Amazon River Exhales Rainforest Carbon: Storage Doesn't Happen at Sea

The Amazon rainforest is known for its ability to store vast quantities of carbon. Its trees absorb carbon dioxide from the atmosphere while exhaling oxygen. Yet when plants die, they eventually decompose and the carbon is washed away. Now, researchers have discovered that while the Amazon Rainforest stores carbon, the Amazon River releases it in high amounts.

Researchers have been able to show in the past that rivers "exhale" huge amounts of carbon dioxide. Yet they haven't been able to figure out how. Bark and stems were thought to be too tough for river bacteria to digest. In order to examine how large rivers exude so much carbon dioxide, the researchers looked at the longest river in the world: the Amazon River.

Tough lignin forms the main part of woody tissue and is the second most common component of terrestrial plants. In the past, researchers believed that lignin was transported by rivers to the sea, where it was eventually buried on the seafloor. In order to see whether or not this was correct, the researchers had to examine the location where the river met the sea: the mouth of the Amazon.

The researchers used flat-bottomed boats to traverse the mouth of the river. The water itself was rich with sediment--so much so that it looked like chocolate milk. Tides raised and lowered the ocean by 30 feet while winds blew at up to 35 miles per hour. Under these conditions, the scientists collected river water samples during all four seasons. They then compared the original samples with ones left to sit up to a week at river temperatures.

"Rivers were once thought of as passive pipes," said Jeffrey Richey, co-author of the new study, in a news release. "This shows they're more like metabolic hotspots."

In fact, the researchers found that about 45 percent of the Amazon's lignin breaks down in soils, 55 percent breaks down in the river system and a mere 5 percent reaches the ocean where it either breaks down or sinks to the ocean floor. The findings completely overturn previous theories about carbon being stored on the ocean floor.

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

The findings could have huge implications for carbon cycle models. In addition, it reveals an entire community of bacteria that's adept at breaking down lignin for its use.

The findings are published in the journal Nature Geoscience.