UC Berkeley press release


Extra carbon stored in high CO2 conditions may be short-lived

by Pat Craig, Carnegie Institution

Berkeley -- Many scientists have predicted that elevated levels of atmospheric carbon dioxide (CO2) -- the result of fossil fuel combustion, deforestation, and other human activities -- will lead to increased photosynthesis and increased carbon storage by plants and soils. If plants and soils can indeed sequester carbon over the long term, stored in large pools with slow turnover, such as wood and soil organic matter, the terrestrial biosphere has the potential to dramatically offset the greenhouse effect.

However, new experiments conducted on grassland ecosystems suggest that while plants and soils do store extra carbon, much of that carbon is stored only temporarily -- for a few years or less. Thus, plants and their soils -- at least those found in grassland ecosystems -- may not be good candidates for long-term carbon storage.

The response of grassland soils to increased CO2 is important for two reasons. First, grasslands cover vast areas, nearly a fifth of the earth's land surface, and their soils contain a quantity of carbon comparable to the total released from fossil fuel combustion -- from the industrial revolution to the present.

Second, some of the lessons from grasslands can be extended to other ecosystems. According to Chris Field, a staff scientist at the Carnegie Institution's Department of Plant Biology and a member of the team conducting the new experiments, "grasslands and other ecosystems, including forests, share many ecological and biogeochemical functions. A grassland is, in many respects, a generic ecosystem -- a foundation for a general understanding."

In a paper published in the 7 August issue of Nature, Field, Bruce Hungate, formerly of the University of California at Berkeley and now at the Smithsonian, and their colleagues present results of elevated CO2 experiments in a natural grasslands ecosystem at Stanford's Jasper Ridge Biological Preserve. As part of a five-year study, the group determined the destiny of carbon in below-ground portions of plants in both ambient CO2 and elevated CO2 plots. They found that elevated CO2 significantly increased carbon pools in roots, surface detritus, and soil microorganisms. Especially significant was the amount of carbon allocated to the roots. Part of this below-ground carbon is released as CO2 through increased root respiration and part is released into the soil.

The scientists found that the extra carbon shipped to the roots is quickly recycled; i.e., it is quickly returned to the atmosphere as CO2, generally in less than two years. They also found that only a minor fraction of the remaining carbon becomes stabilized in soil organic matter.

Soil organic matter consists of slowly decomposing compounds that contain over two-thirds of the carbon in the terrestrial biosphere and over 90% of the carbon in most grasslands. Thus, neither grassland plants nor soils are likely to have significant capacity for storing additional carbon.

As the group writes in Nature, "...the distribution of the extra carbon, particularly the increase in carbon allocation to labile pools belowground, suggests that net C balance results from short-term CO2 enrichment experiments tend to overestimate the potential for grasslands to sequester C in soils in the long term."

Hungate, currently at the Smithsonian Institution's Environmental Research Center, worked with F. Stuart Chapin III, professor of integrative biology at UC Berkeley. Their coauthors are Elisabeth A. Holland of the Natonal Center for Atmospheric Research in Boulder, Col.; Robert B. Jackson of the University of Texas at Austin; Harold A. Mooney of Stanford; and Christopher Field of the Carnegie Institution.

The Jasper Ridge CO2 Experiment is being conducted by Chris Field, a staff member at the Carnegie Institution of Washington's Department of Plant Biology. It is supported by grants from the National Science Foundation and other institutions. The Department of Plant Biology, located on the Stanford University campus, is one of five research departments of the Carnegie Institution, a science research and education nonprofit organization founded in 1902 by Andrew Carnegie. The Department's director is Christopher Somerville. The Carnegie Institution, based in Wasington, D.C., is led by its president, Maxine F. Singer.

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