July 15, 1998 MARICOPA, ARIZ. -- Researchers at The University of Arizona in Tucson are planning to cultivate a crop of the future this summer in Maricopa.
The sorghum they will plant will not differ from the everyday variety. It's the atmosphere around the crops that will give the project its air of distinction.
Equipment to be installed soon after planting will encircle some of the sorghum plots to pipe in additional carbon dioxide. By adjusting the amount released based on wind direction and existing carbon dioxide levels, the elaborate set-up will keep the carbon dioxide level in these experimental plots consistently 200 parts per million above normal levels.
In other words, the circular design will help ring in a new year -- a year when carbon dioxide levels reach hit about 560 parts per million. The researchers will then compare the crops grown within the experimental plots to "control" plots growing under current normal carbon dioxide levels of about 360 parts per million.
The experiment with sorghum is part of the Free Air -- CO2 Enrichment (FACE) project, a years-long effort under way at the UA's Maricopa Agricultural Center (MAC), a 2,100 acre farm located 20 miles south of Phoenix and east of the town of Maricopa.
"About 50 to 100 years in the future, carbon dioxide concentration in the atmosphere is going to be about 200 parts per million greater than it is now," explained Steve Leavitt, a professor of dendrochonology at the UA Tree-Ring Laboratory who is affiliated with the project. "The experiment we're doing is trying to find out the future response of crops to higher CO2 levels."
Scientists have documented an ongoing rise in atmospheric carbon dioxide (CO2), mainly from deforestation and the burning of fossil fuels such as oil, coal and gas. "Greenhouse gases" like carbon dioxide are projected to increase the temperature at the Earth's surface from between 2 to 7 degrees Fahrenheit over the next century.
While many researchers have focused on the global warming potential of carbon dioxide, a smaller but growing group of international researchers has been studying the direct effect of elevated carbon dioxide levels on plant growth. Interestingly, the same gas that humans fear for its heat-trapping effect seems to inspire plants to reach new heights.
For instance, earlier research in Maricopa found that both cotton and wheat thrived when carbon dioxide levels in the field were "enriched" by about 200 parts per million above the usual level.
"That degree of enrichment was increasing cotton growth by about 40 percent, and cotton yield by about 40 percent as well," noted Bruce Kimball, a scientist with the USDA Water Conservation Lab in Phoenix who has been involved with the carbon dioxide enrichment project since its inception in 1989. Similarly, the wheat plants grown under enriched carbon dioxide levels tended to weigh about 20 percent more than those grown under normal conditions, he said.
The advantage of carbon dioxide enrichment stems from the way plants grow. Plants use energy from the sun to transform carbon dioxide and water molecules into carbohydrates, releasing oxygen as waste in the process. However, sometimes plants err by trying to fuel the carbohydrate production factory with oxygen instead of carbon dioxide, a mistake that can waste a lot of energy.
The presence of more carbon dioxide molecules mingling with the 210,000 parts per million of oxygen molecules, then, can improve the odds that a plant will be able to make the carbohydrates that form the bulk of its mass.
A greater abundance of carbon dioxide also has another advantage from a plant's point of view: It can lead to improved water-use efficiency. Carbon dioxide enters the plant through the same openings, known as stomata, from which water can escape. Under higher CO2 levels, the stomata can remain partially closed. So comparisons between experimental plants and controls can be more dramatic when water is scarce.
In the Maricopa wheat experiment, for instance, CO2-enriched plants that were watered adequately yielded about 15 percent more grain and seeds than controls, while CO2-enriched plants grown in drought-like conditions yielded about 25 percent more than controls.
Now for the bad news. It seems the increase in quantity can decrease the quality of grains for some plants enriched by carbon dioxide. Thomas Thompson and Allan Matthias, both of the UA department of soil, water and environmental sciences, found that an increase in the number of wheat grains under enriched-CO2 conditions translated into a decrease in protein content per grain.
"This may take on importance in areas where grains are a main source of protein," Thompson speculated. Americans tend to use grain to feed livestock, while people in developing countries generally depend upon it for their own meals.
The pattern holds true for sorghum, used for cattle here but for cereal in Africa. So the results of this summer's work in Maricopa could have repercussions around the globe.
"Sorghum is number six as a crop in this country, but it's number two in Africa. So it's a very important crop globally," Kimball noted.
Editor's Note: The MAC web site can be found at http://ag.arizona.edu/aes/mac/
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