More atmospheric CO2 leads to less nutritious rice

An international research team has found that increased carbon dioxide in the atmosphere will reduce the nutritional value of rice in the future — an outcome that could have a devastating effect on some of the world’s poorest communities.

The research was instigated by Kazuhiko Kobayashi, a professor at The University of Tokyo and an expert in the effects of air pollution on agriculture. He was responding to previous reports of reduced protein and mineral levels in crops, which curiously corresponded with rising atmospheric CO₂ concentration. Kobayashi decided to grow rice under the higher CO₂ concentrations expected in the second half of this century (568 to 590 ppm), and he was soon joined in his work by scientists from China, the US and Australia.

The rice was grown at research sites in China and Japan using an open-field method where researchers build 17 m-wide plastic pipe octagons elevated about 30 cm above the tops of plants within standard rice fields. A network of sensors and monitors measure wind speed and direction to determine how much CO₂ is released out of the pipes to raise the local CO₂ concentration to the desired experimental level. The technique is known as free-air carbon dioxide enrichment (FACE).

“I first started using this technique in 1998, because we knew that plants raised in a plastic or glass house do not grow the same as plants in normal, open-field conditions,” said Kobayashi. “This technique allows us to test the effects of higher carbon dioxide concentrations on plants growing in the same conditions that farmers really will grow them some decades later in this century.”

Rice within the octagon in this field is part of an experiment started by a University of Tokyo professor and designed to grow rice under different atmospheric conditions. The experiment was conducted by researchers of National Agriculture and Food Research Organization of Japan. Image credit: Dr Toshihiro Hasegawa (National Agriculture and Food Research Organization of Japan).

The researchers analysed a total of 18 different varieties of rice for protein, iron and zinc levels, in addition to vitamin B1 (thiamine), B2 (riboflavin), B5 (panthothenic acid) and B9 (folate). The outcome of the multiyear study was published in the journal Science Advances.

“Whereas our results confirm the declines in protein, iron and zinc, we also find consistent declines in vitamins B1, B2, B5 and B9 and, conversely, an increase in vitamin E,” the researchers wrote. “A strong correlation between the impacts of elevated [CO₂] on vitamin content based on the molecular fraction of nitrogen within the vitamin was observed.”

These results may seem insignificant compared with other consequences of rising CO₂ levels, such as rising temperatures and wild weather, but the 600 million people worldwide who consume at least 50% of their daily energy and/or protein directly from rice would disagree.

“Rice is not just a major source of calories, but also proteins and vitamins for many people in developing countries and for poorer communities within developed countries,” noted Professor Kobayashi. Populations in countries with both high rice consumption and low gross domestic product may thus experience more malnutrition as the nutritional value of low-cost staple foods like rice declines.

The researchers noted that not all varieties of rice responded in the same way, so future research projects may examine the possibility of finding varieties of rice that can remain nutritious despite the change in the atmosphere.