Amazon's mysterious black earth earth: Soil found along region riverbanks; Rich in nutrients, stores more carbon

Tom Miles

Amazon's mysterious black earth: Soil found along region riverbanks; Rich in nutrients, stores more carbon, February 25, 2006

Above ground, rainforests like the Amazon basin flourish as biological hot spots with exuberant growth and a riot of plant and animal species.

But the red and yellow soils below are notoriously poor in nutrients and organic matter. Once the lush vegetation is cleared, the heavy rains and tropical sun quickly decompose even that small reservoir.

Except not in thousands of patches dotted along the Amazon River and its tributaries, where dark, friable soil extends metres deep, fertile in nutrients and organic material. In total, an area the size of France may be covered by this Indian black earth, called terra preta do Indio in Portuguese.

"The textbooks say it shouldn't be there. That's justification enough for me to explore why it is there," says Johannes Lehmann, a Cornell University professor specializing in the chemistry and geology of soils.

Lehmann is one of a small band of researchers in the U.S., Europe and Brazil who are deciphering the mysteries of terra preta after the phenomenon was "discovered" for the third time a decade ago. At a major scientific meeting here last week, they explained that the black earth of the Amazon is exciting for widely disparate scientific reasons:

1) Archaeology and anthropology. Terra preta typically is stuffed full of shards of sophisticated pottery discarded as much as 2,000 years ago by an advanced civilization whose existence wasn't even suspected.

2)Sustainable agriculture. Unlike the typical rainforest soils, the black earth can be worked for years, with minimal fertilization. Yet, it is essentially man-made, created by community activities that could be reproduced today by industrial means.

3)Climate change. The key step in producing terra preta traps large amounts of carbon in the soil, avoiding the release of carbon dioxide, the leading greenhouse gas from human activities. In addition, lab tests demonstrate that black earth also gives off less than regular soils of two other atmospheric bad actors: methane and nitrous oxide.

The crucial step is a method of weed control and land clearing called "cool burning" or slash-and-char, to distinguish it from slash-and-burn, widely used in tropical regions and widely condemned.

In slash-and-burn, dry brush and grass are burned in open fires, spewing vast quantities of carbon dioxide into the atmosphere and leaving only small amounts of nutrients in the ash that's then dug into the ground.

By contrast, slash-and-char involves burning wet vegetation, so it smoulders underneath a layer of dirt and straw. Robbed of oxygen, the fire only partly burns any wood or stalks, leaving most as tiny chunks of charcoal. This bio-char is turned into the soil.

Only 7 per cent of the carbon content of vegetation gets transferred into the soil by the slash-and-burn approach, Lehmann told a news conference at the annual meeting of the American Association for the Advancement of Science. The slash-and-char method transfers almost half the carbon and most of the nutrients like phosphorus and nitrogen.

Why people living on hills overlooking many rivers in Brazil two millennia ago devised this approach is still a subject of debate, as is how they added all the extra organic content and ensured the soil was teeming with beneficial bugs and other micro-organisms.

Because many patches are about half a hectare in size and loaded with pottery artifacts, University of São Paulo archaeologist Eduardo Neves favours the idea of very big communal backyard compost piles. Successive generations could have swept food refuse — especially fish and animal bones — from their dwellings and then added human and animal excrement.

"I don't think the people ever used this soil for agriculture. They may have grown some medicinal plants in a house garden," Neves says.

But geographer William Woods, of the University of Kansas, disagrees.

In addition to smallish tracts of black earth tracts located right in settlements, Woods has also found a second kind of terra preta. These can cover hundreds of hectares and surround settlements, just where you'd expect to find fields for crops. This soil contains no artefacts, has a slightly lower nutrient level and isn't as dark in colour, more a blending of black-and-white.

"I'm calling it terra mulata, deliberately," Woods says, using the Portuguese word for mulatto.

A modern technology called low-temperature pyrolysis can produce bio-char on an industrial scale. Using wood or agricultural wastes for fuel, pyrolysis would generate heat for electricity while also actually reducing net emissions of carbon dioxide since the organic matter would otherwise decompose.

"This might be the beginning of a bio-char revolution," Lehmann says.

Yet, this particular revolution could have come much earlier. In 1903, a German geologist published an account of terra preta, noting that farming this dark earth would feed many people.

"Apparently no one read his book," Woods says.

Even further back, Cornell professor Charles Hartt led an expedition of biologists to the Amazon River basin in 1874. He discovered supporters of the Confederate side in the U.S. Civil War who had selected the super-fertile terra preta locales for their sugar plantations. No one paid any attention to his account, either.