Until recently people believed much of the rain forest’s carbon floated down the Amazon River and ended up deep in the ocean. 91Ě˝»¨ research showed a decade ago that rivers exhale huge amounts of carbon dioxide – though left open the question of how that was possible, since bark and stems were thought to be too tough for river bacteria to digest.

A published this week in resolves the conundrum, proving that woody plant matter is almost completely digested by bacteria living in the Amazon River, and that this tough stuff plays a major part in fueling the river’s breath.

The finding has implications for global carbon models, and for the ecology of the Amazon and the world’s other rivers.

“People thought this was one of the components that just got dumped into the ocean,” said first author , a 91Ě˝»¨doctoral student in oceanography. “We’ve found that terrestrial carbon is respired and basically turned into carbon dioxide as it travels down the river.”

Tough lignin, which helps form the main part of woody tissue, is the second most common component of terrestrial plants. Scientists believed that much of it got buried on the seafloor to stay there for centuries or millennia. The new paper shows river bacteria break it down within two weeks, and that just 5 percent of the Amazon rainforest’s carbon ever reaches the ocean.

“Rivers were once thought of as passive pipes,” said co-author , a 91Ě˝»¨professor of oceanography. “This shows they’re more like metabolic hotspots.”

When previous research showed how much carbon dioxide was outgassing from rivers, scientists knew it didn’t add up. They speculated there might be some unknown, short-lived carbon source that freshwater bacteria could turn into carbon dioxide.

“The fact that lignin is proving to be this metabolically active is a big surprise,” Richey said. “It’s a mechanism for the rivers’ role in the global carbon cycle – it’s the food for the river breath.”

The Amazon alone discharges about one-fifth of the world’s freshwater and plays a large role in global processes, but it also serves as a test bed for natural river ecosystems.

Richey and his collaborators have studied the Amazon River for more than three decades. Earlier research took place more than 500 miles upstream. This time the U.S. and Brazilian team sought to understand the connection between the river and ocean, which meant working at the mouth of the world’s largest river – a treacherous study site.

“There’s a reason that no one’s really studied in this area,” Ward said. “Pulling it off has been quite a challenge. It’s a humongous, sloppy piece of water.”

The team used flat-bottomed boats to traverse the three river mouths, each so wide that you cannot see land, in water so rich with sediment that it looks like chocolate milk. Tides raise the ocean by 30 feet, reversing the flow of freshwater at the river mouth, and winds blow at up to 35 mph.

Under these conditions, Ward collected river water samples in all four seasons. He compared the original samples with ones left to sit for up to a week at river temperatures. Back at the UW, he used newly developed techniques to scan the samples for some 100 compounds, covering 95 percent of all plant-based lignin. Previous techniques could identify only 1 percent of the plant-based carbon in the water.

Based on the results, the authors estimate that about 40 percent of the Amazon’s lignin breaks down in soils, 55 percent breaks down in the river system, and 5 percent reaches the ocean, where it may break down or sink to the ocean floor.

“People had just assumed, ‘Well, it’s not energetically feasible for an organism to break lignin apart, so why would they?'” Ward said. “We’re thinking that as rain falls over the land it’s taking with it these lignin compounds, but it’s also taking with it the bacterial community that’s really good at eating the lignin.”

The research was supported by the , the and the . Co-authors are Richard Keil at the UW; Patricia Medeiros and Patricia Yager at the University of Georgia; Daimio Brito and Alan Cunha at the Federal University of Amap in Brazil; Thorsten Dittmar at Carl von Ossietzky University in Germany; and Alex Krusche at University of SĂŁo Paulo in Brazil.

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For more information, contact Ward at nickward@uw.edu or 858-531-1558 and Richey at jrichey@uw.edu or 206-368-1906.

The rivers of South America’s Amazon basin are “breathing” far harder — cycling the greenhouse gas carbon dioxide more quickly — than anyone realized.

Most of the carbon being exhaled — or outgassed — as carbon dioxide from Amazonian rivers and wetlands has spent a mere 5 years sequestered in the trees, other plants and soils of the surrounding landscape, U.S. and Brazilian researchers report in the July 28 issue of Nature.

Some evidence exists that regions such as the nearly 2.4 million-square-mile Amazon River basin — where tropical forests rapidly gulp carbon dioxide during photosynthesis — are serving as carbon sinks, actively removing carbon from the atmosphere and holding onto it in wood and soils for decades, even centuries, says Emilio Mayorga, 91Ě˝»¨ oceanographer and lead author of the Nature piece with Anthony Aufdenkampe of the Stroud Water Research Center in Pennsylvania.

As policy makers turn increasingly to carbon-credit trading as a means of grappling with the impacts of human-induced climate change, knowing how much carbon can be stored — and where and for how long — is critical, the authors say.

“Our results were surprising because those who’ve previously made measurements found carbon in the rivers that came from the surrounding forests to be 40 to more than 1,000 years old,” Aufdenkampe says. “They assumed that the return of this forest carbon to the atmosphere must be a slow process that offered at least temporary respite from greenhouse effects.

“As part of the largest radiocarbon age survey ever for a single watershed, we show that the enormous amount of carbon dioxide silently being returned to the atmosphere is far younger than carbon being carried downstream,” he said. “Previous studies failed to detect the rapid recycling of forest carbon because they never dated the invisible greenhouse gas as it is literally exhaled by the river organisms.”

“River breath is much deeper and faster than anyone realized,” says Jeff Richey, 91Ě˝»¨oceanographer and another co-author.

Carbon is carried by rains and groundwater into waterways from soils, decomposing woody debris, leaf litter and other organic matter. Once in waterways it is chewed up by microorganisms, insects and fish. The carbon dioxide they generate quickly returns to the atmosphere, some 500 million tons a year, an amount equal to what may be absorbed into long-term storage as wood and soils each year by the Amazonian rainforest.

“Having established that the amount of carbon outgassing is much greater than anyone imagined, the issue then becomes, where does it come from,” Mayorga says. “If it’s young, that indicates the carbon pool is dynamic, which could make the system much more reactive to deforestation and climate change.”

For example, data from a region of active deforestation in the southern Amazon already shows that the carbon leaving rivers has an identifiable isotopic signature of pasture grasses.

“You’re changing the land use, changing vegetation and other conditions. In terms of what’s being respired, the system is responding fairly quickly,” Mayorga says. “Human and natural systems, in turn, will be impacted.”

No previous tropical study has used both radioactive carbon-14 and stable carbon-13 isotopes to address these questions. Funding from the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory made the analysis by Mayorga and Aufdenkampe possible. The samples were collected by Richey’s research group and Brazilian scientists on expeditions going back as far as 1991 that were funded by the National Science Foundation, National Aeronautics and Space Administration and the Research Support Foundation for the State of San Paulo (FAPESP), Brazil.

Other co-authors are Paul Quay and the late John Hedges, both 91Ě˝»¨oceanographers; Caroline Masiello of Rice University; Alex Krusche of the University of SĂŁo Paulo, Brazil; and Thomas Brown of the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory.

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For more information:

Mayorga, (206) 295-5778, emiliom@u.washington.edu

Aufdenkampe, (610) 268-2153 ext. 263, aufdenkampe@stroudcenter.org; Aufdenkampe will be away from his office July 25-29 but reachable through Sandra Hines (contact info at start of release)

U.S. and Brazilian researchers say the amount of carbon dioxide coming off streams, rivers and flooded areas of the world’s tropical forests is triple that of some currently accepted estimates, meaning such forests aren’t the carbon sponges some scientists believe.

The new total of 900 million metric tons (about 2 trillion pounds) of carbon given off globally by tropical-forest waterways is comparable to, for example, about a fifth of the carbon dioxide generated each year by human activities such as deforestation and burning fossil fuels.

It matches the amount that some leading modelers have speculated was missing from tropical forests when they’ve tried to tally up the worldwide movement of carbon, according to Jeffrey Richey, professor of oceanography at the 91Ě˝»¨ and lead author describing the new calculations in the April 11 issue of the journal Nature.

The new calculations are possible because of a NASA-sponsored program giving the researchers access to radar imagery of a kind available only in the last few years. That data was then used in conjunction with river measurements from 13 National Science Foundation expeditions to the Amazon in the 1980s and ’90s. The expeditions also were supported by the Brazilian government.

Global-carbon models come up short when using numbers from efforts in recent years to directly measure carbon dioxide in the air in, and just above, tropical forests. In contrast, Richey and his colleges said that with the new numbers for waterways, the amount of carbon dioxide is actually about even, with as much being absorbed as is released by tropical forests.

Apart from technical problems that may be occurring while trying to directly measure carbon dioxide in tropical forests, “the land-water connection appears to be far more important than anyone thought,” Richey said. He and his group determined, for instance, that 20 percent of the carbon being released from streams and rivers was from aquatic processes, while the other 80 percent originated in the forest.

There the trees and other vegetation draw carbon dioxide out of the air during photosynthesis, using the carbon to create wood cells including roots and releasing oxygen back to the atmosphere. The carbon that ends up in streams and rivers is “swept” there by rains and floods that draw carbon out of the soil and carry woody debris, leaf litter and other matter downstream.

As the material decomposes and is eaten by organisms including insects and fish, and as river conditions change, carbon dioxide is release, or degassed, from surface waters. The resulting “river breath,” as Richey termed it, turns out to be a significant downstream movement of carbon fixed by the forest.

And, although it’s long been assumed that most carbon making its way into waterways just ended up in the ocean, Richey and his co-authors found 13 times more carbon is degassed than reaches the sea.

“If you want to know where carbon from today’s tropical forest goes, look a thousand kilometers downstream in 20 or 30 years,” he said.

Co-authors on the paper are John Melack and Laura Hess of the University of California, Santa Barbara; Anthony Aufdenkampe, who just completed his doctorate with Richey at the 91Ě˝»¨and is now at the University of South Carolina; and Victoria Ballester of the Centro de Energia Nuclear na Agricultura, SĂŁo Paulo, Brazil.

Their calculations are based on water chemistry measurements made by groups led by Richey in the Amazon basin, information that couldn’t be scaled up to the degree now possible because there was no way to know the extent of water in the forest over the course of a year.

That became possible with a new kind of radar, a Japanese system to which the researchers had access as part of the National Aeronautics and Space Administration’s Large-Scale Biosphere Atmosphere project. Used from satellites, the radar can measure the extent of water. The study quadrant was an area of 1.77 million square kilometers (690,000 square miles), which is more than four times the size of California.

Water covers between 8 percent and 20 percent of the study area, depending on the time of year and amount of flooding.

 
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• Richey can be reached at jrichey@u.washington.edu. He has been in Buenos Aires and returns to his office in Seattle the afternoon of April 11, 206-543-7339.
• Contact Sandra Hines for information about reaching other co-authors.
• Scott Denning, assistant professor at Colorado State University, is a modeler interested in these new calculations and is willing to comment on the paper. He’s busy with meetings in Sarasota, Fla., through April 12 but he is checking his e-mail, denning@atmos.colostate.edu, twice a day, and will be checking for phone messages left at 970-491-6936.

Images are at:

In both images, U.S. and Brazilian researchers use floating chambers to measure the flux of carbon dioxide from waters of the forest in the central Amazon basin. Photo credit: 91Ě˝»¨