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This is the VOA Special English AGRICULTURE REPORT.

Increasing levels of carbon dioxide gas in Earth's atmosphere have been linked to warming of the Earth. Now, American scientists say the rising carbon dioxide levels can interfere with the ability of plants to use some forms of the important element nitrogen. They say the higher levels of carbon dioxide have affected plants everywhere and are forcing changes in the use of agricultural fertilizer.

The Proceedings of the National Academy of Sciences reported the findings.

Carbon dioxide levels in Earth's atmosphere have increased more than thirty percent during the past two centuries. For years, scientists believed that rising levels of the industrial gas would help plants. Carbon dioxide is necessary for photosynthesis, the process in which green plants use sunlight to make energy.

However, recent studies found that plants cannot support the increased amounts of carbon dioxide. In experiments, scientists increased carbon dioxide levels by as much as two-hundred percent. At first, the plants used thirty percent more carbon. Yet a short time later, the rate of carbon processing dropped to just twelve percent greater than normal.

Scientists at the University of California at Davis are studying how plants react to fertilizer products made with nitrate and ammonium. Nitrate and ammonium are different forms of nitrogen. Nitrogen is necessary for production of proteins and nucleic acids in plants. Many farmers add nitrogen-rich fertilizers to their crops.

In the new study, the scientists found that ammonium fertilizer is more effective than nitrate fertilizer when carbon dioxide levels in the atmosphere are unusually high. The scientists grew wheat seedlings with either nitrate or ammonium under different levels of carbon dioxide. They found that increased carbon dioxide levels limited the processing of nitrate in the plants.

The scientists also found that the kind of nitrogen did not affect wheat growth when carbon dioxide was at a normal level. However, this changed when they increased the gas to almost two times the normal atmospheric level. The seedlings treated with ammonium grew at a faster rate than those treated with nitrate.

The scientists say they had similar results using tomato plants.

This VOA Special English AGRICULTURE REPORT was written by George Grow.