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Rising CO2 Could Affect Fertilizer Needs

By Don Comis
January 9, 1997

Rising carbon dioxide levels in the atmosphere could prompt a future adjustment in fertilizer recommendations for soybean and other crops.

A study of grain sorghum and soybean by Agricultural Research Service scientists shows that a carbon boost may enable plants to yield more on less nitrogen. But it also can make soybeans appear nitrogen deficient by a standard leaf test.

Researchers grew the crops in air containing twice the current CO2 concentration--now about 360 parts per million--because CO2 levels are expected to double during the next century if they continue increasing at the current rate.

Soybean leaf nitrogen fell below a critical threshold that, by today’s standards, should have caused at least a 10-percent drop in yield. Instead, yield was up 35 percent without any additional fertilizer. That’s because the increase in plant growth and leaf size caused by additional CO2 diluted nitrogen levels. Sorghum yields were up 18 percent, not enough to affect the nitrogen level in the plants.

If yields of soybeans, sorghum and other major crops go up in step with CO2 levels over the next century, scientists will have to alter the estimated critical nitrogen threshold for maximum yields.