Although nearly 40 years have passed since Brazil banned slash-and-burn practices in its Atlantic Forest, the destruction lingers. New research reveals that charred plant material is leaching out of the soil and into rivers, eventually making its way to the ocean. So much of this “black carbon” is entering the marine ecosystem that it could be hurting ocean life, although further tests will be needed to confirm this possibility.
People have used fire to shape Earth’s vegetation for millennia. In Brazil’s Atlantic Forest, Europeans began burning trees to make way for settlements and agriculture in the 16th century. What once blanketed 1.3 million square kilometers and ranked as one of the world’s largest tropical forests had shrunk to 8% of its former size by 1973, when protective laws were put in place.
But that’s not the end of the story, according to researchers led by Carlos Eduardo de Rezende, an aquatic biogeochemist at the State University of Norte Fluminense in Rio de Janeiro, and Thorsten Dittmar, a marine geochemist at the Max-Planck Institute for Marine Microbiology in Bremen, Germany. The team discovered high levels of black carbon in the region’s soil and in the Paraiba do Sul River, the largest river that exclusively drains the area once occupied by the Atlantic Forest. Locals still burn sugarcane each year as a preharvest way of prepping the soil, but the researchers found that this could not account for the amount of black carbon they were seeing.
To figure out how much black carbon the burned forest originally released, Rezende and colleagues looked to the neighboring Amazon forest for clues. Other studies reported black carbon rates for burning tracts of virgin Amazon rainforest, so they extrapolated those figures to match the historical range of the slashed-and-burned Atlantic Forest, which once had similar woody tree species to the Amazon. They calculated that torching the Atlantic Forest released about 200 to 500 million tonnes of black carbon. Given the material’s half-life, they estimate that it will take between 630 and 2200 years for just half of the black carbon to leach out of the region’s soils.
Black carbon typically leaves the soil when rain water carries the material into nearby rivers. From there, the rivers deposit it in the ocean. To calculate just how much carbon this process may be adding to the sea, the researchers collected river samples once every 2 weeks from 1997 to 2008. They found that the dissolved black carbon continues to be exported from the soil at approximately the same levels each year during the rainy season. More than 2700 tonnes of the former forest’s dissolved black carbon enters the ocean each year from the Pariaba do Sul River alone, the team reports online today in Nature Geoscience. Scaling their findings up, Rezende and his colleagues estimate that former forest’s total cleared area sends between 50,000 to 70,000 tonnes of dissolved black carbon to the marine environment.
“This kind of long-term time series is really essential to understanding global environmental change,” says Carrie Masiello, an Earth systems scientist at Rice University in Houston, Texas, who was not involved in the study.
What becomes of this black carbon upon entering the ocean, however, is still unknown. One of the researchers’ previous studies found black carbon in the remote depths of the oceans surrounding Antarctica, and Dittmar suspects that much of the black carbon eventually winds up in deep ocean deposits around the globe. Only further investigation will reveal how much of it makes its way from the river transport to the deep ocean, however, and how it might affect marine life, especially microbial communities that live in and feed on small organic particles.
“What’s exciting about this paper is it shows that tropical deforestation is not a small scale process,” Masiello says. Because slash-and-burn is still rampant in tropical locales around the planet, she explains, deforestation may very well be changing the way carbon cycles through the world’s oceans.