Reducing methane emissions from rice production

Text and photos by Henrylito D. Tacio
Among the greenhouse gases that cause global warming, most reports emphasize more on carbon dioxide.  But methane is equally important.  In fact, about 12 percent of global warming is attributed to increases of methane in the atmosphere.
The journal Science reported that atmospheric concentration of methane has more than doubled during the last 300 years and is increasing at annual rate of about one percent each year.
“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,” says Dr. Constancio Asis Jr., a recipient of the 2011 Norman E. Borlaug International Agricultural Science and Technology Fellowship Award.
Methane gas is created naturally as a waste product of anaerobic bacteria (living with little or no oxygen).  These bacteria produce methane gas in waterlogged soils and wetlands, but also in human-produced environments such as rice paddies and landfills.
The Philippines, being an agricultural country, as contributed much of the methane in the atmosphere.  Most of the methane gasses are released from farming.
H. Steven Dashefsky, in his book Environmental Literacy: Everything You Need to Know about Saving Our Planet, ranked rice fields as the second among the top ten sources of methane as it contributes 19.4%.
With 20.2%, wetlands topped the list. These eight other sources of methane are ruminants or cud-chewing animals like cattle (14%), biomass fires, such as burning forests (9.7%), oil and natural gas pipeline leaks (7.9%), termites (7%), coal mining (6.2%),  landfill (6.2%), animal wastes (5%) and sewage (4.4%).
The Philippines is one of the world’s top producers of rice.  “Rice is a plant that grows best in wet soil, with its roots flooded,” explains L. Hartwell Allen, an American soil scientist at the Crops Genetics and Environmental Research Unit in Gainesville, Florida.  “But flooded rice crops emit substantial amounts of methane to the atmosphere.”
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.
There are three products that come from growing rice: grain, husk and straw. Among these three products, the straw and husks are considered wastes.  “In 2008, about 620 million tons of rice straw and about 125 million tons of husks were produced in Asia alone, and this quantity is increasing every year,” reports the Laguna-based International Rice Research Institute (IRRI).
In most places, these wastes have no commercial value and are disposed of in various ways.  “Around 60% of Asia’s rice straw is just burnt in the field,” says Dr. Craig Jamieson, who works with the IRRI and the World Agroforestry Center.
In the Philippines, a total of 10,680 gigatons (Gg) of rice straw are produced per year.  “Much of this is burnt in open fields or incorporated in the soil in wet condition during ploughing,” says a policy brief paper published by the Economy and Environment Program for Southeast Asia (EEPSEA).
In addition, burning rice straw is unhealthy — not only to the environment but to people as well.  “Rice straw burning is also known to emit particulate matters and other chemicals such as dioxins and furans that have negative impact on human health,” the paper said.
A recent study headed by Cheryll C. Launio of the Philippine Rice Research Institute (PhilRice) found out that early incorporation of both stubble and straw into the soil is “the most cost-effective way of disposing rice straw.”  More importantly, it can help reduce GHG emissions.
Dr. Asis was also involved in the study along with Rowena G. Manalili, and Evelyn F. Javier.  The result of the study was published by the EEPSEA under the report, “Economic Analysis of Rice Straw Management Alternatives and Understanding Farmers’ Choices.”
The study was done in the rice growing areas of Central Luzon, Western Visayas, Cagayan Valley, and Ilocos regions, where around 30% of farmers burnt their rice straw.  The rest of farmers adopted any of the following practices: scattered it in their fields, incorporated it into the soil during land preparation, or just left it in their threshing areas for incorporation in the next cropping season.
Based from the five-year study (from 2006 to 2010), it was found that incorporating stubble less than 30 days before crop establishment is responsible for the largest contribution of GHG emissions.  But incorporation rice stubble more than 30 days before crop establishment and incorporating composted rice straw into the field “yielded the lowest cumulative levels of methane and nitrous oxide.”
However, “simply shifting from the baseline approach of late stubble incorporation and straw burning to early incorporation of both stubble and straw also gave good results and led to reduction in GHG emissions of around 80%.  This was mainly due to reductions in methane emissions.”
The researchers’ conclusion: “Shifting from rice straw burning to rice straw incorporation will not necessarily reduce global warming potential if straw is incorporated less than 30 days before cultivation, especially in flooded conditions.”
In terms of monetary benefits, early incorporation of both stubble and rice straw into the soil more than 30 days before cultivation is more profitable.  “This option gave a net benefit of P21 or around $0.50 per ton of carbon dioxide equivalent reduction,” the paper said.
The study also found that incorporating rice stubble more than one month before cultivation, and rapidly composting rice straw and applying it back to the field, was also a cost-effective option.
“Because this option significantly mitigates greenhouse gas emissions while improving the soil condition, it is cost-effective despite the additional labor cost of piling and composting the rice straw,” the researchers said.