Slash-and-burn clearing of the rainforest leaves behind large quantities of charcoal - which over time ends up in the sea as extremely stable carbon compounds. The consequences for marine microorganisms are incalculable, according to the authors of a recently published study.
Until the middle of the last century, the Atlantic rainforest stretched across large parts of what is now Brazil - from Amazonia to the south to the current Argentinian border. Decades of deforestation followed: in the 1970s, the Atlantic rainforest was almost completely destroyed, mainly to make way for cattle pastures. German scientists led by Dr Thorsten Dittmar from the Max Planck Institute for Marine Microbiology, together with colleagues from Brazil and the USA, have now been able to prove in a study that slash-and-burn agriculture has unexpected consequences.
The slash-and-burn agriculture of the past centuries has left large quantities of charcoal in the forest floor. Over time, they dissolve, are washed out by rainwater and transported into the sea via rivers. This dissolved charcoal consists of extremely stable carbon compounds. This study - now published in the international journal Nature Geoscience - suggests that the amount of this stable form of carbon in the deep sea will increase as a result of human activity. "With unknown consequences for marine microorganisms and the global carbon cycle," says Dittmar.
The beginning of the end of the Atlantic rainforest
Humankind has been using fire to reclaim land since time immemorial. When European settlers arrived in Brazil in the 16th century, this soon became common practice there too - and the beginning of the end of the Atlantic rainforest. Over the centuries, slash-and-burn agriculture has caused the rainforest to shrink from more than 1.3 million square kilometres to just 100,000 square kilometres. This has left 200 to 500 million tonnes of charcoal in the soil.
These combustion residues stored in the soil are extremely stable complex carbon compounds. The masses of water during the rainy season wash parts of the charcoal out of the soil. Rivers carry these into the sea, where they will influence the biogeochemical cycles for centuries and millennia.
Marine research in the rainforest
Dr Thorsten Dittmar and his research group at the Max Planck Institute for Marine Microbiology have a highly sensitive mass spectrometer at their disposal. This enables them to analyse and track the complex carbon compounds. The working group is established at Oldenburg University as a branch of the Max Planck Institute and has been working on the carbon cycle, in particular dissolved organic material in the sea, for several years.
"A few years ago, we were able to detect the typical carbon compounds in the sea that are formed during the carbonisation of plants, i.e. the production of charcoal," explains Dittmar. "We suspected that one of the sources could be the burning of sugar cane plants and forest fires in Brazil." As marine researchers, this is how they came to conduct research in the rainforest and make contact with Brazilian colleagues. "They had been taking soil and water samples around the Paraiba do Sul river area for years."
When the researchers drew up the balances, they were surprised: considerably more carbon compounds were flushed out of the soil during the rainy periods than were replenished by the annual burning. "When we visualised our measurements from the Brazilian colleagues' samples over the years with the rainfall amounts and the occurrence of fires in a clear graph, the connection was obvious. These amounts of carbon can only come from the times of slash-and-burn agriculture."
The scientists were able to confirm their assumption with further experiments and findings. Large-scale slash-and-burn agriculture ended in 1973, and the burning of sugar cane plantations before harvesting, which is now practised, produces combustion residues of only 190 to 740 tonnes of carbon per year. However, the amount of carbon compounds in the Paraiba do Sul river was three to 16 times higher than the new quantities produced each year.
Use of charcoal "not a sustainable concept"
Extrapolated to the entire area of the former rainforest, the researchers estimate that 50,000 to 70,000 tonnes are transported away by rivers every year and end up in the sea. And in the laboratory, the researchers were able to wash out the highest concentrations of soluble carbon compounds from the soil samples of the former rainforest. It became increasingly obvious that the concentrations in the river could not have come from today's sugar cane plantations, as high concentrations were also detected upstream, in areas with a low density of sugar cane plantations.
"Scientists are considering using charcoal as a long-term carbon store to banish this carbon from the global cycle," says Dittmar. However, the results showed that this method could not be a sustainable concept, as this carbon would sooner or later end up in the sea and change the ecosystem there. "And we know nothing about the consequences."