Fertilizers and other nitrogen sources, like the burning of fossil fuels, help plants to grow. That growth, in turn, sops up carbon dioxide, a major greenhouse gas. But counter to what climate modelers tend to assume, says a Report  in today's issue of Science, nitrogen's green thumb may not keep a lid on global warming, because it fosters the growth of plants that store less carbon.
Ecologists David Wedin of the University of Toronto and David Tilman of the University of Minnesota measured carbon stored in plants and soils in 162 grass plots that were fed fertilizer for 12 years. The researchers found that the nitrogen induced plants to gulp more carbon dioxide as they fixed carbon for growth. Over the years, however, the added nitrogen pushed the mix of grasses toward weedy, invasive species that are less efficient at fixing carbon in soils. These invading species decompose more quickly than native plants do and thus contribute less organic matter--and less sequestered carbon--to soils. The result: A grassland's biodiversity drops, and its overall ability to sequester carbon soon levels off. "So in the long term, the higher growth rates observed aboveground are not reflected in greater ecosystem storage [of carbon]," Wedin says.
And that's not all. The nitrogen-induced changes in growth patterns may also be occurring in forests, which are even more important carbon sinks than grasslands, says David Schimel of the National Center for Atmospheric Research in Boulder, Colorado: "This process may be occurring over very large areas of the world." The message, say ecologists, is that while adding nitrogen may be a growth industry, it's not the answer to global warming.