AS carbon dioxide levels in the atmosphere hit their highest level for possibly three million years, researchers have determined that the gas is changing the nutritional quality of food crops.
The effect of rising CO2 levels is already evident across the world’s crops, say researchers from the University of California, Davis, and could reduce the amount of plant protein available globally by three per cent over the next few decades.
The assimilation of nitrogen plays a key role in a plant's growth and productivity. In food crops, it is especially important because plants use nitrogen to produce the proteins that are vital for human nutrition.
Wheat alone provides nearly one-fourth of all protein in the global human diet.
Laboratory studies had demonstrated that elevated levels of carbon dioxide inhibit plants’ assimilation of nitrate into proteins, but there had been no verification of this process in field-grown plants.
A team led by Arnold Bloom, a professor in the UC Davis Department of Plant Sciences, examined samples of wheat that had been grown in 1996 and 1997 in the Maricopa Agricultural Center near Phoenix, Arizona.
While the wheat was being grown, carbon dioxide-enriched air was released in the fields, creating an elevated level of atmospheric carbon at the test plots, similar to what is now expected to be present in the next few decades.
Control plantings of wheat were also grown in the ambient, untreated level of carbon dioxide.
Leaf material harvested from the various wheat tests plots was immediately placed on ice, and then oven dried and stored in vacuum-sealed containers to minimize changes over time.
When Dr Bloom and his team pulled out the preserved leaf material, they were able to do analyses not available when the plants were harvested.
"These field results are consistent with findings from previous laboratory studies, which showed that there are several physiological mechanisms responsible for carbon dioxide's inhibition of nitrate assimilation in leaves," Dr Bloom said.
Other studies have shown that protein concentrations in the grain of wheat, rice and barley, and in potato tubers, can decline by up to eight per cent under elevated levels of atmospheric carbon dioxide.
"When this decline is factored into the respective portion of dietary protein that humans derive from these various crops, it becomes clear that the overall amount of protein available for human consumption may drop by about three per cent as atmospheric carbon dioxide reaches the levels anticipated to occur during the next few decades," Dr Bloom said.
Heavy nitrogen fertilisation could partially compensate for this decline in food quality, but it would also mean higher costs, more nitrate leaching into groundwater and increased emissions of nitrous oxide, a potent greenhouse gas.
Concentrations of CO2 in the atmosphere passed 400 parts per million (ppm) for the first time since recordings began in 2013. Before the 20th century, CO2 levels hadn’t exceeded 300 ppm for at least 800,000 years, according to studies of air trapped in Antarctic ice cores.
The last time CO2 levels passed 400 ppm was probably in the Pliocene Epoch, between 2.6 and 5.3 million years ago, when the planet was about 3-4 degrees warmer than it was in the 19th Century, before the current warming phase began.
Findings from the UC Davis study have been reported in the journal Nature Climate Change.