Farming in the Future

Technician Stephanie Johnson measures the rate of photosynthesis in leaves of an orange tree growing in a CO₂-enriched atmosphere.

Inside growth chambers in L. Hartwell Allen's Florida test field, rice, soybeans, and forage plants such as bahia grass are growing in air with twice the CO₂ found in today's atmosphere. Allen's an ARS soil scientist in Gainesville.

CO₂ gas--pumped into the sunlit chambers, temporary plastic-covered greenhouses, and other structures--creates a mixture of air similar to what scientists predict could be present in the Earth's atmosphere within the next century. The experiments, begun as collaborative studies with the U.S. Department of Energy, are in their 15th year.

Allen and others have found that elevated CO₂ concentrations increase plant photosynthesis. But vegetative growth--roots, leaves, and stems--increases more than seed production.

"That means that in the future, scientists may have to breed plant varieties that are capable of producing more seed in the higher CO₂ atmosphere," Allen says.

Results by ARS researchers nationwide give farmers a glimpse into how their jobs might change as CO₂ concentrations--and possibly temperatures--rise.

Rice growers in temperate areas are likely to see a yield increase as the CO₂ concentration rises, Allen and University of Florida colleagues found. But if temperatures increase too much, yields are expected to decline. In experiments with both current and doubled CO₂ concentrations, today's rice cultivars produced the greatest yield at an average daily temperature of about 80^oF and the least when daily average temperatures rose to 97^oF. Since most rice is grown at temperatures close to the optimum of 80^oF, temperatures would have to rise far more than modeled predictions before yields would be seriously reduced.

Soybeans seem to be able to tolerate slightly higher temperatures than rice, Allen says. With increased CO₂, farmers should see up to 30 percent higher soybean yields--even if temperatures rise as much as 5^oF--as long as rainfall remains adequate

Southern beef producers may have to provide more shade, more water, and high-protein supplements to keep cow-calf operations profitable. That's both because the animals would have to tolerate more heat and because the forage quality may decline in southern areas if temperatures rise significantly, according to the SPUR2 model developed by Jon Hanson. Northern producers would fare better, with increased forage quality.

Crops like barley grown in northern areas could benefit from extended photosynthesis--with increased yields and earlier harvests--if CO₂ levels rise.

Nitrate leaching into groundwater could decrease as CO₂ increases, according to experiments in Auburn, Alabama, by ARS soil scientist H. Allen Torbert and plant physiologists Hugo H. Rogers and Steven A. Prior. They found that soybean and grain sorghum plants grew larger and tied up more soil nitrogen--including nitrogen from fertilizer--under elevated CO₂ concentrations.

Even after the plants died, less nitrogen moved through the soil towards groundwater. "Because the plants are bigger, the residue contains more carbon and a higher carbon-to-nitrogen ratio," says Torbert, who's based in Temple, Texas. "The microbes that decompose the plants tie up more of the nitrogen in order to use the larger amount of carbon." The bottom line: most of the nitrogen stays in the soil.

Increased plant growth under elevated CO₂ and higher temperatures could help reduce water runoff and related soil erosion in the Midwest, based on computer modeling done in West Lafayette, Indiana, by ARS hydrologist M. Reza Savabi. That's because the additional growth provides a larger plant canopy, which reduces the formation of a hard crust on the soil surface. That allows more rainfall to infiltrate the soil.

Environmental stresses that normally decrease crop yields, such as air pollution or moisture stress, could be partially ameliorated with higher CO₂ concentrations, according to work led by ARS plant pathologist Allen S. Heagle in Raleigh, North Carolina. "That means increased CO₂ would have a greater benefit for crop yields during dry seasons and where concentrations of ozone are high," says Heagle.

Alaskan farmers could get greater yields of barley and potatoes, says ARS soil scientist Verlan L. Cochran, who was at Fairbanks until 1995. Today, even under the 24-hour daylight of Alaskan summers, plants stop photosynthesis as the light intensity weakens in the early morning hours. But in experiments with elevated CO₂, photosynthesis didn't stop. That meant higher yields and an earlier harvest.

Increased CO₂ would lead to higher wheat yields--about 10 percent more under well-watered conditions and up to 20 percent more than would be typical during drought, according to research by Kimball in Phoenix. If there's not too much global warming, some farmers may even save irrigation water because wheat plants use less water in CO₂-rich air. Kimball performed these and other experiments as part of the FACE project

Despite the extensive research to date, scientists are still working to better predict the effects of global environmental changes on agriculture. The good news is that we have time to mitigate change and adapt to it, and ARS research will continue to work towards both goals. "In the long run," says Heagle, "we can minimize the effects of global change on agriculture by improving our crop cultivars and modifying our cultural practices."

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