In recent years, there has been a lot of talk about reducing greenhouse gas (GHG) emissions. In the agricultural sector, cow burps are actually the largest source of GHG emissions, but rice paddies are also a major source. This is due to the fact that watering the rice paddies reduces the oxygen in the soil, which causes microorganisms in the soil to produce methane gas.

The concentration of methane in the atmosphere has increased by 150% between the industrial revolution and 2011. Of this, about 11% of the methane originating from human activities comes from rice paddies. In monsoon Asia, where rice cultivation is thriving, 90% of the world's rice is produced. This means that about 90% of the world's methane generated from rice paddies comes from this region, and the spread of GHG reduction technologies is eagerly awaited. 

JIRCAS, in collaboration with Chiba University and Can Tho University in Vietnam, has tested an intermittent irrigation technology to reduce GHG emissions in rice paddies in the Mekong Delta, and found that it can reduce irrigation water and GHG emissions and increase rice yields.

The technique used is called alternate wetting and drying (AWD), a type of intermittent irrigation developed and popularized by the International Rice Research Institute (IRRI). A study on the effects of AWD in a paddy field in Can Tho City showed an increase in yield by 8.9%. A subsequent study on the effects of a simplified version of AWD, referred to as multiple drainage (MD), in paddy fields in An Giang Province adjacent to Can Tho City showed further increase in yield by 22% and reduction in methane gas emissions by 35% compared to normal cultivation method. 

Since AWD was developed to save water in irrigated rice fields in Southeast Asia, it has been introduced in countries and regions where water resources are inadequate. However, the concern has been that although AWD reduces methane emissions from rice paddies, the yield of rice is either the same or decreases. In the Mekong Delta region where this study was conducted, there are many triple-cropping rice paddies, and the paddy soil is always in a low oxygen environment throughout the year. The results of this research have enabled us to verify the effects of MD, which is easier for farmers to implement, and we believe that this will lead to the spread of the developed technology.

The fact that we were able to increase yields even with intermittent irrigation is expected to lead to behavioral changes among farmers. In the Asian monsoon region, including Vietnam, paddy rice cultivation is the backbone of agriculture, as is the case in the Mekong Delta. If the developed technology is adopted by countries in the Asian monsoon region, it will not only increase yields but also contribute to the achievement of GHG reduction targets.

References
Arai et al. (2021) https://doi.org/10.1080/00380768.2021.1929463
Uno et al. (2021) https://doi.org/10.1007/s10333-021-00861-8

Contributors: UNO Ken-ichi and IZUMI Taro (Rural Development Division), MINAMIKAWA Kazunori (Crop, Livestock and Environment Division)