The study, reported in Nature Climate Change​, suggests that the total protein and nitrogen concentrations in plants generally decline under elevated levels of carbon dioxide (CO2) - indicating that the nutritional quality of food crops is at risk as climate change intensifies.

Led by Professor Arnold Bloom from the University of California - Davis , the findings are based on data from field trials involving wheat that was grown in an environment richer in CO2, which showed that crops grown in an environment where CO2 is higher have a reduced ability to process nitrogen.

The processing, or assimilation, of nitrogen by plants plays a vital role in its growth and productivity, said the team - who noted that it is especially important in food crops because these plants use nitrogen to produce proteins that are vital for human nutrition.

"Food quality is declining under the rising levels of atmospheric carbon dioxide that we are experiencing,"​ said Bloom. "Several explanations for this decline have been put forward, but this is the first study to demonstrate that elevated carbon dioxide inhibits the conversion of nitrate into protein in a field-grown crop."​

3% protein decline expected​

Bloom noted that other studies have also shown that protein concentrations in the grain of wheat, rice and barley — as well as in potato tubers — decline, on average, by 8% percent when subjected to higher levels of atmospheric CO2.

"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 3% as atmospheric carbon dioxide reaches the levels anticipated to occur during the next few decades,"​ Bloom suggested.

While heavy nitrogen fertilization could partially compensate for this decline in food quality, it would also have negative consequences including higher costs, more nitrate leaching into groundwater and increased emissions of the greenhouse gas nitrous oxide, he added.

Study details​

To observe the response of wheat to different levels of atmospheric carbon dioxide, Bloom and his team examined samples of wheat that had been grown in 1996 and 1997 in the Maricopa Agricultural Center near Phoenix, USA.

At that time, carbon dioxide-enriched air was released in the fields, creating an elevated level of atmospheric carbon at the test plots. This level was similar to what is now expected to be present in the next few decades, said the team.

Control plantings of wheat were also grown in the ambient, untreated level of carbon dioxide.

More than a decade later, the team have been able to use new chemical analysis techniques not possible to perform at the time.

Bloom's analysis used three different measures of nitrate assimilation to show that that the elevated level of atmospheric carbon dioxide had blocked the processing of nitrate into protein in the field-grown wheat.

"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,"​ Bloom said.

Source: Nature Climate Change

Published online ahead of print, doi: 10.1038/nclimate2183​

"Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat"​

Authors: Arnold J. Bloom, Martin Burger, Bruce A. Kimball, Paul J. Pinter Jr