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Scientists find that grasslands can act as 'carbon sinks'

North Carolina State University

Scientists have long known that forests sometimes act as "carbon sinks," absorbing more of the greenhouse gas carbon dioxide than they release. Now, a team of researchers has identified a mechanism through which grasslands appear to demonstrate the same property.

The research findings, published in the Jan. 11 issue of the journal Nature, may have important implications as scientists and policy-makers around the world debate ways to lower levels of atmospheric carbon dioxide, believed to be a major contributor to the greenhouse effect and global climate change.

The lead author of the paper is Dr. Shuijin Hu, assistant professor of plant pathology at North Carolina State University. The project leaders are Dr. F. Stuart Chapin III, formerly at the University of California, Berkeley, and now at the University of Alaska; Dr. Christopher B. Field of the Carnegie Institution of Washington; and Dr. Harold A. Mooney of Stanford University.

Hu explains that other scientists have proposed that grasslands can act as carbon sinks when atmospheric carbon dioxide is elevated. The research described in the Nature paper, however, identified a mechanism through which grassland soils can sequester carbon, and, in fact, found a trend toward increased soil carbon under elevated carbon dioxide conditions.

"Our data indicate that soil microbes quickly respond to changes in carbon and nitrogen availability and may play critical roles in determining the potential of grasslands - and other terrestrial ecosystems, too - to act as a carbon sink," Hu said.

Carbon sequestration occurs in an ecosystem when the amount of carbon dioxide absorbed by growing plants is greater than the amount of the gas released by decomposing plant material.

The results are from a five-year study conducted at an annual grassland on Stanford University's Jasper Ridge Biological Preserve in central California. Between 1992 and 1997, the researchers maintained two sets of open-top chambers at the grassland, one in which carbon dioxide levels were maintained at their normal level - 360 parts per million (ppm) - and one in which they were doubled to 720 ppm. In 1996 and 1997, the scientists analyzed soil core samples from each of the plots. They found a trend toward higher carbon content in the soil from plots given elevated carbon dioxide levels. Hu says the implications are that grasslands can be carbon sinks - at least for the short term. The magnitude of carbon sequestration in such a grassland is yet to be determined, he notes.

Hu says the implications are that grasslands can be carbon sinks - at least for the short term. The magnitude of carbon sequestration in such a grassland is yet to be determined, he notes.

Soil microbes appear to play a critical role in the process, Hu explains. The increased atmospheric carbon dioxide stimulates the grassland plants to grow more quickly, drawing nitrogen from the soil in the process. That results in less nitrogen available for use by the microbes in the soil, reducing the microbes' ability to decompose dead plant material. With less plant material decomposed, less carbon is released into the air as carbon dioxide. Additionally, the research indicated that under elevated carbon dioxide levels, fungi become a more dominant part of the microbial community, which is also conducive to protecting soil carbon from decomposition.

At the same time, the research indicates that the extent of carbon buildup in the grassland soil may be limited, because the lower rate of plant decomposition reduces the supply of nitrogen for additional plant growth.

Hu notes that forests are probably able to store more carbon than grasslands. "Forests may be of greater potential as a long-term carbon sink than annual grasslands because trees can sequester carbon in above-ground biomass and their roots can exploit nutrients in deeper soils," he said.

In late 2000, carbon sinks were a major sticking point during negotiations over an international climate change treaty. The talks on implementing the Kyoto Protocol broke down in November in part because of a U.S. proposal - opposed by the European Union - to allow developed nations to count carbon dioxide absorbed by forests against goals for reducing greenhouse gas emissions.

Hu says his future research will focus on how microbial-mediated processes in ecosystems respond to combined changes in atmospheric carbon dioxide concentrations, nitrogen deposition and temperature. He also hopes to investigate how the findings from the grassland research can be extrapolated to ecosystems in which carbon dioxide levels increase gradually - rather than dramatically, as in the research reported in Nature.

Hu worked on the project while he was a National Science Foundation postdoctoral fellow at the University of California, Berkeley. His advisers from UC-B - Chapin, and Dr. Mary K. Firestone - are co-authors of the Nature paper. Field and Dr. Nona R. Chiariello from the Carnegie Institution of Washington's Department of Plant Biology are also co-authors. The research was supported by grants from the National Science Foundation to the Carnegie Institution of Washington; the University of California, Berkeley; and Stanford University.

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Note to editors: The Nature abstract follows. For a copy of the full paper after the embargo date, contact Hu at (919) 515-2097 or shuijin_hu@ncsu.edu, or Kevin Potter at (919) 515-3470 or kevin_potter@ncsu.edu. For a copy before Jan. 10, call Nature at (202) 626-4956.

"Nitrogen limitation of microbial decomposition in a grassland under elevated CO2"
Authors: S. Hu, North Carolina State University and University of California, Berkeley; F.S. Chapin II, University of Alaska, Fairbanks, and University of California, Berkeley; M.K. Firestone, University of California, Berkeley; and C.B. Field and N.R. Chiariello, Carnegie Institution of Washington.
Published: Jan. 11, 2001, in Nature

Abstract: Carbon accumulation in the terrestrial biosphere could partially offset the effects of anthropogenic CO2 emissions on atmospheric CO2. The net impact of increased CO2 on the carbon balance of terrestrial ecosystems is unclear, however, because elevated CO2 effects on carbon input to soils and plant use of water and nutrients often have contrasting effects on microbial processes. Here we show suppression of microbial decomposition in an annual grassland after continuous exposure to increased CO2 for five growing seasons. The increased CO2 enhanced plant nitrogen uptake, microbial biomass carbon, and available carbon for microbes. But it reduced available soil nitrogen, exacerbated nitrogen constraints on microbes, and reduced microbial respiration per unit biomass. These results indicate that increased CO2 can alter the interaction between plants and microbes in favor of plant utilization of nitrogen, thereby slowing microbial decomposition and increasing ecosystem accumulation.

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