The 'Terra Preta' phenomenon: a model for sustainable agriculture in the humid tropics

Many soils of the lowland humid tropics are thought to be too infertile to support sustainable agriculture. However; there is strong evidence that per manent or semi-pennanent agriculture can itself create sustainably fertile soils known as 'Terra Preta' soils. These soils not only contain higher con centrations of nutrients such as nitrogen, phosphorus, potassium and calciu m, but also greater amounts of stable soil organic matter. Frequent finding s of charcoal and highly aromatic humic substances suggest that residues of incomplete combustion of organic material (black carbon) are a key factor in the persistence of soil organic matter in these soils. Our investigation s showed that 'Terra Preta' soils contained up to 70 times more black carbo n than the surrounding soils. Due to its polycyclic aromatic structure, bla ck carbon is chemically and microbially stable and persists in the environm ent over centuries. Oxidation during this time produces carboxylic groups o n the edges of the aromatic backbone, which increases its nutrient-holding capacity. We conclude that black carbon can act as a significant carbon sin k and is a key factor for sustainable acid fertile soils, especially in the humid tropics.