High CO2 Makes Crops Less Nutritious

http://news.nationalgeographic.com/news/2014/05/140507-crops-nutrition-climate-change-carbon-dioxide-science/

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In the largest study yet, Samuel Myers of Harvard University and colleagues report that the CO2 levels expected in the second half of this century will likely reduce the levels of zinc, iron, and protein in wheat, rice, peas, and soybeans. Some two billion people, the researchers note, live in countries where citizens receive more than 60 percent of their zinc or iron from these types of crops. Deficiencies of these nutrients already cause an estimated loss of 63 million life-years annually.

Conducted over six growth years on field sites in Japan, Australia, and the United States, the study compared crops grown in normal conditions with ones grown in nearby experimental plots where the air is enriched with CO2 via open-air sprayers. The current atmospheric CO2 level is 400 parts per million; in the enriched plots, it was between 546 and 586 parts per million, a level scientists expect the atmosphere to reach in four to six decades.

In addition to wheat, rice, peas, and soybeans, which all use a form of photosynthesis known as C3, Myers and his colleagues studied corn and sorghum, which use C4 photosynthesis, a faster kind. They found relatively little effect of CO2 enrichment on the nutritional value of the C4 crops.

In the C3 crops, however, they found significant declines in zinc and iron. The largest was a 9.3 percent drop in the zinc level in wheat. They also found reduced levels of protein in wheat, rice, and peas, but not in soybeans.

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CO2 enrichment experiments at Long's university have also shown that rising CO2 levels lower crops' resistance to pests. By exposing the plants to levels of CO2 similar to those used in the Harvard-led study, says Long, crop damage from three major crop pests doubled.

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Recent efforts by the U.S. Agency for International Development and the Bill and Melinda Gates Foundation to breed rice and other crops with enhanced nutrition under current atmospheric CO2 levels have shown some success, he notes. But those efforts haven't been without setbacks. "There's been some indications that when you do that, you often suffer yield declines," Myers says. "So it's not entirely clear that you can have your cake and eat it too."