How Will Rising CO2 Affect Nitrogen Use?

Wheat grown under elevated levels of carbon dioxide over the next half-century will need slightly more nitrogen to grow, but not as much as previously predicted, according to a two-year study by Agricultural Research Service scientists and cooperators.

Atmospheric carbon dioxide (CO2) levels are projected to increase 43 percent by 2050. The increased CO2 makes plants like wheat grow larger. But a bigger plant needs more nutrients such as nitrogen, at least in theory, according to ARS soil scientist Floyd J. Adamsen, who works at the agency's U.S. Water Conservation Laboratory in Phoenix, Ariz.

So the ARS scientists have been trying to determine whether higher CO2 levels will increase the amount of nitrogen that wheat and other crops need to grow. They reported their findings on the interaction between carbon dioxide and nitrogen in the January-February 2005 issue of Agronomy Journal.

At the Maricopa Agricultural Center near Phoenix, the team compared wheat grown under current levels of CO2 to wheat grown with the CO2 levels expected by 2050. A series of tubes injected CO2 into circular, open-air field plots to increase the CO2 concentration in the air during the two-year experiment. The plants grown with higher CO2 levels only used about 3 to 4 percent more nitrogen than the plants grown at current CO2 levels.

The researchers applied fertilizer four times, which spread out the uptake of the nutrients. Based on the study's findings, farmers in the future may need to apply fertilizer four times on wheat, instead of the traditional one or two applications.

The scientists believe growers need to understand how rising levels of CO2 may affect their crops. Accordingly, farmers may have to adapt their farming practices--such as altering the timing and amounts of nitrogen fertilizer--to produce crops in the changing environmental conditions of the future.

ARS is the U.S. Department of Agriculture's chief scientific research agency.