Micromorphology and electron microprobe analysis of phosphorus and potassium forms of an Indian Black Earth (IBE) Anthrosol from Western Amazonia

Abstract

The Indian black earth (IBE) anthrosols (Terra Preta) overlying deep weathered kaolinitic soils of Western Amazonia represent one of the most interesting features of the Amazon Basin, with broad implications to human ecology. We studied one IBE site, in particular the chemical composition and forms of phosphate and potassium, micropedological attributes, and their ecological implications. In the IBE anthrosol, high levels of available P were due to the presence of comminuted fish and animal bone apatite. Flakes of 2 : 1 layer silicates rich in K occurred in pottery remains in the IBE, indicating that they were manufactured from neighbouring 'Várzea' soils. Amazon 'Várzea' environs were the only source of the soil material for pottery, since K-rich 2 : 1 minerals are not present in Terra Firme kaolinitic sediments. High available and total P contents of IBE were related to abundant, very small particles (5–60 μm) of Ca-P and Al-P forms, resolved by s.e.m./EDS at very high magnification. These features originated in animal bones and fish spines that have chemically altered in the soil environment. Earthworm and other biological channels exhibited high Al/P contents, due to apatite ingestion, comminution, and alteration. In transitional horizons, abundant secondary Al/P was present in biological channels, indicating intense biological activity down to 1.5 m. In deeper B horizons, apatite fragments are rare. Charcoal residues of incompletely combusted wood are widespread down to the B horizon. No spatial or concentration relationships exist between P and total free-iron in the soil.