Flooded fields

The dilemma of Asia's staple food (7 minute read)

When I walk into a supermarket, I find the choices dizzying. The shelves are stocked with so many tempting products that sometimes I find myself standing there, unable to decide. Most of what is available, I’ll never try. But I could, if I wanted to. I could reach out and put any number of different cooking oils, sauces, biscuits, cuts of meat, salty snacks or sugary sweets into my trolley.

For most of the world’s population, the picture is quite different. The dizzying choices I face are very much the experience of someone in a wealthy country who earns enough to live on. For half the world’s population there is no choice. Every day, they depend on a single crop as their main source of energy – rice. Unlike many of our other staple crops, we don’t waste rice by feeding it to animals to produce meat. 95% of the world’s rice crop is eaten directly by people.

The origin of rice is the subject of controversy, with scientists from China arguing that it comes from China, and scientists from India arguing, unsurprisingly, that it comes from India. In fact, the evidence is starting to suggest that both are partly correct. Long-grained rice and short-grained rice appear to have different genetic origins, with long-grained rice linked to India and short-grained rice linked to China. But it’s not as simple as that, because they have some genes in common.

Rice, like wheat, oats, barley and corn, is a type of grass, and the part that we eat is the seed. The wild relatives of rice have seed heads which shatter, scattering the grains when they are mature. This helps the plant to disperse, but makes it difficult for people to harvest. Cultivated rice has a mutation which keeps the seed attached to the stalk when the grains are mature. This mutation appeared first in rice grown in China, but spread from there to rice grown in India. Once it had this trait, rice became much easier to harvest.

Much of the rice produced today is still grown in China and India, with significant amounts also produced in Indonesia, Bangladesh, Vietnam and neighbouring countries. In comparison with crops like wheat and soybean, where production is often highly mechanised, the cultivation of rice is labour-intensive. Rather than sowing seed directly where it is to be grown, seeds are germinated in seed beds then transplanted to flooded paddy fields where they complete their growth. This transplanting process is still mostly done by hand (I’ve linked to a video which shows the rice being transplanted here). Although there have been some developments in mechanisation, many are still fairly low-tech, for example devices for spreading fertiliser which are pulled through the fields by farmers.

While the production of rice is labour-intensive rather than fossil fuel-intensive, there’s a strange irony about rice production. Compared with other grains and staple crops such as potato, it has a high carbon footprint. Although emissions from rice are less than those for beef, lamb and dairy, they can’t be ignored when rice is such an important crop for so many people.

But why does rice have a high carbon footprint in comparison to other grain crops? After all, the process of rice growing doesn’t use a lot of machinery, so it doesn’t use a lot of fossil fuel.

The carbon footprint of rice doesn’t have anything to do with fossil fuel at all. Instead, it has to do with the breakdown of organic material, such as the old leaves and stems from the previous year’s crop, in the rice fields. In normal soil, which is neither too wet nor too dry, organic material breaks down to release carbon dioxide. This might sound like a problem for the climate, but it’s not, because the carbon in this kind of organic matter came from plants absorbing carbon dioxide out of the atmosphere in the first place, during the process of photosynthesis. So, in a normal soil, the breakdown of organic matter is effectively neutral in terms of the climate.

But in a saturated soil, such as in a flooded rice field, there’s not enough oxygen for organic material to break down to form carbon dioxide. Instead, it forms methane, which is a much more potent greenhouse gas. This methane is the reason for rice’s high carbon footprint.

But does rice need to be grown in flooded fields at all? There are varieties of rice which are more resistant to dry conditions, but in the past they have mostly been varieties which give low yields, so they aren’t going to solve the problem just yet. However, there is work underway to breed better varieties, with good yields and tolerance of drought. In the longer term, this may provide one solution.

There are other solutions too, and some of them can be used immediately. Rice grows best in flooded fields, but the fields don’t need to be flooded for the entire growing season to grow a good crop. Scientists have found that draining rice fields part of the way through the growing season can lead to a large reduction in the methane omitted. This is now a common practice in China, not because it reduces methane emissions, but because it increases yields and saves water. Wider adoption of this cultivation method would make a real difference to methane emissions from rice fields.

There’s a catch, though. Rice fields which are drained part of the way through the growing season emit more nitrous oxide, another greenhouse gas. When rice fields are flooded, nitrogen tends to stay in the soil, in the form of ammonium, which is available for plants to use for growing. Nitrogen which escapes into the atmosphere is released as nitrogen gas, which has no effect on climate change. But when the soil is drier, there is more oxygen available to combine with the nitrogen, creating nitrous oxide.

All is not lost, however. The problem of nitrous oxide release particularly relates to the overuse of nitrogen fertilisers. Less than half of the fertiliser applied to rice fields is taken up by plants – in fact, sometimes as little as 20% of the nitrogen gets used by plants. Part of the problem simply relates to timing – if the fertiliser is applied too early, before the rice plants have formed enough of a root system, the nitrogen is wasted, and can potentially be released as nitrous oxide into the atmosphere, or as nitrates into waterways. Using less fertiliser, but making sure it is applied at the right place and the right time, can make a real difference. It’s good for farmers too, because it saves them money.

One approach to rice cultivation, developed in Madagascar and now being used in other areas such as India and Vietnam, shows that it is possible to grow rice with a lower impact on the climate. Farmers have been making simple adjustments to the way rice is grown, such as planting younger rice seedlings, spaced further apart. The result is a better crop of rice from the same area, with less fertiliser input. This approach uses intermittent flooding rather than leaving the fields flooded for the whole growing season, so it reduces the methane emitted. And because there is less fertiliser used, there’s no corresponding increase in nitrous oxide emissions. Estimates from India suggest that this approach can lead to substantial reductions in greenhouse gas emissions per kilogram of rice produced. Importantly, this approach also increased yields, so it’s better for the farmers too.

When so many of the climate change problems we face seem intractable, rice gives us cause for hope. Although the solutions aren’t easy – somehow, these new approaches need to be brought to millions of smallholder farmers – they do show that rice can continue to be a crucially important crop without such a great cost to the climate.