Over the past two centuries, atmospheric carbon dioxide concentrations have risen by nearly 30 percent. Global warming concerns aside, scientists long thought the trend would benefit plants, which use CO2 to manufacture the chemical energy they need. But in recent years, research has shown that it's not quite that simple. For one thing, it appears that although plants initially process the additional CO2, the accelerated assimilation declines after a few days or weeks. Now new research sheds light on why this happens. The findings, published today in the Proceedings of the National Academy of Sciences, could have important implications for crop fertilization.

In laboratory studies of wheat seedlings (right), Arnold Bloom of the University of California, Davis, and colleagues discovered that elevated levels of CO2 inhibited the processing of nitrate fertilizerone of two kinds commonly used by farmers. As it turns out, the plant uses the same resources to process CO2 and nitrogen, and it gives priority to CO2. Furthermore, elevated levels of CO2 can block the transfer of nitritethe compound the plants produce from nitrateinto their photosynthesizing structures. The decreased processing of nitrate thus offsets the benefits of increased CO2. Under the same high CO2 conditions, however, the plants made ample use of ammonium-based fertilizer. Indeed, whereas plants receiving nitrate fertilizer increased in size by only 24 percent, those fed the ammonium variety grew by nearly 49 percent. In addition, the protein content of the plants that received ammonium rose by 73 percent, as compared with only 32 percent for the nitrate-nourished plants. Unpublished work on tomatoes has yielded similar results.

"We expect that the data from this study will have real-world implications for crop production," Bloom comments. "In well-drained soils generally devoted to wheat production, nitrate is the common form of nitrogen available in the soil. This study suggests that a shift to increase ammonium availability might be needed." He further notes that faced with continued increases in atmospheric CO2, plants that naturally rely on ammonium for nitrogen would probably have a competitive advantage over those that depend on nitratea differential that could significantly affect the distribution of plants in the wild.