Dark Earth

Black Gold of the Amazon
Secrets of that ancient “dark earth” could help solve the Amazon’s ecological problems today.

 
Before the Europeans arrived, this peninsula in the heart of the Amazon was home to communities with roads, irrigation, agriculture, soil management, ceramics, and extended trade. These civilizations were as complex as the southwestern Native American cultures that inhabited ChacoCanyon and Mesa Verde. But due to the scarcity of stone in the Amazon, the people built with wood, and over time the structures disintegrated, leaving little evidence of the culture.
 
One legacy remains, however: their soil. Terra preta de Indio—Portuguese for “Indian black earth”—is prized among local farmers, and it is a direct contribution of the vanished Amazonian cultures.
 
While most Amazonian earth is notoriously nutrient poor, yellowish, sterile, and unscented, there are extensive patches of soil that are mysteriously dark, moist, fragrant, and filled with insects, microbial life, and organic matter. Scholars have come to realize that by devising a way to enrich the soil, the early inhabitants of the Amazon managed to create a foundation for agriculture-based settlements much more populous than scholars had thought possible.
 
Petersen had called the soil a gift from the past; he believed that studying it would reveal the region’s past cultures in a new, much more complex light. At the time of his death, he and his colleagues had been developing a workshop for teachers in the region on the science and archaeology surrounding terra preta.
 
The discovery held meaning for more than archaeologists, however. Figuring out the composition of dark earth and how it was formed offers a way to improve soil fertility for today’s small farmers and also curb carbon emissions from the fires that these farmers set to clear the forest.
 
Yet after Petersen’s murder the project was shut down, and those who had gathered for both the workshop and the August field season were sent home. Had the Amazon turned too dangerous for its inhabitants to learn their own ancestors’ secrets?
 
Although the central Amazon may not have been the heart of the massive empire that Donald Lathrap envisioned, these cultural traces suggest that the Amazonians managed to flourish in a formidable environment—and terra preta may have been an important component of that success.

As thrilling as this evidence is to archaeologists, it may also have very practical importance as a modern weapon against some of our most urgent ecological problems. Soil scientist Johannes Lehmann of
CornellUniversity believes that the mysterious dark earth holds clues to creating sustainable farming practices and even to combating global warming.
 
Lehmann explains that nutrients from plant and animal remains—like nitrogen, phosphate, and potassium—bind to charcoal or biochar, drastically reducing how much is washed away by the constant rains. It is a gradual process that begins with the charcoal breaking down in the soil over time.
 
Tiny pores in the charcoal, along with changes in its chemistry, provide more surfaces for nutrients to adhere to, which in turn encourages microorganisms to colonize the soil. “With a handful of biochar you can keep many more nutrients in the soil than with a handful of mulch or compost. It is like mopping up nutrients with a magnet that looks like a sponge—that is, it has high surface area like a sponge but can attract a thin layer of material like a magnet,” Lehmann says.
 
Although the complete transformation of soil ingredients into true terra preta may take several years, soil scientists have shown that the mixture can have immediate benefits when added to nutrient-poor soils. 

Experiments outside of
Manaus have shown that the yield in plots treated with charcoal and fertilizer (whichs contains plant nutrients), a mix similar in composition to terra preta, was double the yield of plots treated with fertilizer alone.
 
What does this mean for fighting global warming? Brazil is the world’s eighth-largest emitter of greenhouse gases, and most of those emissions come not from industry and cars but from loggers, ranchers, and farmers burning the forest. Just substituting slash-and-char for slash-and-burn could reduce human-produced carbon emissions in the Amazon by 12 percent.
 
Even better, burning agricultural wastes in a controlled process called pyrolysis can convert wood and other organic waste into useful volatile gases, heat, electricity, and bio-oil. The process is win-win: Burning the biomass produces substantial amounts of rich biochar from waste material like peanut shells and rice husks, and mixing this biochar into soil could more than offset the carbon that is emitted into the atmosphere—not only during the burning process itself, but also when the derived fuels are used.
 
“You wouldn’t just be carbon neutral, you would be carbon negative, drawing carbon dioxide out of the atmosphere, producing energy and improving the climate in the process,” Lehmann says. Through workshops with other scientists, he is trying to spread the message about terra preta worldwide, carrying on where Petersen left off.

Kemo D. (a.k.a. no.7)
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