Forests may grow more slowly than expected as CO2 levels rise

Soil microbes can outcompete plants for vital nutrients, which could limit the amount of carbon dioxide forests are able to remove from the atmosphere.

Higher levels of CO2 generally increase plant growth by stimulating photosynthesis, but this CO2 fertilisation effect boosts growth only up to a point. Eventually, growth is limited by available nutrients in the soil. In between a third and half of all ecosystems, the limiting nutrient is phosphorus, says Kristine Crous at Western Sydney University in Australia.

However, researchers remain uncertain about where those phosphorus limits are. One key unknown is how the amount of available phosphorus might change as plants and soil microorganisms respond to rising levels of CO2.

Crous and her colleagues collected six years of data on changing phosphorus levels in a mature forest in New South Wales, Australia, as part of a long-standing experiment called the Eucalyptus Free Air CO2 Enrichment. Plots there are exposed to artificially increased levels of CO2 using long pipes hanging around the trees.

The team found that the amount of available phosphorus didn’t increase with added CO2, despite the plants releasing more carbon into the soil through their roots. Some had thought this would spur soil microbes to recycle more phosphorus from dead and decaying matter, says Peter Reich at the University of Michigan, a member of the team.

The researchers attribute this to the microbes outcompeting the plants for any available phosphorus: the microbes contained more than triple the amount of phosphorus held within the plants.

If this microbe-driven phosphorus limit is widespread, forests might respond less than expected to CO2 fertilisation, says Crous. “Most models do not take the effects of low phosphorus into account and therefore overestimate ecosystem productivity.” Nutrients may need to be added to some ecosystems to allow them to reach their full carbon storage potential, she says.

However, it is an open question how much these results apply to forests elsewhere, says César Terrer at the Massachusetts Institute of Technology.

And nutrients are just part of the picture. Increased drought, heat and fires associated with climate change are changing carbon storage in forests more than their direct response to CO2, says Terrer.