New Worries Associated With Rising CO2 Levels | Chemical & Engineering News
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Web Date: March 24, 2011

New Worries Associated With Rising CO2 Levels

Climate Change: Plants grown in high CO2 draw more contaminants out of the ground
Department: Science & Technology | Collection: Climate Change
News Channels: Environmental SCENE
Keywords: climate change, carbon dioxide, greening theory, plants, metals, agriculture
Plants exposed to high levels of atmospheric CO2 grow fast, but also accumulate metals from the soil.
Credit: Environ. Sci. Technol.
Plants exposed to high levels of atmospheric CO2 grow fast, but also accumulate metals from the soil.
Credit: Environ. Sci. Technol.

Plant growth accelerates in response to high levels of carbon dioxide in the atmosphere. This well-known phenomenon inspired the "greening theory," which holds that plant growth and food production could increase as CO2 levels go up. But new research reports a downside to this faster growth: Plants absorb more toxic metals from the soil when atmospheric CO2 levels are high (Environ. Sci. Technol., DOI: 10.1021/es102250u).

Benjamin Duval, now a post-doctoral researcher at the University of Illinois, Urbana-Champaign, knew that as CO2 levels rise, plants consume more nutrients from soil to feed their enhanced growth. He wondered if a similar trend might hold true for contaminants. "Plants can't always distinguish toxic elements from nutrients," he says. "For instance, arsenic can look a lot like phosphorous, which plants need for their metabolism."

To investigate this question, Duval, then a doctoral candidate at Northern Arizona University, and his colleagues collected soil and oak tree samples from the Smithsonian Environmental Research Center's site at the Kennedy Space Center, in Florida. At this site, researchers had grown trees for 11 years in open enclosures either with normal CO2 levels or with 700 ppm CO2, which is roughly twice the normal level. Duval's team used mass spectrometry to measure the samples' concentrations of 13 metals, including the toxic elements chromium, cadmium, and lead. These metals existed naturally in the sites' soil at normal background levels, Duval says.

The researchers found that the amounts of all 13 metals were 1.5 to 2 times as high in the elevated-CO2 trees as in the controls. Moreover, spikes in plant concentrations matched declines in the amounts of contaminants found in the soil samples. "There's an interaction between pollutant uptake in plants and climate change," Duval says. "We're seeing that they co-occur."

Duval emphasizes that the levels they detected in plant biomass didn't exceed toxic thresholds defined by health agencies. "Health threats would increase if the soil contaminant levels were higher," Duval says.

Robert Miranda, a doctoral candidate in biology at Northern Arizona University who was not involved in the research, says that the findings also have ecological implications. Soil contaminants in plant tissue could biomagnify up the food chain, he says, and threaten reproduction and animal behavior.

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