June 22, 2006 — Extra amounts of key
nutrients in tropical rain forest soils cause them to release more
carbon dioxide into the atmosphere, according to research conducted by
scientists at the University of Colorado (CU) – Boulder.
Results of the research, conducted by Cory Cleveland and CU
scientist Alan Townsend, are published this week in the journal
Proceedings of the National Academy of Sciences.
"The large change in carbon dioxide emissions from tropical forest
soils due to soil nutrients is a new dimension in understanding these
important ecosystems," said Martyn Caldwell, program director in the
National Science Foundation’s (NSF) Division of Environmental Biology,
which funded the research.
"Tropical rainforests have received considerable attention related
to the global carbon balance, but that has largely revolved around
rainforest vegetation and its ability to ‘take up’ carbon dioxide,"
said Caldwell. "This is a new look at tropical rainforests and their
relationship to carbon dioxide levels on Earth."
The study showed that when phosphorus or nitrogen — which occur
naturally in rain forest soils — were added to forest plots in Costa
Rica, they caused an increase in carbon dioxide emissions to the
atmosphere by about 20 percent annually, said Cleveland.
"The study is important because human activities are changing the
amount of phosphorus and nitrogen in ecosystems all over the globe,
including the tropics," Cleveland said. "Tropical rain forests play a
dominant role on Earth in regulating atmospheric carbon dioxide."
One big question, said Cleveland, "is how tropical rain forests are
responding to climate change. What we have demonstrated is that even
small changes in nutrients could have a profound impact on the release
of carbon dioxide from tropical forest soils."
The study, which took place in 2004 and 2005 in Costa Rica’s Golfo
Dulce Forest Reserve, included a series of 25 meter-square plots that
were fertilized with phosphorus, nitrogen, or a combination of the two.
Tropical forests contain up to 40 percent of the carbon stored on
Earth’s continents and account for at least one-third of the annual
exchange of carbon dioxide between the biosphere and the atmosphere,
said Cleveland. Earth’s soils are believed to store several times more
carbon than all the planet’s vegetation.
"This is the first time anyone has taken a close look at how changes
in key nutrients may alter soil carbon dioxide emissions in tropical
forests," said Cleveland. "Processes in the tropics affect what is
happening around the globe, so this study has some big implications."
Phosphorus is known as a "limiting nutrient" because its
availability can govern the growth rate of many organisms. While
slash-and-burn agriculture in the tropics often reduces soil phosphorus
in the long run, the practice can initially make more phosphorus
available to tropical soil microbes, increasing their metabolism and
the amounts of carbon dioxide they emit.
Phosphorus and many other nutrients are regularly transported around
the Earth by global wind patterns, sometimes riding on huge
transcontinental dust clouds, said Townsend. "There is strong evidence
that humans are increasing the size of these dust clouds as changes
occur in both land-use patterns and climate, which in turn can alter
the availability of nutrients to forests," he said.
Nitrogen pollution also is increasing around the world, including in
tropical forests, a result of fossil-fuel combustion and crop
fertilization activities, said Townsend.
Source : National Science Foundation