Higher CO2 levels could lead to slower growth rates in forests

But the researchers Not sure where phosphorus limits areOne key unknown is how available phosphorus changes as plants and soil microbes respond to elevated carbon dioxide concentrations.

Crowth and her colleagues collected data on changes in phosphorus concentrations in mature forests in New South Wales, Australia, for six years as part of a long-term experiment called Eucalyptus Free-Air CO2 Enrichment, in which they suspend long pipes around the trees and spray the plots with artificially elevated concentrations of CO2.

The team found that adding carbon dioxide did not increase the amount of available phosphorus, even though plants were releasing carbon into the soil through their roots. Some had thought this might encourage soil microbes to recycle more phosphorus from dead and decaying matter. Peter Reich Team member from the University of Michigan.

The researchers think this is because microbes outcompeted plants for available phosphorus: They had more than three times as much phosphorus as plants had.

If microbial phosphorus limitation is widespread, forests may respond less than expected to carbon dioxide fertilization, says Crouse. “Most models don't consider the effects of phosphorus deficiency, and therefore overestimate ecosystem productivity.” Some ecosystems may need to add nutrients to reach their full carbon storage potential, Crouse says.

But the extent to which these results apply to other forests remains an open question, he said. Cesar Terrell At Massachusetts Institute of Technology.

And nutrients are only part of the picture: Increased drought, heat, and fires associated with climate change are altering forest carbon stores beyond a direct response to carbon dioxide, Teller says.