Methane emissions raised by planting in flooded rice paddies

Rice is considered a staple food for more than half the world’s population. But according to scientists who are studying the crop, the traditional method of growing rice in flooded paddies, called ‘puddling’, damages the soil, affects the quality of future crops, and increases methane emissions.

Directly seeding rice into fields rather than transplanting it into flooded paddies would dramatically reduce methane emissions and slow down climate change, the scientists said.

Experiments in Asia, particularly in the Philippines and Japan, also show that a change in the way rice is grown would have considerable other benefits in saving water and improving yield. However, in Asia, which accounts for 90% of rice production, only a quarter of the crop is currently produced by direct seeding.

It is estimated that a 25% increase on current crops, amounting to an extra 100 million tons of rice yield, will be needed by 2035. But it will not be reached if the method of growing rice is not changed.

After carbon dioxide, methane is considered the second major greenhouse gas. Agriculture accounts for 40% of methane emissions. Flooded rice paddies emit as much as 500 million tons, which is around 20% of total manmade methane emissions.

The Intergovernmental Panel on Climate Change has estimated that rice cultivation is a major contribution to global warming. When rice is grown under puddled transplanted conditions, paddy soil becomes anoxic − depleted of dissolved oxygen − and then, in the absence of oxygen, microbes that break down plant matter produce methane.

Large quantities of methane that are trapped in flooded soil escape into the atmosphere during wet tillage, harrowing, transplanting and weeding. The rice plants also have a system of releasing methane through their leaves called methanogenesis. The rest escapes either by bubbling upwards or when the soil finally dries out.

Ninety percent of rice land worldwide is at least temporarily flooded even though not all rice is grown in flooded conditions. On average, the soil is fully waterlogged for about four months each year.

The research shows that direct seeded rice has a shorter flooding period (roughly a month) that ensures there is oxygen in the soil through most of the rice season. There is also decreased soil disturbance compared to transplanted rice seedlings.

Jagdish K. Ladha, principal scientist for soils and agronomy research at the International Rice Research Institute in New Delhi, says direct seeding of rice always reduces methane emissions from the crop and can be up to 90%, but that depends on methods used and the water management system.

A field experiment in the Philippines showed that the direct seeding techniques reduced methane emissions by 18% as compared with transplanted rice. Another study, in Japan, showed that global warming potential declined by 42% just by changing puddling of rice seedlings to zero tillage.

Ladha says that it is imperative to promote the technique of direct seeding throughout Asia to reduce methane emissions.