Rising CO2 could threaten human nutrition
No free lunch Increased carbon dioxide levels could cause a global drop in nutrients in rice, wheat and soy, a new study has found.
Experimental results published today in the journal Nature have found that zinc, iron and protein levels all fall in these crucial crops when CO2 levels rise.
"This relationship holds across different ecological zones. It's pretty universal," says co-author agronomist Dr Glenn Fitzgerald, of the Department of Environment and Primary Industries, in Horsham, Victoria.
The idea that rising CO2 levels can impact on the nutritional composition of grains and legumes has been around for a while, but Fitzgerald says this is the largest study of its kind to support the idea.
Fitzgerald and colleagues analysed over 20 field-based experiments that looked at the effect of elevated CO2 on crops in a number of countries including Australia, Japan and the United States.
"In Australia we had different types of wheat or field peas. In Japan they had rice, in the US they had maize and soy bean and sorghum," says Fitzgerald.
The experiments involved small research plots fertilised with CO2 to just above the height of the plant via a network of tubes.
"As the plant grows, we raise the network of tubes," says Fitzgerald.
A computer connected to sensors in the crop maintained a CO2 concentration at 550 ppm, which is that atmospheric level expected around 2050, he says.
Once the plants were harvested they were analysed for protein, zinc and iron levels.
The researchers found that plants which use a C3 pathway for photosynthesis had roughly 5-10 per cent lower levels of these nutrients under elevated CO2, compared to control plots that were exposed to current day levels.
The yield of C4 crops such as sorghum and maize were not affected by the CO2 levels. While nutrients in maize did appear fall, the low number of maize trials meant it was not possible to be sure of this result, says Fitzgerald.
Relationship with yield
Fitzgerald says the researchers don't fully understand why higher CO2 causes a drop in nutrients in C3 crops.
But such crops are known to grow bigger under elevated CO2 and this appears to boost carbohydrates but "dilute down" protein and micronutrients, he says.
"You have more carbon going into the grain, why don't you have more nitrogen going into it as well? We don't understand well why that happens."
Fitzgerald says that while developed nations could adapt to such a drop in nutrients, developing countries like those in Africa would find it more of a challenge.
"Do they have to eat 10 per cent more food? Do they have to grow 10 per cent more food or import it? Where are they going to get that protein from? Where are they going to get those micronutrients from?"
Fitzgerald says the problem may be overcome by breeding crops that are less affected by atmospheric CO2.
He says current studies are now looking at other factors that will affect the global nutrient levels in C3 crops, such as temperature and rainfall.
EDITOR'S NOTE: Since publication of this story, we have been made aware of another large study that supports the findings of the research reported on above.