Crop residue burning
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Agricultural crop residue (ACR), or stubble, as it is informally known, is defined as “any vegetative material remaining in the field after harvest, or vegetative material produced on agricultural lands. It includes whole fields, pastures, spots within a field or pasture, broken bales in the field that they were generated during the time of harvest”(DEQ). Fire is therefore used to facilitate land clearing by setting a blaze all the crop residues in either combustion towers or in open fields. Agricultural crop residue burning (ACRB) is a common practice as it is the most cost effective way, in the short term, of disposing of residues from farms especially in developing countries. This is mostly done in rural China, where large amounts of agricultural residues are produced and are disposed of by burning in open fields. It is also the most important source of biomass burning as well as “domestic fuel for at least 21 provinces in China that are produced mainly from 25.5%, 20.3%, and 24.7% for rice, wheat, and corn straws, respectively” (8437). According to Guoliang et al. before 1979, the crop residue energy had covered about 70% of the rural energy consumption in China. Today, crop residue energy is still an important part of the rural energy for cooking or heating in China even though the energy conversion efficiency is only 10% to 20% (50).
 Effects of ACRB
China has faced and is still facing severe air pollution problem, with particulate matter as the principal pollutant in most cases. Crop residue burning significantly reduces the amount of trash that needs to be dealt with, but pollutes the surrounding neighborhood with smoke and ash. In addition, residue combustion is a source of particulate and gaseous emissions such as “Carbon dioxide, Nitrogen oxide, Nitrogen dioxide and Nitrous oxide(CO2, NO, NO2 and N2O) to the atmosphere that may contribute to the 'greenhouse effect', and the associated global warming” (Hemwong et al.85). For example, crop residue burning process can go on for months depending on combustibility, and in cases of unfavorable weather, “large amounts of smoke are produced forming what looks like large dark clouds over fields and residential areas” (Awasthi et al. 4440). Also, it is a process that has been considered as an important source of atmospheric trace species with significant impacts on global atmospheric chemistry and global climate change which possesses a greater threat to human health according to Zhang et al. (8432). “Active trace gases emitted from crop residue burning such as CO and nitrogen oxide (NO) are precursors of tropospheric ozone (O3) and can reduce the tropospheric concentration of the extremely reactive hydroxyl radical (OH)” (Yang et al.1962).
 Effect on health
The resulting smoke from crop residue burning therefore becomes a health hazard as the smoke produced causes multiple and lasting effects particularly on children's lung function (Awasthi et al. 4440). For instance , it brings about respiratory infections due to “chemical agents and particulates that are smaller than 10 microns (PM10), that are able to penetrate deep into the lungs causing significant increase in levels of heart and respiratory problems” ( 4443). That is, these small particles (PM2.5 and PM10) which make up the majority of the smoke produced by the burning process were according to EHP (Environmental Health Perspectives), more closely associated with decreases in lung function than suspended particulate matter (SPM), which can contain particles 100 μm or larger (475).
 Effect on the environment
Streets et. al have estimated that the emissions of Black Carbon (BC), roughly about one-fourth of global anthropogenic, originates from China. This is attributed to “China’s high rates of usage of coal and biofuels” (4281). Because of the extensive use of coal and biofuels in the country, much of which is poorly controlled by Western standards, Streets et al. suggest that these emissions of primary particles may exert considerable influence on the East Asian atmosphere (4282). It is also important to note that the amount produced vary significantly from year to year as crop residues are burned in the fields as a means of disposing off the waste, rather than as an energy source. For instance, in order to estimate future emissions, several factors were examined. According to Streets et al., the World Bank has projected that 636 million tons of grains (rice, wheat, and ‘‘coarse’’ grains) will be harvested in China in 2020. This is expected to produce a total of 737 million tons of grain residue that will require clearing (4291). “This has resulted in some serious environmental consequences, including temporary closings of airports and highways due to heavy smog”(Yang et al. 1962). In addition to polluting the air and affecting the O zone layer, crop residue burning also “adversely affect soil fertility due to the fact that it causes losses of some nutrients and organic matter over time” (85).
 Causes of ACRB
Crop residue burning has also been attributed to the rapid economic development in China which has led to increased rural access to commercial energy, and less use of bio-fuel causing farmers dispose off crop residues by use of fire. With the economy growing, crop residue is no longer a necessary fuel and collecting and storing them would not only be very troublesome, but also finding use for them (Yang et al.1967). Moreover, the busyness of peasants in China especially during the summer harvest, (June) is an important reason why crop residue burning in the field is serious. For instance, in Suqian in Jiangsu province, “many peasants are so tired in this month that they are unwilling to spend the valued time on callback crop residue” (Yang et al. 1968). To add on to, according to Lu & Zhang, the Chinese government has been conducting long-term and wide-ranging research and development on crop residue energy conversion technologies since 1960 (4026). “By 2005, the main uses of crop residues in China were for energy, forage, manure and industrial raw materials, accounting for 24.0%, 21.5%, 17.2%, and 2.9%, respectively. The remaining 34.5% was accounted for by field burning and by a collection-loss component” (4026). Today as it was then, open field burning of crop residues has resulted to serious environmental impacts as well as freeway traffic accidents due to heavy smokes.
Another cause of crop residue burning in open fields is the use of combine harvester which harvests faster and effectively and therefore requires faster and effective field clearing . As Yang et al. have noted, before 1990 s, in Suqian, most of wheat and rice were harvested by man, transported to settled place, then was threshed. “At that time, crop residue was a byproduct which did not need any extra labor and cost” (1967).
 Alternatives to burning of ACR
Also, crop residues are an important category of biomass. According to Lu & Zhang, numerous “competing uses have been developed using conversion technologies to transform crop residues into energy, forage and industrial raw materials”(4026). However, results indicate that these various technologies have different environmental impacts and economic benefits, suggesting that with only limited investment at a regional or national scale a combination of crop residue burning technologies could be utilized to maximize benefits. Not all crop residues however, are available for these uses because some of them need to be returned to the soil to reduce erosion and maintain soil fertility. Also, studies have reported that there are beneficial effects of crop residue burning on soil nutrient availability. For instance, “lash-and-burn of a tropical forest resulted in transformations of non-plant-available; Phosphorous and Nitrogen in the soil into mineral forms readily available to plants” (Hemwong et al. 85). However, the “economic and environmental consequences of the competing uses of crop residues must be assessed objectively with a holistic approach and a long-term perspective”(Lu & Zhang 4026).
The effect of retaining crop residues in farming systems is generally thought to be advantageous over burning and physical removal from the point of view of nutrient cycling. “Significant increases in crop yield after retaining crop residues due to the combined effects of improved soil organic matter, soil structure, increased water use efficiency and reduced numbers of some plant pathogens”(Hemwong et al. 85). According to Yang et al., peasants are encouraged by the Chinese government to return crop residue to agricultural soil as organic fertilizer. They are encouraged to plow under more of these wastes to increase the organic content and fertility of the soil. Streets et al. have made an assumption that these efforts by the government will have “a significant impact on farmers’ behavior, reducing the percentage of residue burned in the fields in 2020 to one-half the rate in 1995; from 23% to 11.5% (4284). Also, these assumptions have estimated that BC emissions from field combustion of 75Gg in 1995, would fall to 56Gg in 2020.(Streets et al. 4290). However, this operation “increases labor, cost and has some side effects on crop growth; thus, is not very popular with most peasants, and a large proportion of crop residue is still burned in the field” (Yang et al. 1962). Instead, Hemwong et. al, have suggested that farmers could grow green manure legumes during fallow periods. Grain legumes such as groundnut and soybean are could be good alternatives because they can provide cash income as well as improve soil fertility. “Retention of legume crop residues has been shown to increase soil organic matter and nutrient content in several cropping systems” (Hemwong et al. 85). Also, the Chinese government is supporting the commercialization and expansion of biomass energy conversion technologies to reduce open field burning( Lu & Zhang 4026). By complete utilization of crop residues and their return to the field, it is possible to avoid open-field burning of residue biomass, with its associated environmental impact (Lu & Zhang 4026)
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