Limited storage: lack of nutrients will constrain carbon uptake.

Plants take carbon dioxide out of Earth's atmosphere and use its carbon to promote their growth. However, if human activities continue to increase atmospheric concentrations of the planet-warming gas, vegetation won't sequester large amounts of carbon dioxide in the long term, two new analyses suggest. That's because plants will quickly run out of other nutrients.

In the short term, plants store carbon in their tissues. Eventually, some of that carbon makes its way into the soil through the roots or via fallen leaves and stems. Those phenomena had raised the possibility that plants would decrease the buildup of carbon dioxide in the atmosphere.

Lab and field experiments had shown that plants grow more quickly in the presence of higher-than-normal concentrations of carbon dioxide in the air, says Peter B. Reich, an ecologist at the University of Minnesota in St. Patti. Unfortunately, results of along-term experiment by Reich and his colleagues show that the trend doesn't last.

In their 6-year study, the researchers measured carbon storage in nearly 300 patches of Minnesota grassland cultivated under various conditions. Some plots were exposed to an atmosphere with 50 percent more carbon dioxide than the current concentration, some received extra nitrogen via fertilizer, some received both treatments, and others received neither. The plots contained between 1 and 16 species of grasses, herbs, wildflowers, and legumes.

As expected, for the first 4 years of the experiment, plants exposed to higher-than-normal concentrations of carbon dioxide grew faster and became larger than those that didn't get extra carbon, says Reich. However, unless they were also receiving nitrogen supplements, growth of such plants slowed substantially in the fifth and sixth years of the experiment. Reich and his colleagues report their findings in the April 13 Nature.

Another group of researchers also finds that plants getting extra carbon dioxide run out of other nutrients. That team, led by ecologist Johan Six of the University of California, Davis, reports in an upcoming Proceedings of the National Academy of Sciences an analysis of earlier experiments by several research groups.

In the presence of nitrogen-producing legumes and higher-than-normal concentrations of atmospheric carbon dioxide, soil continues building up carbon only when other nutrients, such as phosphorus, potassium, and molybdenum, are added. In other ecosystems, high concentrations of atmospheric carbon dioxide increase soil carbon only when researchers add nitrogen, the Davis group concludes.

If nutrient limitations cause plant growth to slow, as the new studies suggest, carbon dioxide may build up in Earth's atmosphere faster than scientists previously expected, says Reich.