Wheat's Nutritive Value Decreases Under Elevated CO2

Amber waves of less nutritious grain

Lately you hear a lot of, well, trash talk about how CO2 is good for plants and CO2 fertilization will be a boon for agriculture, and all that. (If you can't deny climate change, then you argue it will be beneficial.) In reality the changes to plants under climate change are complicated, with higher CO2, higher temperatures and changes in precipitation comingling in complex ways.

Plant tissues also change, and this recent paper in Nature Climate Change is very interesting:

“Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat,” Arnold J. Bloom et al, Nature Climate Change, April 6 2014.

This paper presents a direct test of the hypothesis, pointed to by meta-analyses, that more CO2 inhibits nitrogen assimilation into proteins. Higher CO2 increases yields, but protein concentrations decrease. Who wins?

In a trial of field-grown wheat in Arizona, under conditions of elevated CO2 (up to 559 ppmv), this research group found that nitrate assimilation was indeed slower under elevated than ambient CO2. The authors conclude

These findings imply that food quality will suffer under the CO2 levels anticipated during this century unless more sophisticated approaches to nitrogen fertilization are employed.

The Times of India quotes the lead author, Arnold Bloom of UC Davis:

"Food quality is declining under the rising levels of atmospheric carbon dioxide that we are experiencing," said lead author Arnold Bloom, a professor in the department of plant sciences at University of California, Davis.

"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," he said.

The article says that wheat "provides nearly one-fourth of all protein in the global human diet." It concludes:

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 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 3 per cent as atmospheric carbon dioxide reaches the levels anticipated to occur during the next few decades," Bloom said.

That is, 3% is the difference between the increase from CO2 fertilization and the decrease in protein density.

Of course it's possible that scientists and farmers will find better methods of nitrogen fertilization to overcome this decrease. And now it's starting to look necessary....