March
12, 2007 Researchers at the The scientists said
these findings add a new perspective on
the capacity of Earth's soils to store carbon, and a measure of caution
suggesting that elevated carbon dioxide, by altering microbial
communities, may
turn the soil from a potential carbon sink into a carbon source. This
could
offset some of the gains in carbon storage in plant biomass due to
increased
growth at elevated carbon dioxide. Previous studies
(including the present study) have shown
that plants will respond to higher carbon dioxide by increasing growth
and
taking up much of the excess carbon. This has led some to speculate
that plants
may be able to mitigate increases in atmospheric carbon dioxide and
that soils,
which represent the largest and most stable terrestrial carbon pool,
also may
serve as a sink for excess carbon. During the course of
their study, Smithsonian scientists
found that the amount of carbon in the ecosystem as a whole increased.
However,
they also saw a consistent loss in soil carbon under high carbon
dioxide
conditions. The carbon dioxide loss from soils offset about 52 percent
of the
additional carbon that had accumulated in the plants above ground and
in the
roots. "We were surprised
to find that these soils were losing
soil carbon despite the fact that there was more plant growth," said
Patrick Megonigal, a microbial ecologist at SERC and one of the study's
authors. "We thought that higher plant growth at elevated carbon
dioxide
would either add more carbon to soils, or at least leave it the same.
We now
need to consider a third possibility-the carbon already in soils will
end up
back in the atmosphere as a greenhouse gas." The study will be
published this week in Proceedings of the
National Academy of
Sciences. Working at a
long-term Smithsonian experimental carbon
dioxide site in a Florida scrub oak ecosystem, the researchers compared
core
samples from test plots that had been exposed to six years of elevated
carbon
dioxide and core samples from plots exposed to ambient carbon dioxide.
They
also performed laboratory experiments on soils from both elevated and
ambient
plots to understand microbial composition and activity within each type
of
soil. Their study reveals
that added carbon dioxide has a
so-called "priming effect," stimulating certain microbes and
increasing decomposition. Soils exposed to the elevated carbon dioxide
had
higher relative abundances of fungi and higher activities of a soil
carbon-degrading enzyme. As the fungi and enzymes decompose the organic
matter
in the soil, they free up stored carbon and release it through
respiration as carbon
dioxide. With the priming effect of added carbon dioxide, more soil
decomposition results in higher respiration rates, an overall loss of
carbon
and an increase in the release of carbon dioxide from the soil.
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