Rising CO2 levels cut nutrients in crops

Rising levels of carbon dioxide (CO2) in the atmosphere may cut the nutritional quality of some of the world’s most important food crops, researchers reported on Wednesday after conducting experiments simulating conditions expected by mid-century. 

The amounts of two important nutrients – zinc and iron – were found to be lower in wheat, rice, soya beans and field peas grown in open-air fields where the scientists created CO2 concentrations at the level they forecast for Earth by roughly 2050, about 550 parts per million (ppm). They grew 40 varieties of six different grains and legumes, including corn and sorghum, at seven locations on three continents, in Japan, Australia and the United States. 

“This is important because almost two billion people globally receive most of these two nutrients [zinc and iron] by eating crops,” said University of Illinois plant biology professor Andrew Leakey, one of the researchers. The researchers said these findings point to one of the most important health threats shown to be linked to climate change. 

Dr Samuel Myers of the Harvard School of Public Health, who led the study published in the journal Nature, said there is already a significant public health problem in parts of the world because of inadequate intake of zinc and iron. Myers said that zinc insufficiencies affect the immune system and makes people more vulnerable to premature death from maladies such as malaria, pneumonia and diarrhoea. He said iron deficiency is linked to increases in maternal mortality, anaemia, reductions in IQ and reduced work productivity. 

Scientists have sought to gauge the impact of climate change on humankind in the coming decades, including the effects of CO2 levels that have been rising owing to the burning of fossil fuels since the advent of the Industrial Revolution. 

Rising emissions
Earth’s atmospheric CO2 concentration is about 400ppm at present and continues to rise. 

The study found that in wheat grown under elevated CO2 conditions, there were about 9% lower levels of zinc and 5% lower levels of iron compared with wheat grown under normal conditions. The rice grown with elevated CO2 levels had 3% less zinc content and 5% less iron. In wheat and rice, there was also a lower protein content at the elevated CO2 levels, the researchers said. 

Nutrients in sorghum and corn remained stable at the higher CO2 levels because these crops use a kind of photosynthesis that concentrates CO2 in their leaves, the researchers said. 

The scientists simulated higher CO2 levels in open-air fields using a system known as Free Air Concentration Enrichment (Face), which pumps out, monitors and adjusts ground level CO2 in the air to simulate future conditions. 

“You have an open agricultural field that looks like any other except that in the middle of the field you have rings of carbon dioxide-emitting jets. At the centre of the ring you have a sensor that tells you the CO2 concentration and the wind direction. When the CO2 falls below your prescribed level, the upwind jets emit some more CO2 to keep it at a constant level,” Myers said. 

Carbon variable
These crops were grown in identical conditions to crops grown outside the ring in terms of weather, pests, pathogens and soil nutrients, with CO2 concentration as the only variable. The scientists then compared nutrient content using those crops grown in matching plots without the added CO2. 

Leakey said rice, wheat and soya beans made more sugars through photosynthesis at the elevated CO2 levels and produced about 15% more seeds, but saw decreased zinc and iron content. 

He said more work is needed to nail down the details of why higher CO2 levels drive down the nutrients. He also said it would be important to study how crops in tropical countries will respond to elevated CO2 and other aspects of global environmental change. – Reuters


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