Maria de Lourdes P. Ruivo, MPEG, Belém, Brazil, Maria de L. Oliveira, UEPa, Belém, Brazil, and Dirse Kern, Museu paraense emílio Goldi, Av.Perimetral, 1901, Belém, Brazil.
Many aspects of the origin of Amazonian Dark Earths (ADE) are still unclear. Knowledge of past land-use practices of Amazonian inhabitants and the induced soil processes that may have caused the sustainable fertility of ADE is very important to the study of these soils. A group of researchers at the Museum Paraense Emílio Goeldi in Brazil have compared soils from several sites in Amazonia. The sample ADE soils were collected in Pará State: Caxiunã (0-10 and 10-20 depth), Santarém/Belterra (0-20 depth), Juruti/Tabatinga (0-5; 5-10 and 10-20 depth), and in Amazonas State: Manaus (0-5; 5-10 and 10-20 depth). These soils had predominantly sandy textures. Also, the soils demonstrated a similar mineralogy (kaolinite and quartz) in the soil matrix and high fertility. For comparison of the microbial population the work included results from a Yellow Latosol from Roraima State (Yanomami/Homoxi Area) under sandy and clay soils and the site of the LBA Project - Esecaflor in Caxuanã. In comparison with the microbiota of the Yellow Latosol the Dark Earths showed higher diversity, including a distinct higher number of the fungal and bacteria genuses, a lot of actinomycetes, and much occurrence of the organic substances and micelles distribution. These results show that ADE soils from Caxiuanã, Santarém and Manaus compared with at soils of the Roraima and Juruti. These organisms, important decomposers of organic matter, in the Dark Earths have more occurrences and more production of the organic substances and micelles. The identification tests showed the presence of gram-negative bacteria of the Achromobacter, Flavobacterium, Nitrobacter, Nitrosomonas, Pseudomonas, Escherichia, Enterobacter and Celovibrio genera; and gram-positive bacteria of the Arthrobacter, Bacillus, Micrococos, Streptomyces and Sarcina genera. Among these genera we can find cellulolythic, humic acid producers, lignine decomposers, starch decomposers and nitrogen producers. The fungi that were identified were from the genera: Rhizopus, Rhizomucor, Trichoderma, Cladosporium, Penicillium, Mucor, Aspergillus, Fusarium and Chaetomium. The analysis of Amazon anthropogenic soils indicate that alterations by human actions, such as the incorporation of organic residues and the effects of fire in the superficial horizon influenced some of the chemical (carbon, phosphorus,..) and physical (aggregation,..) characteristics. Studies of the soil micromorphology, chemical and biological show that the high fertility of anthropogenic soils are the result of a favorable combination of mineral and organic components, making these soils highly enriched in exchangeable forms. The organo-mineral stabilization of soil organic matter showed that is mainly stabilized via chemi-sorption to mineral surfaces, as well as physical stabilization via entrapment into interior of aggregates.