Forms of soil organic phosphorus at black earth sites in the Eastern Amazon

ABSTRACT Soils containing archaeological black earth (ABE) are formed by the continuous deposition of organic residue, and maintain their high fertility even after years of cultivation. The aim of this study was to characterise and quantify the forms of organic phosphorus in areas of archaeological black earth (ABE), with a view to understanding the dynamics of the element and contributing to the development of sustainable practices of land use. Samples of 10 profiles were used from Latosols, Argisols and Gleysols located in the eastern Amazon with an anthropogenic A-horizon (ABE), using adjacent, non-anthropogenic soils as reference. The samples relative to the A, transitional and B-horizons, were subjected to sequential fractionation of P in an acid base extraction, and to further physicochemical characterisation. The acid-extracted P fraction in the A1 and B-horizons predominated over the basic-extractant labile and soluble fractions in all areas, displaying the highest percentages for the inorganic form. An increase in the labile organic phosphorus content (Pol) was found in the A1-horizon, with a reduction between the A and B-horizons of 97.6%. The amount of total phosphorus (TP) was significantly higher (6,778 mg dm-3) in the A-horizons of the ABE in comparison with the soil in the reference area (168 mg dm-3). The predominance of inorganic phosphorus over organic phosphorus was found for the total fraction, while the opposite occurred with the labile fractions. Most of the total labile P is therefore accumulated in the organic fractions of the soil, and represents an active means of supplying the nutrient to plants as it mineralises.