Nora Schultz, New Scientist 17 Aug 09;
You may have thought that the silver lining of rising carbon dioxide levels would be a boost in crop yields. But evidence is mounting that we may trade quantity for quality.
The discovery that staple crops like wheat have less protein when grown in high concentrations of CO2 has already caused concern, but the bad news doesn't stop there.
Ramping up CO2 also changes the balance of amino acids and several trace elements, says Petra Högy from the University of Hohenheim in Germany.
Together with Andreas Fangmeier, also at the University of Hohenheim, and his team, Högy grew wheat in open fields over three years while blowing extra CO2 over the plots to achieve the concentrations of the gas that are expected to be reached by around 2050.
Iron out
They found several changes in the wheat grains, including an 8 per cent drop in iron and a 14 per cent increase in lead.
"Both of these changes would be bad for human health. The drop in iron is particularly worrisome as half of the world's population are already iron deficient, and this is going to get worse," says Högy.
The team also looked again at the drop in protein that had already been seen and found that essential amino acids – including those important for children – are affected by it, not just non-essential protein components.
In addition, wheat grown under high-carbon conditions was worth less money, with smaller grains that are harder to sell for good prices and different dough properties due to the changed protein composition.
No quick nitrogen fix
The researchers do not know all the reasons for the changed balance of nutrients and minerals but argue that other experiments show that simply increasing nitrogen fertiliser is unlikely to restore the lack in grain protein.
Not all the changes are bad however, says Högy: "The heavy metal cadmium also decreased by 14 per cent, which might be positive," she says.
Iraki Loladze of the University of Nebraska-Lincoln, who predicted that rising CO2 would lead to smaller amounts of micronutrients in crops almost seven years ago (PDF), says he is concerned by the continuing lack of awareness of this problem: "The effect of rising CO2 on our food quality is [one of] stealth – it's there, but our scientific radars are not tuned to it.
"This study is important because it brings into sharp focus this effect on wheat – one of the largest sources of calories and nutrients for humans."
Wheat is not the only crop to suffer carbon-induced changes beyond having less protein. Ros Gleadow of Monash University in Melbourne, Australia, who recently reported rising cyanide levels in cassava, says that plants such as eucalyptus respond to rising CO2 levels by making more defensive chemicals, which may make the plants a worse food source for farm animals and wildlife in the future.
Journal reference: Plant Biology, DOI: 10.1111/j.1438-8677.2009.00230.x
