Scientists Overestimate Ability of Soil to Store Carbon

A new study suggests that global carbon models are getting it wrong when it comes to soil carbon.

The concentration of carbon dioxide in the atmosphere is expected to rise steadily through the end of the century. This is good news for plants and trees, which thrive on CO₂ and can serve as important sinks to absorb carbon and mitigate climate change.

But a higher level of carbon dioxide is likely to have the opposite effect on carbon stored in soils, according to the results of an 11-year study on vegetation in an experimental oak woodland in Florida. Soil carbon tends to escape in a high-CO₂ environment, which means the overall uptake of carbon is much less than previously thought, said Bruce Hungate, the lead author of the paper.

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"We've known they were wrong for some time," said Hungate, a professor of biological sciences at Northern Arizona University, about models that foresee a high uptake of carbon in soils in the future. "This study provides new and strong empirical evidence that we need to rethink this."

Hungate and his colleagues observed that increased carbon input in the soil leads to higher carbon losses because it stimulates tiny microorganisms that digest soil carbon and release carbon dioxide. The efficiency of the microbes in a high-CO₂ environment was surprising, said Hungate.

The researchers raised the CO₂ concentration to 720 parts per million -- 320 ppm over today's levels -- over 16 open-top chambers in the woods. That concentration is in the range of what the Earth may experience by the end of the century, said Hungate. The study was published online earlier this year in New Phytologist and will be published in print as part of a special feature on long-term responses to carbon in ecosystems.

In addition, the study disproves the theory that higher carbon dioxide concentrations in the air will lead to plants taking in more nutrients in the form of nitrogen. In the experimental plot, nitrogen was leached away and wasted.

"The CO₂ is tipping the balance toward nutrient losses," said Hungate.

Can climate change be a silver lining?
The findings hinge on a debate on how well environments will respond to climate change, and whether climate change will offer a silver lining to some landscapes. A study published earlier this year found that carbon fertilization -- the boost that CO₂ gives plants -- may do tropical forests more good than extreme weather from climate change will do bad (ClimateWire, March 11).

Another recent study found that satellite images indicate deserts around the world are becoming greener because of the carbon fertilization effect (ClimateWire, June 10).

The woodland has been the setting for a series of studies on environmental responses to higher carbon concentration, said Hungate. One study based on observations in the experimental site found that trees respond more positively to a high-carbon setting after a disturbance, like a fire or hurricane.

This experiment isn't the first to show that increased CO₂ in the air does not lead to better carbon storage, said Joe Berry, a staff member in the Department of Global Ecology at the Carnegie Institution for Science. A similar project on grasslands in central California showed parallel results.

"Models are often put together based on how we think ecosystems work. Experimental manipulations, in contrast, inform how the ecosystem actually responds," wrote Berry in an email. "They don't have to agree. In fact, that is what science is all about."

The Intergovernmental Panel on Climate Change, the authority on climate for the United Nations, will release its fifth assessment report on the science of climate change late this year and next year. Hungate said it is unlikely that experiments like his will make it into the next assessment report to challenge the existing models, but he hopes they will make it in eventually.

"I'm more hopeful there," he said. "Ever the optimist."