Controlling water in rice fields cuts methane emissions

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Monday, January 13, 2014


FARMERS, particularly those growing rice, can help reduce methane emissions into the atmosphere by adopting controlled irrigation or alternate wetting and drying (AWD) technology.

Developed by the Laguna-based International Rice Research Institute (Irri), AWD is a technology that allowed rice fields to dry for a certain period before applying irrigation water.

Also called controlled irrigation or intermittent irrigation, AWD technology can actually save farmers almost one-third of irrigation water without sacrificing yields. It also saves farm inputs like oil, fuel, and labor being utilized on the operation of water pumps.

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Rice fields using this technology are alternately flooded and dried. The number of days of non-flooded soil can vary from one day to more than 10 days, according to IRRI. It uses an "observation well" that is made of bamboo, plastic pipes, or any hollow indigenous material. Perforations are made in the lower half of the tube.

The AWD technology can be started a few days after transplanting (or with a 10-centimeter tall crop in direct seeding). When many weeds are present, AWD can be postponed for two to three weeks until weeds have been suppressed by the ponded water. Local fertilizer recommendations as for flooded rice can be used. Nitrogen fertilizer maybe applied preferably on the dry soil just before irrigation.

"A practical way to implement AWD technology is by monitoring the depth of the water table in the field using a simple perforated field water tube," Irri explains. "When the water level is 15 centimeters below the surface of the soil, it is time to flood the soil to a depth of around five centimeters at the time of flowering, from one week before to one week after the maximum flowering."

The water in the rice field is kept at five centimeters depth to avoid any water stress that would result in severe loss in rice grain yield. The threshold of water level at 15 centimeters is called "safe AWD," as this will not cause any yield decline because the roots of the rice plants are still be able to take up water from the saturated soil and move it to root zone.

"The field water tube used in this technology will help to measure the water level in the field so that incipient water stress in the rice plants can be anticipated," the Irri points out.

As such, the AWD technology does not only save water but can greatly reduce emissions of methane.

Studies conducted at the Irri have shown that AWD technology reduces methane emissions by about 30 percent or even up to 70 percent, depending on water usage and management of rice stubble.

"Rice production also contributes to global warming as it emits methane," said Dr. Constancio Asis Jr., supervising science research specialist at the Philippine Rice Research Institute (PhilRice) in Muñoz, Nueva Ecija.

After carbon dioxide, methane is the second most important greenhouse gas that is responsible for trapping the heat from the sun at it radiates back into space. Methane is created naturally as a waste product of anaerobic bacteria (living with little or no oxygen). These bacteria produce methane gas in waterlogged soil and wetland, but also in human-produced environment such as rice paddies.

Scientists explain that long-term flooding of the fields cuts the soil off from atmospheric oxygen and causes anaerobic fermentation of organic matter in the soil. During the wet season, rice cannot hold the carbon in anaerobic conditions. The microbes in the soil convert the carbon into methane which is then released through the respiration of the rice plant or through diffusion of water.

It wasn't until in 2001, when the United Nation's Intergovernmental Panel on Climate Change (IPCC) submitted its report that methane was given much attention.

"One of the most potent greenhouse gases on Earth," the report said.

"Methane absorbs heat 21 times more than carbon dioxide and it has 9-15 year life time in the atmosphere over a 100-year period," said Dr. Asis Jr., a recipient of the 2011 Norman E. Borlaug International Agricultural Science and Technology Fellowship Award.

"The Journal Science" reported that atmospheric concentration of methane has more than doubled during the last 300 years and is increasing at an annual rate of about one percent each year.

Although atmospheric methane levels declined during 1983-99, they remained relatively constant during 1999-2006, according to a new report released by the Washington, D.C.-based Worldwatch Institute. "And they have been increasing since 2007," it said.

A new study, which appeared in the journal "Geophysical Research Letters," said that methane's effect on warming the world's climate may be double what is currently thought. The new interpretations reveal methane emissions may account for a whopping third of the climate warming "from well-mixed greenhouse gases" between the 1750s and today.

Rice fields are one of the major contributors of methane in the atmosphere.

"An estimated 19 percent of world's methane production comes from rice paddies," admitted Dr. Alan Teramura, a botany professor at the University of Maryland. "As populations increase in rice-growing areas, more rice, and more methane, are produced."

According to Dr. Asis, rice crops emit methane starting from the transplanting to harvesting stage, peaking at two weeks after transplanting up to the tillering stage.

The AWD technology is one of the farming systems that can help minimize methane emissions. But there's more: "AWD technology can reduce the number of irrigations significantly compared to farmer's practice, thereby lowering irrigation water consumption by 25 per cent, reducing diesel fuel consumption for pumping water by 30 liters per hectare, and producing 500 kilograms more rice grain yield per hectare," the Irri said in a statement.

However, the AWD technology has its shares of disadvantages. For one, rice productivity is reduced using AWD technology if moisture stress condition is induced. But studies have shown that the reduction of yield was less compared to the yield reduction due to the direct moisture stress effect.

Another disadvantage: emissions of nitrous oxide, also a greenhouse gas, are increased.

Published in the Sun.Star Davao newspaper on January 13, 2014.

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