Saturday, June 16, 2012

Paddy Fields in a Globalized World 2: Methane

Methane. A powerful greenhouse gas; human activities pump it out like crazy. By now educated readers are aware of the problem of methane from production of cattle and other ruminants, which worldwide accounts for 28% of methane production (EPA). Last year, I wrote a post on paddy fields in a globalized world and learned that rice paddy fields, 90% of which are in Asia, account for roughly 20% of global methane production according to most sources, though there is much variability in the estimates. They are also known to be major producers of another greenhouse gas, nitrous oxide.

The connection between human activities and a warming world, robustly-established in the climate sciences, is important because by 2050 the world will be feeding 9 billion people, meaning that rice production and its associated methane production will likely have to increase.

In Taiwan rice is generally planted twice a year. The vast majority of methane emissions take place during the second planting (here, here). Because in every country cropping practices are different, local factors affect methane output from rice farming. This paper on the methane emissions in all 15 of Taiwan's irrigation districts observes:
The amount of methane emitted in the second crop season was 7– 16 times that emitted during the first crop season. If the reduction in planted acreage proposed (12%) is during the second crop season, this will reduce the amount of methane emitted by 21% annually.

Additionally, the rice straw following the first crop harvest is ground into the soil before the rice is planted in the second crop season. The addition of ground rice straw to the soil significantly increases the rate of emission of methane in the hot and humid summer. Removing the rice straw from the soil greatly reduces the rate of emission of methane.
The authors recommend that rice straw be removed from fields prior to the second planting to reduce methane emissions. The paper dates from 2003, when Taiwan had just entered the WTO and the government had taken thousands of hectares of rice fields out of production to comply with agreements to import rice. This ironically reduced methane emissions. The paper observes that in Taiwan temperature is the major determinant of rice field methane output; it is highest in Pingtung fields and lowest in Nantou.

Another factor in rice field methane output is water and water depth. In China methane emissions fell 40% when water-saving practices were implemented that drained fields of their water three times annually instead of keeping them continually flooded. In Taiwan the effects are similar though not so dramatic. This is because when the fields are flooded, soil bacteria shift to anaerobic fermentation of organic material in the field, which creates methane. Upland rice fields, which are not flooded, produce negligible amounts of methane. Recall, however, that as my post on paddy fields noted, flooding rice fields recharges underground water sources.

Another methane reducer in Asia was the shift to artificial fertilizer and away from animal and organic waste. Differences in fertilizer type also affect methane production. Nitrogen-rich fertilizers result in much higher methane emissions, in one 2003 study in Taiwan large reductions were found if non-nitrogen fertilizers were used (another study of methane and nitrous oxides emissions from Taiwan).

Because every region has its own special methods and practices for producing rice, there is no global fix for the problem of methane production from rice fields. But it is surely something humanity will have to consider in the coming decades.

REF: Want to reduce weeds, snails and methane in rice paddies? Try ducks!
Daily Links:
  • Travelscope Introduces Taiwan's Tea and Aboriginal Cultures. 
  • American surfers hit Taiwan
  • AsiaEye under the radar news
  • Ag losses from rain to top NT$500 million; expect higher food prices in local markets.
  • Five day boycott by DPP blocks vote on ractobeef. The KMT has been calling for an executive order. Note that the Ma Administration could have ended this impasse by executive order. That Ma has not suggests that he wants to use the legislature as a fig leaf for permitting ractobeef into Taiwan ("Look, the legislature decided to let beef in, my hands are tied") or as an excuse for not letting it into Taiwan. Either way, the Administration shifts the blame for the decision onto someone else. The really terrifying thing is that Taiwan's voters will return this legislature in the next election even if it votes to let in beef laced with cyanide.
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Anonymous said...

鴨禾米 is a Taiwanese rice that uses the duck method. 公視 did a story on its growers not long ago. Google it for more info.

StefanMuc said...

Even better than avoidance would be if we could trap the Methane and burn it to generate electricity.