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Dark earth: How humans enriched the rainforests a.. 06 June 2011 by Fred Pearce b.. Magazine issue 2815. Subscribe and save Read full article Continue reading page |1 |2 The lushest patches of some jungles are rooted in enigmatic black soil - with unexpected origins TO FIND it, you have to go digging in rainforests. And to the untrained eye, it does not seem special at all - just a thick layer of dark earth that would not look out of place in many gardens. But these fertile dark soils are in fact very special, because despite the lushness of tropical rainforests, the soils beneath them are usually very poor and thin. Even more surprising is where this dark soil comes from. "You might expect this precious fertile resource to be found in the deep jungle, far from human settlements or farmers," says James Fraser, who has been hunting for it in Africa's rainforests. "But I go looking for dark earth round the edge of villages and ancient towns, and in traditionally farmed areas. It's usually there. And the older and larger the settlement, the more dark earth there is." Such findings are overturning some long-held ideas. Jungle farmers are usually blamed not just for cutting down trees but also for exhausting the soils. And yet the discovery of these rich soils - first in South America and now in Africa too - suggests that, whether by chance or design, many people living in rainforests farmed in a way that enhanced rather than destroyed the soils. In fact, it is becoming clear that part of what we think of as verdant virgin rainforest is actually long-abandoned farmland, enriched by the waste created by ancient humans. Enduring mystery The story starts in the Amazon. Most of the rainforest soils there are poor in nutrients because any organic matter decomposes rapidly and the nutrients get leached away by rainwater. The soils have a characteristic red colour because they contain iron oxides, and so are known as oxisols. But it has long been known that there are patches of dark, fertile soil. The richest type is known as "terra preta", from the Portuguese for "dark earth". While dark earths underlie less than 1 per cent of the Amazon rainforest, individual patches can be as large as several square kilometres and more than a metre thick. In recent decades, it has become clear that far from being natural, these soils were created by humans. The evidence is overwhelming. They are full of pottery and the charred remains of burnt wood from fires set by humans, along with organic waste from crop residues, and animal and fish bones. The black carbon from charcoal is thought to be a key ingredient of dark earths. It can persist in soils for more than a millennium, and its porous structure is thought to trap nutrients. Some archaeologists argue that such "anthropogenic dark earths" solve an enduring mystery - how the large populations that existed in parts of the Amazon in pre-Colombian times grew enough food in poor jungle soils. Now it seems that people in Africa created dark earths, too. After spending just four months in Liberia in west Africa last year Fraser, an anthropologist at University of Sussex in the UK, located more than 150 sites. The areas of dark earth form distinctive rings around existing and abandoned settlements. Close in are soils that form at active middens created by dumped waste, which are often used as kitchen gardens. Further out are old midden soils, which are planted with crops such as cocoa and kola. The dark soils are very different from the rust-red oxisols beneath the surrounding forests, says Fraser, who has returned to Liberia to continue his research. Dark soils are turning up elsewhere in Africa, too. Kojo Amanor, a land-use -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
