Pedogenesis and pre-Colombian land use of “Terra Preta Anthrosols” (“Indian black earth”) of Western Amazonia

Abstract

The “Terra Preta de Índio” (Indian black earth) or Terra Preta of Western Amazonia is a thick, dark-coloured, anthropic epipedon, usually rich in nutrients. It occurs mostly at the fringes of the Terra Firme, along the Amazon river banks, overlying deep strongly weathered soils. We studied selected chemical, physical and mineralogical properties of seven soils, ranging from the Tertiary Plateau down to the Amazon river floodplain in the Iranduba district, near Manaus, Amazonas, Brazil. Three Terra Preta soils were classified as anthropogenic (Anthropic Xanthic Kandiudult, Anthropic Xanthic Kandiudox and Anthropic Dystropepts). Chemical, mineralogical and micropedological attributes, such as high total and available P and mica flakes in pottery remains found in the Terra Preta, indicate that the origin of soil materials of these anthrosols is closely associated with neighbouring floodplain (várzea) soils and sediments. Amazon floodplain soils were the source of soil material for pottery, since 2:1 clay minerals are not found in the Tertiary Plateau (Terra Firme) sediments. Total and available P contents of Terra Preta are associated with microfragments of bone apatite with high P and Ca values. In the anthrosols under cultivation, these values are less, with increasing Al release suggesting acidification and losses of nutrients. Large amounts of Mn and Zn occur in the anthrosols and in high-fertility floodplain soils. It is unlikely that well-drained Tertiary Plateau (Terra Firme) area far away from lowland Amazon floodplain soils could develop high-fertility Terra Preta on the top of nutrient-poor Oxisols (Latosols). The suggested model of Terra Preta formation between the Tertiary Plateau and nutrient-rich Amazon floodplain does not extend to other nutrient-poor, smaller, floodplains draining the deep-weathered interfluves of the Brazilian Uplands. This raises reservations about estimates of precolonial human population densities for the Amazon basin as a whole, assuming the widespread occurrence of such anthrosols farther inland.