The Heat Is Online

CO2 Diminishes Nutritional Value of Crops

More Carbon Dioxide Could Reduce Crop Value

COLUMBUS, Ohio, October 3, 2002 (ENS) - Rising atmospheric carbon dioxide levels may increase agricultural productivity, but reduce the nutritional quality of some crops, a new study suggests.

Carbon dioxide (CO2), a greenhouse gas linked to global warming, could help crop plants grow and reproduce more in the temperate zones that now produce most of the world's food.

But the price of that bonus could be a reduction in the nutritional value of crops, said Peter Curtis, a professor of evolution, ecology and organismal biology at Ohio State University.

"If you're looking for a positive spin on rising CO2 levels, it's that agricultural production in some areas is bound to increase," Curtis said. "Crops have higher yields when more CO2 is available, even if growing conditions aren't perfect. But there's a tradeoff between quantity and quality. While crops may be more productive, the resulting produce will be of lower nutritional quality."

Nutritional quality declines because while the plants produce more seeds under higher CO2 levels, the seeds contain less nitrogen.

"The quality of the food produced by the plant decreases, so you've got to eat more of it to get the same benefits," Curtis said. "Nitrogen is a critical component for building protein in animals, and much of the grain grown in the United States is fed to livestock. Under the rising CO2 scenario, livestock - and humans - would have to increase their intake of plants to compensate for the loss."

Curtis and his colleagues conducted a meta-analysis, a technique in which researchers pull together data from a large number of similar studies and summarize the results. The 159 studies the researchers reviewed were published between 1983 and 2000.

The studies included data on crop and wild plant species' reproductive responses to the atmospheric CO2 levels predicted by the end of this century. Scientists expect CO2 levels to almost double by 2100.

The researchers analyzed eight different ways plants respond to higher CO2 levels:

number of flowers; number of fruits; fruit weight; number of seeds; total seed weight; individual seed weight; the amount of nitrogen contained in seeds; and a plant's reproductive allocation, a measurement of a plant's capacity to reproduce.

Plants grown at higher CO2 levels had more flowers, more seeds, greater individual seed weight, greater total seed weight and lower concentration of nitrogen in their seeds than those grown at current levels of atmospheric CO2.

Under higher CO2 levels, crop plants showed a notable increase in reproduction while wild plants did not. On average, crops produced more fruits and seeds than did wild species.

"Wild plants are constrained by what they can do with increased CO2," Curtis explained. "They may use it for survival and defense rather than to boost reproduction. Agricultural crops, on the other hand, are protected from pests and diseases, so they have the luxury of using extra CO2 to enhance reproduction."

Individual crops varied in their response to increased CO2 levels. Rice seemed to be the most responsive, as its seed production increased an average of 42 percent. Soybeans followed with a 20 percent increase in seed, while wheat increased 15 percent and corn five percent.

Nitrogen levels decreased by an average of 14 percent across all plants except cultivated legumes, such as peas and soybeans.

"That's bad news," Curtis said. "Nitrogen is important for building protein in humans and animals. If anything, plant biologists want to boost the levels of nitrogen in crops. A growing global population demands more food, but humans would have to eat more of the food to get the same nutritional benefits."

The study was funded by a grant from the National Science Foundation. It appears in the current issue of the journal "New Phytologist."