Carbon Dioxide Fertilization is Neither Boon Nor Bust

[Guest Ron Walli]

Trees absorb more carbon dioxide when the amount in the atmosphere is higher, but the increase is unlikely to offset the higher levels of carbon dioxide (CO2), according to results from large-scale experiments conducted at Oak Ridge National Laboratory and elsewhere.

 

“Some people have used carbon dioxide fertilization to argue that this is a boon of the fossil fuel era and that it will lead to greater agricultural yields and carbon sinks,” said Richard Norby of the Department of Energy’s ORNL. “Some recent experiments, however, have suggested that there will be no lasting effect of carbon dioxide fertilization. As is often the case, the truth may lie in between.”

 

Norby is among several scientists participating in a panel discussion titled “CO2 Fertilization: Boon or Bust?” Feb. 16 at the American Association for the Advancement of Science annual meeting in Seattle.

 

For the last six years, Norby and colleagues at ORNL have examined the responses to elevated carbon dioxide levels in a stand of sweetgum trees a few miles from ORNL. The experiment consisted of pumping tons of carbon dioxide into the plots, raising the concentration of carbon dioxide in the tree stand from the ambient level of about 370 parts per million (ppm) to 550 ppm, and studying the effects.

 

One of the goals of the project, called the Free-Air CO2 Enrichment (FACE) experiment, is to examine how forests will respond to the increasing level of carbon dioxide in the atmosphere and whether it will alter the development of the greenhouse effect. Among the findings is that young trees and other green plants respond favorably to elevated concentrations of atmospheric carbon dioxide. The relevance to the role of the terrestrial biosphere in the global carbon cycle, however, has long been subject to debate.

 

There are lots of questions, including: Do the responses of seedlings and young trees predict those of large mature trees in a closed forest? Does the response diminish over time? Can increased carbon assimilation by the global forest prevent the continued increase in atmospheric carbon dioxide concentration?

 

The long life and large size of trees make direct assessment of forest responses to carbon dioxide enrichment difficult. FACE and other similar research, however, allow for multi-year studies of forest stands and enable researchers to get a peek at the future with respect to atmospheric carbon dioxide.

 

In every year since the FACE project began, net primary productivity, which is the total amount of carbon dioxide fixed into organic matter such as leaves, stems and roots, has been higher in plots given extra carbon dioxide. The average increase has been 24 percent, and there is no indication that the increase will not continue. But, Norby notes, while his colleagues have observed a sustained increase in leaf photosynthesis, the response to carbon dioxide fertilization would not be apparent if only above-ground growth were measured. Wood production increased significantly during only the first year of treatment.

 

While Norby and colleagues have learned a great deal about above-ground allocation of carbon dioxide, in recent years they have focused their efforts on impacts on fine roots and soil sequestration of carbon dioxide. Fine root production has increased substantially in response to elevated carbon dioxide.

 

Fine roots are important for water and nutrient uptake, but they have a short life and their carbon returns to the soil within a year. Initial results suggest that the increase in carbon supply to fine roots has increased the carbon content of the soil. Norby cautions, however, that the positive effect of carbon dioxide fertilization is insufficient to halt the rising level of atmospheric carbon dioxide.

 

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Contact:

 

Ron Walli

DOE/Oak Ridge National Laboratory

865-576-0226