Phosphorus is known as a "limiting nutrient" because its availability can govern the growth rate of many organisms, said Cleveland. 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 CO2 they emit, he said.
Phosphorus and many other nutrients are regularly transported around the Earth by global wind patterns, sometimes riding on huge transcontinental dust clouds, said Townsend, who also is associated with CU-Boulder's ecology and evolutionary biology department. "There is strong evidence that humans are increasing the size of these dust clouds as both land-use patterns and climate change, which in turn can change 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. "Human activity has changed the availability of nitrogen all over the world, especially in the last 50 years," he said. "This study was surprising because of the large effect both of these nutrients had on the release of CO2 to the atmosphere."
About three-quarters of anthropogenic emissions of CO2 to the atmosphere during the past 20 years are thought to be due to fossil-fuel burning, the researchers said. The rest is predominantly due to land-use changes like deforestation.
The new 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, said Cleveland. Soil respiration was measured using plastic tubes in the ground running into vented, closed chambers.
Source:University of Colorado at Boulder