In the recent study, the researchers documented that three different measures of nitrate assimilation affirmed that the elevated level of atmospheric carbon dioxide had inhibited nitrate assimilation 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.
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Bloom noted that other studies also have shown that protein concentrations in the grain of wheat, rice and barley — as well as in potato tubers — decline, on average, by approximately 8% 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 3% as atmospheric carbon dioxide reaches the levels anticipated to occur during the next few decades," Bloom says.
While nitrogen fertilization could partially compensate for this decline in food quality, Bloom believes it would have negative consequences – including higher costs and increased emissions of the greenhouse gas nitrous oxide.
Previous research has indicated that higher CO2 levels could also favor weed strength.
Findings from the wheat field-test study are online in the journal Nature Climate Change. Funding for the study was provided by the USDA's National Institute of Food and Agriculture.