option as explained in Section 4.1.2, and consequently it has not been much studied and only limited data are available.

136

Kerala, India: Sandy loam soil with pH 5.5, fertilizer N applied at 100 kg ha–1, continuously flooded Same as above but intermittently flooded Indonesia: Acidic soil Acidic soil, fertilizer N at 120 kg ha–1 Acidic soil, no fertilizer N applied Muara, Indonesia: Latosol

Experimental details

2.75 6.12 2.45 2.78

4 4 4 20 10 10 10 10 10 10 10 20

28

28 14 7 28 14 7 – –

4.27 4.22 4.00 3.49 3.22 2.84 2.17 2.17

3.04 6.15 2.90

Rice residue removed

– – – – – –

– – – –

– – –

Rice residue burned

Rice grain yield (t ha–1)

4 4 4

Days residue incorporated before transplanting

Amount of rice residue incorporated (t ha–1)

3.95 3.97 3.81 3.04 3.15 2.95 2.58 2.44

2.92 6.25 2.45 3.06

3.18 6.64* 2.10

Rice residue incorporated

– – – – – – – –

WS 1972 DS 1973 WS 1973 –

WS 1972 DSb 1973 WS 1973

a

Season

Table 4 Effect of rice residue incorporation on grain yield of rice in rice–rice cropping systems in Asia

Ismunadji (1978)

Vamadevan et al. (1975)

Reference

137

West Bengal, India: Sandy clay loam soil with pH 7.8, 60 kg N ha–1 in dry season and 40 kg N ha–1 in wet season Los Banõs, Philippines: Maahas clay, averaged for 5 cultivars after 16th crop Tropaqualf, clay soil with pH 6.6, 9-year study Tropaqualf, clay soil with pH 6.6, 9-year study Indonesia: Vertic Tropoquept, pH 6.5, no NPKS applied Same as above with NPKS applied 5.7

–

–

8.3c

8.3c

6

–

–

–

8.2c

2.4

–

–

3.4

– –

3.2

3.88 5.38

6

–

–

0

10 10

28 35

5.2

2.7

8.7c

8.7c

4.1*

4.11 6.04

–

–

WS+DS

WS+DS

–

WS DS

(continued)

Le Cerff et al. (1985)

A.B Capati, IRRI, cited by Ponnamperuma (1984)

Chatterjee et al. (1979)

138

Uttar Pradesh, India: Soil with pH 8.5 South Korea Shao Shing County, Eastern China: 150 kg N ha–1 Punjab, Pakistan: Soil pH 8.0 Hangzhou, Southeast China: Soil with pH 6.2


Table 4 (continued)

5

35

20

153

Equal to 600 kg organic C ha–1 Equal to 600 kg organic C ha–1

7.5 3 6 10

– –

0

–

30



– –

–

6.20

6.20

– – – –

–

Rice residue burned

5.6

4.3 5.97 5.97 5.38

2.21



6.13

6.19

5.9

4.8* 6.15 6.17 6.04

2.92*


Early rice

WS

DS

–

Season

Lu et al. (2000)

Zia et al. (1992)

Han et al. (1991) Li (1991)

Pandey et al. (1985)

Reference

139

Central Thailand: Ustic Endoaquerts with pH 6.2, rice residues

Los Banõs, Philippines: Aquandic Epiqualf, silty clay with pH 6.6 Central Java, Indonesia: Acric Tropoqualf, silt loam soil with pH 4.7, rainfed rice Central Thailand: Vertic Tropaquept with pH 5.8, rice residues (C:N¼67:1) contained 21.7 kg N ha1 – –

3.75 (þ70 kg N ha1) 3.75 (þ70 kg N ha1) 3.75 (+no N) 3.75 (+no N) 5 (+70 kg N ha–1) 5 (+70 kg N ha–1) 5 (+no N)

– –

7

7

7

7

7

7

7

–

–

3.7

3.8

–

4.1

–

–

4.4

3.0

3.9

4.0

3.7

4.2

4.7

3.7

–

– –

4.8 4.7

5.4 3.0

3.4

4.2

4.1

4.1

4.0

3.9

4.8

5.3 4.6

3.5 3.0

DS 1998

WS 1998

DS 1998

WS 1997

DS 1997

WS 1997

DS 1997

WS DS

DS WS

(continued)

Phongpan and Mosier (2003b)

Phongpan and Mosier (2003a)

Setyanto et al. (2000)

Wassmann et al. (2000a)

140

(C:N=67:1) contained 25.4 kg N ha–1 Central Thailand: Ustic Endoaquerts with pH 6.7, rice residues (C:N=67:1) contained 25 kg N ha–1 Andhra Pradesh, India: Sandy clay loam soil with pH 7.5 Anhui, Guangde, China: Loamy clay soil

14 3 14 3

7 7

4.4 5.7 4.4 5.7 3

5 (+70 kg N ha–1) 5 (+70 kg N ha–1) 5 (+no N) 5 (+no N)

7

7

5 (+no N)


7


–

3.9

6.2 3.1 6.3 3.2 6.2

6.9 3.4 7.5 3.5 –

– –

–

6.0

4.8 3.9

–

Rice residue burned

3.7



7.0 3.5 7.3* 3.7* 6.7

5.0 4.4

4.3

5.6

4.3*


b

Wet season. Dry season. c Yield of dry season+wet season rice crops. * Significantly more than grain yield with residue removed or residue burned treatments at P < 0.05.

a

(continued)


Table 4

DS 1999 WS 1999 DS 2000 WS 2000

DS 1999 WS 2000

WS 2000

DS 1999

WS 1998

Season

Li et al. (2003)

Surekha et al. (2003)

Phongpan and Mosier (2003c)

Reference

141

Uttar Pradesh, India: Soil with pH 8.5, 2-year study West Bengal, India: Silty clay loam acid laterite soil, 2year study Haryana, India: Clay loam soil, 3-year study Uttar Pradesh, India: Clay loam soil with pH 8.6


Wheat, 0

Wheat, 5

Wheat, 5.3

Wheat, 10

10

7–10

30

Kind and amount of upland crop residue incorporated (t ha–1)

30

Days residue incorporated before transplanting of rice

–

– –

4.10 2.29

7.23 4.84

– –

– –

Upland crop residue burned

4.10

6.97 4.65

3.74 1.80

2.21 4.48

Upland crop residue removed

Grain yield (t ha–1)

2.72

4.08

4.45

7.01 4.43

4.17* 2.0*

2.82 4.59

Upland crop residue incorporated

Rice (100% NPK)b Rice (50% NPK)c Wheat (R)

Rice Wheat (R)

Rice Wheat (R)

Rice Wheat (R)a

Crop

(continued)

Agrawal et al. (1995) Rajput (1995)

Sharma and Mitra (1992)

Pandey et al. (1985)

Reference

Table 5 Effect of incorporation of upland (non-flooded) crop residue on grain yield of rice and residual effects on yield of the following upland crop in rice–upland cropping systems in Asia

142

(continued)

New Delhi, India: Sandy clay loam with pH 8.1 Guizhou, China: Loamy clay, rice–rapeseed Haryana, India: Sandy loam soil, 25% N applied at the residue incorporation, 3-year study Punjab, India: Typic Ustochrept, sandy loam soil with pH 7.9, 2-year study Punjab, India: Typic Ustochrept loamy sand


Table 5

Wheat, 5.5 Wheat, 6.1 Rapeseed, 7.5 Wheat, 0

Wheat, 6

Wheat, 6 (90 kg N+13 kg P+13 kg K ha–1)

30

7

60

51–60

14

Kind and amount of upland crop residue incorporated (t ha–1)

42

Days residue incorporated before transplanting of rice

– –

–

6.20c 4.48

3.3 2.3 3.9 3.6

Upland crop residue burned

5.6 4.8

6.83 4.01

3.3 2.4 3.8 3.6 6.7

Upland crop residue removed

Grain yield (t ha–1)

5.10 4.33

5.5 4.9

6.85 4.04

3.6* 2.5 4.0* 3.7 5.9

Upland crop residue incorporated

Rice Wheat (R)

Rice Wheat (R)

Rice Wheat (R)

Rice 1992 Wheat (R) Rice 1993 Wheat (R) Rice

Crop

Bhandari et al. (2002)

Aulakh et al. (2001)

Zhao and Zhu (2000) Dhiman et al. (2000)

Prasad et al. (1999)

Reference

143

Wheat, 4.2

10

8.0

Wheat, 4.4

7.1

5.74 4.41

6.2

Wheat, 1.5

Wheat, 6.4 0.5

7.2

52–55

2

6.20c 4.48

Wheat, 3 (105 kg N +20 kg P +19 kg K ha–1) Wheat, 7.5

– –

– –

7.3

5.37 4.32

8.8

6.6

7.8*

5.72 3.88

b

(R) denotes that the crop was grown to study the residual effect of crop residues applied to the previous crop listed above. 100% NPK is the blanket recommendation of 120 kg N+13 kg P+25 kg K ha–1 for the region. c 120 kg N+26 kg P+25 kg K ha–1 * Significantly more than grain yield with residue removed or residue burned treatments at P < 0.05.

a

Shandong, Lingyi, China: Sandy loam soil Anhui, Guangde, China: Loamy clay soil Shanghai, China: Loamy clay soil Punjab, India: Loamy sand soil with pH 7.6, 12-year study Jiangsu, Wuxi, China: Loamy clay with pH 6.8

with pH 8.2, 14-year study

Rice

Rice Wheat (R)

Rice

Rice

Rice

Rice Wheat (R)

Yang et al. (2003) YadvinderSingh et al. (2004a) Zhu et al. (2004)

Li et al. (2003)

Ma et al. (2003)

144

Bijay-Singh et al.

5.1.1. Grain yield for rice A summary of 51 data sets is reported in Table 4 from rice–rice cropping system experiments designed to assess the effect of incorporating from 3 to 20 t ha–1 rice residue at 0–153 days before establishment of the following rice crop. Only 7 (~14% of the experiments) showed statistically significant increases in grain yield associated with residue incorporation. Table 5 lists studies in which 1.5–10 t ha–1 of residue from an upland (non-flooded) crop was incorporated 2–60 days before establishing the rice crop in India and China. Only in 4 out of 17 comparisons was rice yield significantly increased by incorporation of residue. The effect of residue incorporation on yield of lowland rice can depend on incorporation method, amount of residue, soil characteristics, and timing and amount of fertilizer application (Ponnamperuma, 1984). It was difficult to identify a similarity in terms of region, amount of residue incorporated, or time of incorporation among the data sets showing significant yield increases. In most cases, the effect of residue incorporation was assessed with application of fertilizers, suggesting that benefits in nutrient supply from the residue could have been masked by application of sufficient fertilizer to overcome nutrient limitations to rice. On the contrary, in studies with no application of fertilizer N (Le Cerff et al., 1985; Phongpan and Mosier, 2003a,b,c; Thuy, 2004) there is frequently no significant increase in grain yield associated with residue incorporation. Thuy et al. (2008) in a 3-year study at two locations in China found no significant increase in rice yield following incorporation of wheat or rice residue with and without fertilizer N. Thuy et al. (2008) concluded that incorporated residue had no benefit on N supply during the vegetative growth phase of rice, but N supply at later rice growth stages especially with long-duration rice could be slightly increased. A combined analysis of all data sets for effect of incorporating rice and upland crop residues on yield of the following rice (Tables 4 and 5) revealed no significant trend of increasing yield due to residue incorporation (Fig. 1). The slope and intercept of linear regressions were not different than 1 and 0, respectively, at P < 0.001. Residue of wheat incorporated into rice also did not have a residual effect on the wheat crop that followed rice (Table 5, Fig. 1). A multicountry coordinated research project on management of crop residue for sustainable production concluded that residue incorporation did not lead to higher yields (IAEA, 2003). In some cases the incorporation of crop residues, especially without application of fertilizer N, can reduce rice yield (Thuy, 2004). This is often attributed to short-term immobilization of N following the incorporation of residue with high C:N ratio (Bird et al., 2001; Buresh et al., 2008). The anaerobic decomposition of added residue and associated intensively reduced soil conditions can lead to production and accumulation of aliphatic aromatic acids that can inhibit rice root growth (Chung, 2001; Tanaka et al., 1990) particularly under low temperatures (Cho and


145

A Rice yield (t ha−1) with rice/upland crop residues incorporated

9 With rice residue With upland cropresidue 1:1 line

6

3

0 0

3

6

9

Rice yield (t ha−1) with no residue incorporation

Wheat yield (t ha−1) with wheat residue incorporated in the preceding rice

B 6 Wheat yield 1:1 line 4

2

0 0

2

4

6

Wheat yield (t ha−1) with no residue incorporation in the preceding rice

Figure 1 (A) Relationship between rice yield with and without incorporation of rice or upland crop residue, and (B) wheat yield with and without incorporation of wheat residue into the preceding rice crop. Data are from the literature listed in Tables 4 and 5. The slope and intercept for the linear regressions from A and B were not different than 1 and 0, respectively, at P < 0.001.

Ponnamperuma, 1971), to production of small-molecular-weight organic acids that can have some toxic properties (Rao and Mikkelsen, 1977), and to induced deficiencies of micronutrients especially zinc (Bijay-Singh et al., 1992; Nagarajah et al., 1989). One option for reducing the potential

146 Amount of residue mulched (t ha–1) Kind of residue

Sichuan, China: Fluvaquent grey flood plain soil with pH 7.8

0

–

Wheat

Crop residue mulching in conventionally tilled rice fields 0 Equal to Hangzhou, Rice 600 kg Southeast organic China: Soil C ha1 with pH 6.2, rice straw mulched before transplanting of rice 0 – Rice Guangzhou, China: Grown as upland rice, lowland rice without mulching yielded 6.96 t ha–1 Bhairahawa, – 1.5 Wheat Nepal


Days residue mulched before planting

–

4.2

5.98

3.75

4.63

6.25 (flooded rice)

6.44

Crop residue incorporated

6.31

Crop residue removed

Grain yield (t ha1)

Rice 5.38 (nonflooded rice) 5.74*

Wheat (R)a with 60 kg N ha–1

Rice

Rice

Late rice

Crop

4.75*

6.59*

6.44

Crop residue mulched

Table 6 Effect of returning crop residue as mulch to rice fields on grain yield of rice in rice-based cropping systems in Asia

Duxbury and Lauren (2002) Liu et al. (2003)

Fan et al. (2002)

Lu et al. (2000)

Reference

147

0 Sichuan, China: Loam soil with pH 6.5, nonflooded rice 0 Jiangxi, Yujiang, China: Fluvisol, pH 5.5, organic matter 25.5 g kg–1, rice–rice rotation, nonflooded rice Crop residue mulching in no-till rice Sichuan, Chengdu, China: Sandy loam soil 0 Sichuan, China: Heavy clay soil, permanent bed planting with double zero tillage for rice and wheat Sichuan, Jianyang, China: Loamy clay soil with pH 6.45, no tillage, rice seedling broadcasting

Wheat

Rice

Wheat

Wheat, rice (in ditches)

Wheat

–

5

7.5

–

5.3

–

5.36

8.9

–

–

4.72c

6.6

– –

6.45b 4.62

9.3

5.72

6.8

6.75*

6.84 5.29*

Rice

Rice on permanent beds

Dryland rice

Rice

Rice Wheat (R)

(continued)

Zheng et al. (2005)

Tang et al. (2004)

Ai et al. (2003)

Qin et al. (2006)

Liu et al. (2005)

Days residue mulched before planting Kind of residue

Rapeseed

Amount of residue mulched (t ha–1)

5.3

8.8

Crop residue removed

Crop residue incorporated

Grain yield (t ha1)

c

b

(R) denotes that the crop was grown to study the residual effect of crop residues applied to the previous crop listed above. The no mulch flooded treatment yielded 7.2 t ha–1 and was not significantly different than mulched non-flooded yield. –1 The no mulch flooded treatment yielded 6.8 t ha and was not significantly different than mulched non-flooded yield. * Significantly more than grain yield with residue removed treatments at P < 0.05.

a

Sichuan, Jianyang, China: Loamy clay soil with pH 6.45, rice– rapeseed, no tillage, rice seedling broadcasting


Table 6 (continued)

9.4*

Crop residue mulched

Rice

Crop

Zheng et al. (2005)

Reference


149

detrimental effect of decomposing crop residues on rice seedlings is the type of in situ composting in which the residues are next to but not touching the seedlings in ditches parallel to the rows. This practice, however, did not increase yield of rice in a barley–rice rotation (Zhong et al., 2003). A summary of data from 10 mulching experiments across Asia shows a significant positive effect of rice or upland crop residue applied as mulch on rice grain yield in 4 out of 10 experiments (Table 6 ). In Sichuan Province in China, research has examined the mulching of rice with residue of preceding wheat. The mulch is applied after rice transplanting, and then the mulched rice is not kept flooded to save irrigation water. In two studies (Liu et al., 2003, 2005) the mulch did not increase rice yield, but yield of the wheat crop following rice was significantly increased. The partially decomposed mulch from the rice crop, incorporated during land preparation for the next wheat crop, presumably benefited wheat. The application of mulch after transplanting rice seedlings requires removal of residue from the field and then return as mulch. Following this approach, mulching of rice with wheat residue in Nepal significantly increase rice yield compared to plots without mulch (Duxbury and Lauren, 2002). The application of rice residue before transplanting rice, on the other hand, as examined in Southeast China did not increase rice yield (Lu et al., 2000), probably because the mulch to some extent got incorporated into soil during transplanting of rice. Reduced tillage rather than conventional puddling before rice can result in soil conditions less suitable for transplanting of rice. The throwing of rice seedlings and direct seeding of rice into mulch have consequently been examined as alternatives to transplanting in reduced and no-till rice systems. In a study in Sichuan Province in China, rice yield was significantly increased with establishment by seedling throwing into mulch from residue of a preceding rapeseed crop (Zheng et al., 2005). No benefit of mulching on rice yield was observed when rice and wheat residues were applied as mulch in ditches to rice grown on permanent beds in a double no tillage rice–wheat cropping system (Tang et al., 2004). In a wheat–rice system in Korea, Cho et al. (2001) examined the simultaneous harvesting of wheat and sowing of rice with a sowing device mounted on the combine harvester. Rice seeds broadcast onto the untilled soil surface were covered with wheat straw chopped into mulch by the combine harvester, and comparable rice yields were obtained with no-till, direct sowing of rice and conventional till, transplanting of rice. 5.1.2. Fertilizer use efficiency for rice An important indicator of fertilizer use efficiency is the increase in grain yield per unit of nutrient applied as fertilizer, which is usually referred to as agronomic efficiency. An increase in the agronomic efficiency for a given nutrient occurs when the crop response to the nutrient (i.e., the difference in yield between treatments with and without addition of the nutrient) increases

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per unit of applied nutrient. As indicated in Section 5.1.1, the incorporation of crop residues seldom significantly increases the yield of rice with and without fertilizer application (Tables 4 and 5). In such cases of no increase in grain yield, the incorporation of crop residues would not increase either the agronomic efficiency of a nutrient or the partial factor productivity of the nutrient (i.e., total grain yield with added nutrient per unit of applied nutrient) unless the incorporation of residue was associated with a reduced rate of applied nutrient. Most studies assessing the effect of residues on rice, however, use similar rates of fertilizer in the treatments with and without the residue. Both increases and decreases in agronomic efficiency of fertilizer N (AEN) have been reported for rice when crop residue is incorporated rather than removed with no change in the rate of fertilizer N. Incorporation of rice residue with 70 kg N ha–1 to dry season rice in Central Thailand reduced AEN by 20–50% (Phongpan and Mosier, 2003a,b,c). This was attributed to a reduced yield response of rice to fertilizer N following incorporation of residue, which slightly increased grain yield without fertilizer N but not with fertilizer N. In a 3-year study at two locations in China, the incorporation of rice or wheat residue typically had no effect on AEN (Thuy et al., 2008). In an experiment in the Philippines, the incorporation of rice residue 20 days before transplanting dry-season rice significantly increased AEN irrespective of whether soil was flooded or aerobic for the 2 months from harvest of the previous rice crop to land preparation for rice (Buresh et al., 2007, unpublished data). This increase in AEN was associated with increased response of rice to fertilizer N with incorporation of residue, which arose because the incorporation of residue significantly reduced yield without fertilizer N. Lower yield without fertilizer N was associated with reduced supply of plant-available N due to immobilization of N following residue decomposition (Thuy, 2004). The higher frequency of yield gains with mulching (Table 6) than incorporation of residue (Tables 4 and 5) suggests mulching might be more likely than incorporation to increase fertilizer use efficiency when fertilizer rates are not adjusted for residue management. An approach for increasing AEN while maintaining or even increasing rice yield is to combine the retention of crop residues with improved management of fertilizer N. Xu et al. (2007) found markedly increased fertilizer N use efficiency for rice when the incorporation of residue from a previous wheat crop was combined with timing and rates of fertilizer N that better matched the needs of the rice crop. This approach of improved matching of fertilizer N to crop needs, referred to as site-specific nutrient management (SSNM) (IRRI, 2007), involves adjusting fertilizer N during early vegetative growth to match crop needs as determined by relatively slow early crop growth and immobilization of N from decomposing residue, and it involves adjusting fertilizer N during tillering and at panicle initiation based on the N status of rice leaves. At locations with excessive use of fertilizer N for rice, such as eastern and southern China, an increase in AEN is largely associated with marked reductions in the use of fertilizer N while


151

maintaining or slightly increasing yield (Peng et al., 2006a). At locations in which existing fertilizer N rates are near or below the optimal, an increase in grain yield, often with the same or more fertilizer N, is required to increase AEN. When fertilizer N is optimally managed for rice, the incorporation of crop residue typically has negligible or only small savings in fertilizer N (Linquist et al., 2006; Thuy et al., 2008). The vegetative portion of mature rice contains ~80–85% of the total plant K, and most of this K is not lost during open-field burning of residue (Dobermann and Fairhurst, 2000). The management of crop residue consequently has a strong effect on the requirements of rice for fertilizer K (Witt et al., 2007). Recommended rates for fertilizer K should consequently be adjusted based on the supply of K from crop residues, even when burnt, in order to ensure high rice yields with high efficiency of fertilizer K use. 5.1.3. Water use efficiency for rice There have been several reports in China of savings in irrigation water associated with mulching rice with crop residue and then growing rice under non-flooded conditions. Yields are often comparable for mulched, non-flooded rice and conventional flooded rice, but water use efficiency (i.e., grain yield per unit of water used) can be markedly higher for nonflooded mulched rice (Fan et al., 2002). Qin et al. (2006) found that total water use in non-flooded rice with and without rice straw mulch was 3.3 and 2.4 times less than for rice grown under flooded conditions. Yields were comparable for mulched, non-flooded rice (6.7 t ha–1) and conventional flooded rice grown without mulch (6.8 t ha–1). But growth of non-flooded rice without mulch significantly reduced yield (4.7 t ha–1). Mulching rice to save water can be particularly attractive in regions with limited rainfall or irrigation water. 5.1.4. Pest and disease pressure for rice Weed pressure is typically minimal in flooded soil, decreasing the importance of mulch as a weed suppressant in lowland rice. However, mulch might help minimize weed competition in production systems without soil flooding when rice seedlings are small. In non-flooded rice mulched with wheat residue in the rice–wheat cropping system in southwestern China, Liu (2005) recorded total weed biomass of 1.3 t ha–1 in mulched plots as compared to 4.4 t ha–1 in plots without mulch. The corresponding uptake of N by weeds was 25 kg N ha–1 in mulched plots and 55 kg N ha–1 in plots without mulch. Residue incorporation in rice monocropping systems has been shown to aggravate fungal diseases including stem rot (Sclerotium oryzae) and sheath spot (Rhizoctonia oryzae-sativae), historically leading to the recommendation of infield residue burning as the best means of disease control (Miller and Webster, 2001; Webster et al., 1981). Residue incorporation at the beginning of the flooded winter fallow was identified as an alternative means of control for stem

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Bijay-Singh et al.

rot, because the pathogen decomposed faster in the flooded soil ecosystem than on the aerobic surface (Cintas and Webster, 2001). Relatively little is known about the effects of mulch on disease in flooded systems because of the impracticality of mulching after puddling the soil. For sheath rot, the sclerotia floating to the surface of the floodwater during the cropping season were most readily able to inoculate and infect the plant (Miller and Webster, 2001), implying that using infected residue as mulch would be much worse than incorporating it. There is still insufficient knowledge about how residue management of rice–upland crop rotations affects rice diseases. Mosquitoes are another important pest affected by residue management in cropping systems with flooded soil. They require standing water to complete their life cycle, and their larvae grow better in flooded fields in which rice residue has been incorporated (Lawler and Dritz, 2006). Therefore, from the perspective of disease and mosquito control, it would be better to remove residue than incorporate or mulch it. When weeds are a significant problem, which usually occurs only when soil is not flooded during crop establishment and early crop growth, mulch would be a good option although it might incur increased disease pressure.

5.2. Profitability Profitability considers the potential economic gains or losses resulting from observed changes in productivity. In situ incorporation of crop residue during normal tillage before establishment of rice results in no extra cost for managing crop residue provided the normal tillage does not involve more time or energy due to the presence of residue. If cost of land preparation is not altered by the incorporation of residue, then any increase in production can result in net profit for the farmer. Because of potential short-term detrimental effects of anaerobic residue decomposition on the young rice crop, such as immobilization of N and release of organic acids, the preferred practice is typically to incorporate residue several weeks before establishing rice. In such case, a change in the timing of tillage or land preparation practices to accommodate the incorporation of crop residue could result in extra expenditure. The profitability of residue incorporation then depends on the extra costs for field operations rather than only the effect of residue incorporation on grain yield. Even though much information is available on the effect of residue on rice yield (Fig. 1), corresponding information on any additional costs associated with residue incorporation is typically not available. Dawe et al. (2003) examined the profitability of incorporating rice and wheat residue using data from two rice–rice experiments (in China and Malaysia) and 12 rice–wheat experiments (in India) conducted for 10–17 years and representing a wide variety of soil types, climatic conditions, and crop management practices. In the two long-term experiments on rice–rice cropping systems, 5–6 t ha–1 of rice residue were incorporated before


153

each crop. In the other 12 experiments, wheat residue (6–15.8 t ha–1) was incorporated before the rice crop. Recommended levels of fertilizer N, P, and K were applied to all crops. The break-even cost (BEC), defined as the maximum cost that farmers can incur in managing crop residue without losing money, was computed using the average farm-level prices of US$ 0.15 kg–1 for paddy rice and US$ 0.12 kg–1 for wheat and differences in crop yields between NPK and NPK plus crop residue treatments for the annual rotation. Residue incorporation would be profitable when the sum of all additional costs of incorporating residues above the normal operating cost of the farm are less than the BEC. For rice–wheat experiments, the BEC ranged between –23 and 8 US$ ha–1 and averaged –3 US$ ha–1 per crop, suggesting it was usually not profitable to incorporate wheat to rice. In the two rice–rice experiments, the BEC was 45 and 40 US$ ha–1 per crop, indicating the incorporation of rice residues before transplanting of rice was profitable for rice fertilized with NPK provided there were little or no additional costs associated with incorporation of the residue. These positive BEC arose because of slightly higher yields (mean ¼ 0.2–0.6 t ha–1) when residue was incorporated. Reports of increased yield associated with rice residue incorporation are, however, relatively rare (Table 4). Already in the 1970s, Tanaka (1974) observed the economics of residue incorporation did not encourage farmers to regularly adopt the practice even though the incorporation of residue reportedly improved soil conditions for flooded rice. The tendency for more frequent increases in rice yield when residue of the previous crop is mulched (Table 6) rather than incorporated (Tables 4 and 5) suggests mulching could be profitable when the cost of managing residue as mulch is relatively low. The profitably of mulching could also be influenced by other factors such as potential savings in irrigation water. Savings in irrigation cost without loss in rice yield were reported in China when rice was mulched with wheat residue and grown on non-flooded soil (Fan et al., 2005; Liu et al., 2003). Reports of significantly increased yield for wheat following mulched, non-flooded rice (Liu et al., 2005) suggest additional scope for increased profitability of the production system.

5.3. Environmental impact 5.3.1. Air quality Smoke is one of the most serious environmental problems associated with large-scale, open-field burning of crop residues. It pollutes air with a mixture of gases and fine particles, which can lodge deep in our lungs when we breathe. The peak in asthma admissions to hospitals in India coincides with the annual burning of rice residue in surrounding fields (Bijay-Singh and Yadvinder-Singh, 2003). Smoke particles are less than a micron in diameter, which allows them to remain in the atmosphere up to

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Bijay-Singh et al.

several weeks (Cooke and Wilson, 1996) and therefore spread hundreds to thousands of kilometers before falling back to earth. Large patches of black C aerosol can be detected by satellites over India and China just after the harvest season during large-scale residue burning. As a result of aerosol–radiation–temperature–CO2 interactions, these patches can lead to reduced biomass and grain yield of field crops. Almost any alternative to open-field burning of residue can substantially reduce harmful environmental and health effects of smoke. Streets (2004) predicted that black C emissions in China could be reduced from 75 Gg in 1995 to 56 Gg by 2020 by enforcing laws against residue burning. 5.3.2. Greenhouse gas emissions for rice Another important environmental impact associated with rice residue management is greenhouse gas emissions, including the three main agricultural contributors to climate change: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Atmospheric concentrations of these gases have been increasing in recent decades due to human activity including agriculture, and they have been shown to contribute to increases in average global temperatures (Houghton et al., 2001). When returned to the field, some of the C in crop residue might be retained in the soil organic matter (SOM) with the rest lost as CO2 or CH4. When residue-C is retained as SOM, there is a net benefit for the greenhouse gas balance because some of the CO2 taken in by the plant is no longer in the atmosphere. When residue-C is lost as CO2, the greenhouse gas balance is neutral because the C came in and out of the plant as CO2. However, when residue-C is lost as CH4, there is a strongly unfavorable consequence for the greenhouse gas balance because each molecule of CH4 has 62 times greater global warming potential than a molecule of CO2 (20-year horizon, Houghton et al., 2001). When soil is anaerobic, the end product of microbial decomposition shifts toward CH4 instead of CO2. Hence, flooded rice systems with retained crop residues are sources of CH4 emission. N2O, which has an even higher global warming potential than CH4 (275 times that of CO2 on a 20-year horizon, Houghton et al., 2001), is formed during aerobic nitrification of ammonium and anaerobic denitrification of nitrate. Under extended periods of soil flooding—as is typical during rice cropping—the predominantly anaerobic soil conditions are not favorable for rapid nitrification– denitrification and emission of N2O (Buresh et al., 2008). The emission of N2O can, however, be important during periods of alternate soil drying and wetting and when soil following a prolonged aerobic period is flooded, such as during land preparation for rice cultivation. The effects of major residue management options on CH4 and N2O emissions from lowland rice are summarized in Table 7. CH4 emission from rice paddies has been measured many times, testing effects of diverse

Return Removal

Crop residue use

Incorporation Surface mulch

Beginning of preseason fallow Just before rice establishment

How residue is returned

Timing of residue return

Water Continuous management flooding (including preseason fallow or not) Mid-season drainage (one or more drainage periods, intermittent flooding) Mostly aerobic

Major options

Management decision

Continuous flooding

Return

Worst

Close to flooding

No effect Several months before flooding

Aerobic

Removal

Best

a

CH4

No effectb

Continuous flooding

?

No clear effect

Best

N2O

Intermittent flooding

Worst

Table 7 Trends in CH4 and N2O emitted under different residue management options in lowland rice

(continued)

Yan et al. (2005), Li et al. (2006)

Ishibashi et al. (2005)

Bronson et al. (1997b), Yan et al. (2005), Li et al. (2006)

Yan et al. (2005)

References

High C:N (rice straw) Low C:N (legume residue) Fresh plant material Partially decomposed plant material (compost, manure) Removal, minimum flooding

Partially decomposed plant material

Best

a

CH4

Fresh residue applied incorporated just before flooding

Fresh plant material

Worst

Continuous flooding

High C:N

Best

N2O

b

For both gases, ‘‘best’’ means the management practice that results in the lowest gas emission. Lack of timing effect on N2O is based on a model rather than actual data (Li et al., 2006). Note: Methane is in bold because it represents the greater global warming threat in systems that are predominantly flooded

a

Overall combination

Type of residue

Major options

(continued)

Management decision

Table 7

Mulch with low C:N residue, intermittent flooding

Low C:N

Worst

Yan et al. (2005), Zou et al. (2005)

References


157

variables including residue management, water management, temperature, and soil physical and biochemical properties. In a review of ~1000 seasonal measurements from sites across dominant agroecological zones in Asia, Yan et al. (2005) found the two most important factors controlling CH4 emission were presence or absence of residue followed by water management. Residue return by any method or timing of application or incorporation caused a statistically significant (P < 0.0001) and often very large increase in CH4 emission compared with residue removal. Residue provides a source of easily decomposable organic C, which means (1) anaerobic bacterial populations increase, using up oxygen (O2) followed by other reducible soil components and driving the redox potential down between the transition where CH4 rather than CO2 is end product of decomposition, and (2) methanogenic bacteria have sufficient substrate-C to form CH4 (Yagi and Minami, 1990). The more decomposed the residue before flooding, the less CH4 emitted. The decomposition of residue before soil flooding for rice production can be accomplished by (1) incorporating crop residue soon after harvesting a crop and allowing it to decompose aerobically before soil flooding for the next rice crop (Wassmann et al., 2000a,b,c; Yan et al., 2005), (2) composting the residue off-field (Corton et al., 2000; Yagi and Minami, 1990), or (3) feeding the residue to cattle and returning it as manure (Setyanto et al., 2000; Wang et al., 2000). Although these options were not all directly compared in one study, each of them resulted in lower CH4 emission as compared with the application of fresh straw just before flooding. Because methanogenic bacteria are anaerobes, CH4 formation is minimal under aerobic conditions, meaning that aerobic water management practices mitigate CH4 emission. In many of the experiments cited by Yan et al. (2005), brief mid-season drainage periods as compared to continuous soil flooding significantly reduced total seasonal CH4 emission. The emissions of CH4 from mulched relative to incorporated residue can be strongly influenced by tillage and water management (Hanaki et al., 2002; Harada et al., 2007; Ishibashi et al., 2001, 2005; Xu et al., 2004). When the surface of reduced or no tillage soil was drier than puddled soil due to more rapid percolation, much less CH4 was emitted from the mulched than incorporated residue. But when soil water content was similar between residue management practices, the CH4 emission was also comparable between practices (Ishibashi et al., 2001). We conclude that water management differences have a larger effect than residue placement on CH4 emission, and there is insufficient information to differentiate between incorporated and mulched residue per se (Table 7). In lowland rice cropping systems with continuous soil flooding, N2O emission is not a significant risk regardless of residue management. However, any time the soil is not flooded—during fallow, mid-season drainage, harvest drainage, or water shortage—N2O formation and emission become likely (Table 7). The water management strategies that mitigate CH4 emission simultaneously exacerbate N2O emission. While many have

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reported significant increases in N2O emission following fertilizer N application in partially flooded systems (Xu et al., 2004), there is not a consistent trend in the effect of residue removal or return. Of the available direct comparisons in field or pot experiments, most are not significant and those that are significant are in both directions with returned residue sometimes showing higher and sometimes lower N2O than with residue removed, occasionally even within the same data set (Bronson et al., 1997b; Lou et al., 2007; Ma et al., 2007; Zheng et al., 2000; Zou et al., 2004a, 2005). In the comparison between residue incorporation and surface mulch, N2O emission is often affected more by the soil water regime than the management of residue per se (Harada et al., 2007). One model predicted, without measured data, no effect of the timing of residue return on N2O emission (Li et al., 2006). After water and fertilizer N management, the most important factor determining N2O emission was the type of residue. Incorporation of a high C:N ratio residue like wheat straw decreased N2O emission, presumably through N immobilization, while a low C:N residue like rapeseed cake increased it (Zou et al., 2005). From the perspective of minimizing greenhouse gas emissions from lowland rice, incorporation of fresh residue would be the worst option for CH4 emission, especially where soil is flooded continuously for the month following incorporation (Table 7). However, regardless of residue management, it is important to keep the soil flooded as much as possible to minimize N2O emission. Therefore, a compromise could be to keep the soil flooded most of the time to minimize N2O emission and to remove residue to minimize CH4 emission. Prolonged aerobic decomposition of residue before rice cropping might not be feasible in intensively cropped rice-based cropping systems, and prolonged fallows with aerobic soil can favor formation of nitrate that is subsequently denitrified with formation of N2O upon soil flooding for rice cultivation (Buresh et al., 2008).

5.4. Sustainability 5.4.1. Yield trends with flooded residue management In 14 long-term experiments (2 on rice–rice and 12 on rice–wheat cropping system) explained in the Section 5.2, Dawe et al. (2003) observed that the value of the F-statistic for testing the null hypothesis of identical yield trends in NPK and NPK plus crop residue treatments was never significant at the 5% level, indicating no statistically distinguishable differences in yield trends between the two treatments. The yield trend in the residue treatment was more positive or less negative than the trend in the NPK treatment for 11 of the 16 cases of rice cropping, which included both rice crops in the two rice–rice systems. These differences in yield trends were not statistically significant at the 5% level. Across all sites, there was no consistent yield


159

increase from application of straw. For the rice crops in the rice–rice cropping systems, the average rice yield increase during the 11–12-year duration of the experiments due to crop residue application was 0.43 t ha–1 per crop. For the 12 rice crops in the rice–wheat cropping systems, rice yields during the 11–17-year duration of the experiments were reduced in the crop residue treatments by an average 0.40 t ha–1 per crop. 5.4.2. Soil changes with flooded residue management The long-term incorporation of crop residues in flooded rice soil can increase SOM, total soil N, fractions of soil C, and soil biological activity; but it can decrease the availability of zinc (Yadvinder-Singh et al., 2005). Continuous incorporation of crop residues after each crop can eventually increase the N-supplying capacity of rice soils (Eagle et al., 2000; Verma and Bhagat, 1992). Long-term studies indicate the supply of plant-available soil N can increase after 5–10 years of continuous incorporation of crop residues in tropical (Cassman et al., 1996) and temperate area (Bird et al., 2001). The benefits of incorporated residues on SOM and soil N supply, however, seldom translate into increased yield (Section 5.1) or profit (Section 5.2) for flooded rice. The effect of crop residues on properties of flooded soil has already been extensively reviewed by Yadvinder-Singh et al. (2005), and it is consequently not covered in this chapter. A noteworthy feature of flooded rice soils with continuous and intensive rice cropping is the maintenance and even buildup of SOM (Bronson et al., 1997a; Cassman et al., 1995; Witt et al., 2000). Appreciable inputs of N from biological N2 fixation (BNF) in continuous flooded rice production systems contribute to the maintenance of soil N even in the absence of N fertilization (Ladha et al., 2000). Prolonged soil submergence and anaerobic soil conditions can lead to the buildup of phenolic compounds that can immobilize N abiotically, thereby reducing net N mineralization and supply of plant-available soil N (Olk et al., 1996, 2000). However, long-term experiments with continuous cropping of flooded rice in the Philippines reveal no decline in rice yield during the past 10–20 years in zero–N plots receiving ample supplies of other nutrients (Padilla, 2001). Yield of flooded rice without fertilizer N, which presumably reflects the supply of plant-available soil N, was maintained even when all above-ground crop residues were removed for each crop. The results suggest that the inputs of N during continuous soil flooding via BNF and biological N mineralization matched or exceeded any decline in N-supplying capacity arising from abiotic immobilization of N associated with buildup of phenolic compounds. Long-term experiments in the tropics indicate the incorporation of crop residue is not essential for maintenance of SOM and soil N-supplying capacity in continuous rice cultivation on puddled and flooded soil. Pampolino et al. (2008b) examined trends in total soil C and N during 15 years of

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Bijay-Singh et al.

continuous cultivation with two or three flooded rice crops per year in three long-term experiments with incorporation of crop residues and one long-term experiment with removal of all above-ground biomass after each crop. Soil C was maintained during the 15 years in each experiment. In the experiment with removal of all above-ground crop residues and four rates of fertilizer N, soil C increased by 1.5–2.3 g kg–1 and soil N increased by 0.09– 0.15 g kg–1 during the 15 years. Soil N-supplying capacity as determined by anaerobic N mineralization was statistically similar at the start and end of the 15-year period regardless of fertilizer N management. The input of N via BNF as estimated from N balances in a treatment without fertilizer N averaged 81 kg N ha–1 year–1 during the 15 years. Whereas SOM can be maintained in flooded rice–rice systems regardless of residue management, SOM significantly decreased when one flooded rice crop was replaced by conventional till maize with retention of rice and maize residues (Witt et al., 2000; Pampolino et al., 2008a). On the basis of above research findings, we conclude residues from the crop preceding rice on puddled and flooded soil can be considered for removal for off-field uses, without loss in productivity or sustainability of the flooded rice provided fertilizer is appropriate increased to compensate for nutrient removal in the residue. The management of K is particularly important when crop residues are removed because crop residues can markedly increase K availability in soil and decrease the crop response to K application (Chatterjee and Mondal, 1996; Ning and Hu, 1990; Patil et al., 1993; Sarkar et al., 1989).

6. Evaluation of Options with Residues Managed During a Non-Flooded Crop In this section, we evaluate residue management options for the category of cropping system described in Section 4.2(non-flooded crop following rice) in which residues are managed during the period of a crop grown on aerobic soil without flooding. Our evaluation is based on criteria for productivity, profitability, environmental impact, and sustainability as described in Section 2.

6.1. Productivity The two main options available for in-field management of crop residue during the non-flooded (upland) crop in rice-based cropping systems are incorporation into the soil and leaving the residue on the soil surface as mulch. Incorporation typically involves conventional tillage of soil, whereas mulching usually involves reduced or no tillage. While these options are comparable to those available for flooded rice (Section 5), their feasibility and


161

implementation markedly differ between a non-flooded crop and flooded rice because of differences in land preparation and tillage options between crops. Mulching of crop residue is much more feasible during a non-flooded crop than flooded rice because of greater opportunities for reduced and no tillage. Most of the information on managing rice residue for non-flooded crops in rice-based cropping systems comes from rice–wheat systems. Although mulching rice residue in conventionally tilled wheat has been attempted by some researchers, mulching in no-till wheat is favored and now facilitated by recent developments of appropriate machinery. 6.1.1. Grain yield for non-flooded crop The incorporation of rice residue before wheat or rapeseed significantly increased yield of the nonrice crop in only 1 of 16 data sets examined from China and India in which 3–7.9 t ha–1 of rice residue was incorporated into soil 10–40 days before sowing of wheat or rapeseed (Table 8). A combined analysis of all data sets revealed no trend of greater yield when rice residue was incorporated rather than removed (Fig. 2). The slope and intercept for the linear regression comparing yield with and without residue incorporation were not significantly different from 1 and 0, respectively, at P < 0.001. Combined data for on-farm experiments with rice–wheat cropping in northwestern India similarly reveal no trend of greater yield of wheat when rice residue was incorporated rather than burnt or removed (Fig. 2). Some of the studies reported for rice–wheat systems in Table 8 investigated the residual effect of rice residue incorporated to wheat on the yield of the next rice crop after wheat, but on all seven cases the incorporated residue had not significant residual effect on yield of the following rice. Table 9 summarizes results for rice–wheat systems in which residue from each crop was incorporated before the following crop, resulting in large amounts of incorporated residue. Wheat yield was significantly increased by incorporated residue in only 1 out of 13 cases. When all data for rice and wheat were combined, no effect of residue on grain yield was detected (Fig. 3). The incorporation of crop residue can have adverse effects on the following crop (Cannell and Lynch, 1984), although in some studies the negative effects of residue incorporation in a rice–wheat cropping system diminished after a few initial years (Dhiman et al., 2000). But in other studies the negative effects were not reversed even after 11 years (Beri et al., 1995). The negative effects on wheat yield can result from immobilization of N by the decomposing residue. This is supported by the observation of Beri et al. (1995) of greater decline in wheat yield at a low rate of N application (0.5 t ha–1 decline at 60 kg N ha–1) than at a high rate of N application (0.08 t ha–1 decline at 180 kg N ha–1). The magnitude of N immobilization depends on the extent of straw decomposition before N fertilization (Bhogal et al., 1997). The immobilization of N is temporary, and it can be followed later in the cropping season by release of N through mineralization. In such case the

162

30

Haryana, India: Sandy loam soil, 25% N applied at the time of residue incorporation

–

– –

3.7 –

3.8 2.91

–

Punjab, Pakistan

4.01 6.83

3.6 2.2

3.7 2.2

5

–

2.6

2.6

5

– –

30

2.76 2.37

Rice residue burned

–


Grain yield (t ha1)

28


Himachal Pradesh, India: Acidic clay loam soil, chopped rice straw incorporated Himachal Pradesh, India: Silty clay loam soil with pH 5.9, chopped rice straw incorporated up to 20 cm soil depth

Days residue incorporated before planting

3.72 7.11

3.9 3.51*

3.8 2.2

2.1

2.79 2.47


Wheat Rice (R)

Wheat (first 3 crops) Rice (R) Wheat (next 2 crops) Rice (R) Wheat

Wheat Rice (R)a

Crop

Salim (1995) Dhiman et al. (2000)

Verma and Bhagat (1992)

Sharma et al. (1985, 1987)

Reference

Effect of rice residue incorporation on grain yield of upland crop and residual effects on the following rice in rice–upland crop systems


Table 8 in Asia

163

5.0–7.0

7.1–7.9

10 21

7.1–7.9

20

1.7

3

4.94 6.19 4.94 6.19 4.94 6.19 4.3 4.5 4.5 4.6 4.4

4.7

7.5

7.1–7.9

5.06 5.06

6.4

40

20 40

– – – – – – – – – – –

5.1

5.17 6.34 5.22 6.29 4.95 6.33 4.5 5.1 5.0 4.3 3.7

1.9

5.1

5.00 4.89

a (R) denotes that the crop was grown to study the residual effect of crop residues applied to the previous crop listed above. * Significantly more than grain yield with residue removed or residue burned treatments at P < 0.05.

Punjab, India: Sandy loam—silt loam, on-farm experiments

Punjab, India: Typic Ustipsamment, loamy sand with pH 7.3 Shanghai, China: Loamy clay soil Anhui, Guangde: China: Loamy clay soil Punjab, India: Sandy loam soil with pH7.2, 7-year study Wheat Rice (R) Wheat Rice (R) Wheat Rice (R) Wheat Wheat Wheat Wheat Wheat

Rapeseed

Wheat

Wheat

Sidhu et al. (2007)

YadvinderSingh et al. (2004b)

BijaySingh et al. (2001) Yang et al. (2003) Li et al. (2003)

164

Bijay-Singh et al.

Upland crop yield (t ha−1) with rice residue incorporated

6 Upland crop yield 1:1 line

5

4

3

2

1

0 0

1

2

3

4

5

6

Upland crop yield (t ha−1) with rice residue removed or burned

Figure 2 Relationship between yield of a non-flooded crop (wheat in most cases, and oilseed rape) with and without rice residue incorporation. Data are from the literature listed in Table 8 combined with unpublished data from on-farm experiments with recommended NPK levels in northwestern India (P. R. Gajri, Department of Soils, Punjab Agricultural University). The slope and intercept for the linear regression were not different than 1 and 0, respectively, at P