Research Projects

Project at a Glance

Project Status: complete

Title: Alternative uses of rice-straw in California.

Principal Investigator / Author(s): Bainbridge, David A.

Contractor: California Polytechnic University

Contract Number: 94-330

Research Program Area: Ecosystem & Multimedia Effects, Atmospheric Processes

Topic Areas: Agriculture, Ecosystem Impacts


Interconnectedness and complexity are the hallmarks of almost every environmental problem and opportunity including tie challenge of rice straw management in California. Although attempts are often made to solve environmental problems by working on single aspects. this rarely works, just as treating symptoms may do little to resolve diseases. The rice straw problem includes the physical systems of the atmosphere, air basins, soils, and local and regional watersheds and reaches the global scale with concern over atmospheric contribution of methane and implications for global warming. It includes the biological systems of the rice crop, soil organisms, crop pests and wildlife (both beneficial and harmful). And finally, it includes the economic and social systems of the rice growers, farm families, farm service industries, rural communities, the regional population, rice consumers around the world, fishermen and women, hunters, manufacturers of harvesting equipment, medical services and potentially, builders and home buyers in the region. Complex linkages between the physical, biological and social systems are welI illustrated in the rice straw problem, and like most problems these complex interactions make finding solutions challenging. They also can make it possible to find win-win solutions where multiple parties benefit from improvements in management strategies.

An outline of the rice straw burning problem, rice production has become an important economic activity in California and in a typical year 400,000 acres of rice are planted (almost 500,000 in 1994). From two to three tons of straw are left per acre after the grain is harvested. The traditional grower practice has been to burn the residue. This is inexpensive, as little as $3 acre, and provides added benefits of reducing weed problems and more critically minimizing rice stem rot and other rice diseases. In recent years 75-135,000 tons of rice straw out of a total of 1-1.5 million tons produced on 300-400,000 acres have been burned in the fields in the fall. Over the entire year total amount burned can be two to three times as much. The rice straw burning in the fall is spread over a period of weeks or months and is regulated to discourage burning when meteorological conditions are likely to lead to smoke accumulation. Yet even under this careful control the smoke can cause health and safety problems, including asthma, allergies, bronchitis, and respiratory distress. Smoke can also contribute to highway accidents. No detailed epidemiological workup has been done on the health costs associated with rice straw burning.

Health risks are minor due to burning management, but may be significant locally near burns. These risks include exposure to the various gases (notably carbon monoxide and nitrous oxides) and particles created by recombination of gases, ash and dust raised from the soil surface. These include known carcinogens and mutagens, gases that are hazardous or lethal at high concentrations, and a range of potentially harmful particles and byproducts of burning. These pollutants and other more dangerous materials are also generated by using firewood to heat homes, automobiles and wild land and forest fires. Rice straw burning is a contributor, but other sources are more important on a regional basis. Concern has grown recently over the danger posed by small panicles (less than 10 microns, PM < 10, and more critically particles < 2.5 microns. Health studies in other areas have showed a clear relationship between increased particulate levels and increased mortality. Silica content of rice straw is high and the silica rich ash particles may pose a slightly higher risk than other types of ash, but the organic condensates on these particles may be of more concern and would be similar for other types of straw burning. More detailed studies are needed to accurately assess heath risks.

Rice straw burning and soil incorporation have global environmental risk implications. The carbon content of rice straw is about 40 percent, and the burning of 500,000 tons of rice straw may return 200,000 tons of carbon into the atmosphere. This carbon is fixed during the growing season by photosynthesis and there is little net gain. If the straw is incorporated in the soil it increases methane emissions. which are more damaging than the byproducts of burning. Methane is a special concern for global warming, because each methane molecule has 20-25 times the heat capturing potential of a carbon dioxide molecule. Even allowing for the lower level of emissions, the net impact on global warming would be 10-15 times worse than the effects of carbon dioxide from field burning, The use of rice straw for other purposes that would store or quester carbon would decrease emissions and reduce global warming risks.

The obvious problems associated with rice smoke led to restrictions on burning beginning in 1971 under provisions of the Health and Safety Code, these were revised and made more flexible in the early 1980s. Sacramento Executive Airport records show smoky conditions 24 percent of the time in October-November in many years, although with restrictions in burning this dropped to less than 4 percent of the time in October-November 1990. Current restrictions base allowable burn acres on atmospheric conditions. Predictions of weather conditions and measurements of particulate levels are used by the Air Resources Board to set allowable burn times and amounts. The specific allowable burns are made by the air pollution control districts or their agents. In 1991, an act was passed to phase down rice straw burning by 2000, with the exception of burning essential for disease control.


For questions regarding research reports, contact: Heather Choi at (916) 322-3893

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