Planting trees to curb the effects of global warming is unlikely to work.
The Saleska team used huge booms above the canopy to record CO2 levels
A US-Brazilian team has found that some parts of the Amazon rainforest emit more carbon dioxide (CO2) than they absorb in very wet conditions.
Their report, published in the journal Science, says previous studies have almost certainly over-estimated how much CO2 the Amazon can take in.
And their study is backed by other work which shows newly planted trees will not grow fast enough to mop up CO2.
The Science papers are pertinent because the idea of using forests to curb global warming forms a central plank of the Kyoto Protocol, which world governments will discuss next week in Milan.
The protocol allows countries to plant new trees and conserve old forests rather than cut the amount of greenhouse gases they produce.
But the latest research undertaken over three years in the Amazon, by Scott Saleska, from Harvard University in Cambridge, Massachusetts, provides graphic new evidence that the relationship between trees and carbon dioxide is a complex one.
Saleska's study of old-growth Amazonian rainforest shows clearly that drought or other disturbances that kill trees can lead to higher levels of carbon dioxide release.
These increases in carbon loss occur during wet seasons when the dead wood breaks down, not during the dry season as has been generally found.
It is supported by another study in Science which looks at the subject of so-called carbon sinks - the idea that excess CO2 in the atmosphere can be locked up in soil organic matter and faster growing trees.
The research by Bruce Hungate, from North Arizona University, and colleagues finds the ability of trees to take in more carbon depends on many factors, including the availability of other nutrients in the soil such as nitrogen.
The team says the nutrient levels do not exist to support the level of carbon absorption for which some have been hoping.
"In a garden limited by water, a gardener would not expect a big increase in growth from adding potassium," explained co-author Jeffrey Dukes, from the Carnegie Institution.
"Similarly, plants in natural ecosystems limited by nitrogen may not grow much faster when they are exposed to increased levels of carbon dioxide.
"Plants will need more nitrogen if they're going to lock up more carbon. The models used by the [Intergovernmental Panel on Climate Change] just didn't acknowledge that to a sufficient extent."
According to another member of the team, Christopher Field: "Even with generous assumptions about future increases in biologically available nitrogen, we still couldn't find enough nitrogen to support the range of carbon storage discussed in the IPCC report."
Many environmentalists believe that politicians have run ahead of scientific understanding in giving forestry such prominence in the Kyoto Protocol.
They argue tree planting has been seized on not because it is good science, but because it is politically expedient.