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Tropical rainforest nutrients linked to global carbon dioxide levels

June 21, 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.

National Science Foundation