Rising carbon dioxide could make crops less nutritious

[BEIJING] Rising levels of carbon dioxide in the atmosphere could make rice and wheat grow faster but be less nutritious, say Chinese scientists. The impact on agriculture could be profound.

“The protein level will decrease by ten per cent by about 2050, and elements such as iron and zinc will also decline,” says Zhu Jianguo, a senior scientist with the Nanjing-based Institute of Soil Sciences, part of the Chinese Academy of Sciences.

Researchers led by Zhu spent three years studying the impacts of greenhouse gas emissions on the growth of rice and wheat crops.

A technique called free-air carbon dioxide enrichment allowed the researchers to create an artificial local atmosphere around crops growing in open fields.

By surrounding the study plants with pipes emitting carbon dioxide, they mimicked conditions predicted for 2050 if current levels of increasing carbon dioxide emissions are not checked.

This equates to a 50 per cent increase in the current concentration of carbon dioxide in the atmosphere.

The researchers found that, under these conditions, crop yields increased by 15 per cent for rice and 14 per cent for wheat. The crops also grew more rapidly: 10-14 per cent faster in the case of rice, and 12-20 per cent faster for wheat.

However, protein levels in both rice and wheat grains dropped by about ten per cent.

Plants take up carbon dioxide and combine it with water to create carbohydrates that are essential for growth, so increased concentrations allow them to grow faster.

But by growing faster, the rice and wheat grains accumulate less protein and other nutrients, Zhu told SciDev.Net.

Earlier elements of the research were published in international journals including Global Biogeochemical Cycles, but the overall findings have so far only been published in the Chinese-language Journal of Biology.

According to Zhu, in the study's next phase, the researchers will enlarge their experimental fields and change the level of carbon dioxide to get a wider range of statistics.

In the long term, they will also study how air temperature and the level of ozone in the atmosphere affect the growth and nutritional value of rice.

Zhu says scientists in Europe, Japan and the United States have performed free-air carbon dioxide enrichment experiments, but that his team's study is the first in a developing country. It is also the first undertaken both in rice and wheat fields.

Zhu's research complements research by scientists at the Institute of Subtropical Agriculture, in Changsha City.

Their study predicts that if air temperature continues to increase by 0.04 degrees Celsius each year, the amount of organic matter — derived from decaying animal and plant life — in rice fields will fall by seven per cent by 2050.

Xiao He'ai, one of the researchers at the Institute of Subtropical Agriculture, says Zhu's research is significant in that it helps people better understand the impacts of global warming on their daily lives.

On a practical level, Zhu says his research will help policymakers make long-term estimates of future grain production, and enable them to make policy decisions to ensure food security.

"The research results can also be used to guide farming practices such as irrigation and fertilisation of crops," he says.

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