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Greener pastures. The higher protein levels in plants fertilized with ammonium (left) gives them a deeper green than plants fertilized with nitrate (right).

Fertilizing in the Future

The fertilizers farmers use on their crops today may become less efficient if carbon dioxide levels in the atmosphere keep rising. A new study suggests that in carbon-dioxide-rich environments, plants given the nitrate fertilizers widely used today will grow slower and make less protein than plants fertilized with ammonium.

Researchers once thought that extra carbon dioxide in the atmosphere might allow crops to bulk up and build more protein. But in recent years, that assumption has been turned on its head. Although plants grown in high levels of carbon dioxide--say, double the current atmospheric concentrations--initially grow rapidly, the growth tapers off within weeks and the plants wind up with a low protein content. Researchers hypothesized the reason might be that the plants don't get enough nitrogen--a crucial building block for proteins--to sustain their spurt.

To find out if how the nitrogen is delivered makes a difference, Arnold Bloom, a crop physiologist at the University of California, Davis, and his team monitored the growth of wheat plants raised in high levels of carbon dioxide. Plants fertilized with ammonium grew almost twice as much as the plants fertilized with nitrate, and their darker green leaves attested to higher levels of protein, the team reports in the 5 February issue of the Proceedings of the National Academy of Sciences. This happened despite the fact that both groups of plants took in the same amount of nitrogen through their roots, suggesting that the plants fertilized with nitrate were having trouble converting the nitrogen in nitrate into a more useful, organic form.

This led the team to suspect that in the plants fertilized with nitrate, a traffic jam might be occurring in the chloroplasts, sites inside plant cells where nitrogen is converted to the organic variety. Indeed, when the researchers exposed chloroplasts extracted from wheat cells to nitrate, they found that high carbon dioxide levels inhibited the amount of nitrogen that got inside the chloroplasts to be converted.

The findings suggest that farmers will need to reevaluate their use of fertilizers in a "higher-CO2 world," says Bruce Kimball, a soil scientist who works at the U.S. Department of Agriculture in Phoenix, Arizona.

Related sites
Arnold Bloom's home page
The Fertilizer Institute