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Concepts of phosphorus transformations in soils and landscapes have largely been developed in temperate regions on moderate slopes. Little is known about the P dynamics and availability in semi-arid tropical soils where rainfall extremes cause limited but periodically intensive leaching and erosion. We therefore studied the different forms of inorganic P (Pi) and organic P (Po) as related to mineralogy and landscape position in semi-arid northeastern Brazil.

Samples were collected from a catena of Lithosols at upper, and Cambisols at mid and lower slopes. All soils were derived from syenite which had a high total P content near 5 500 mg kg-1. Phosphate-rich particles were identified by X-ray microprobe as fluor-apatite. The apatite contents of the A horizons were reduced in the weathering sequence from Lithosols to the older Cambisols, while the total P contents diminished to between 1200 and 2300 mg kg-1. The lower-slope soils had also lost two thirds of the Ca, Mg and Fe contents from sand and silt fractions.

Chemical fractionation of P showed the lowest Ca-P contents in the C horizons of the Cambisols, indicating a maximum transformation of primary Ca-P in these lowermost horizons. Deposition of partly weathered materials from upper slopes explained some of the differences in the Ca-P contents between R and C horizons and the overlying solum at the mid and lower slope. Similarly, some Lithosols showed an exceptionally high Ca-P contents suggesting that these soils have been replenished with unweathered material from rock outcrops above.

Microprobe analysis revealed P-rich silt particles containing Fe, Al and Ca, which may be explained by the impregnation of primary Ca-P with Fe-oxyhydroxides. This absorption of Fe by mineral particles would represent a short-cut in the transformations of primary P to secondary, resistant P forms. The observation of Fe-oxyhydroxide coatings in feldspars from saprolite indicates that such transformations did occur and may have general importance where primary and highly weathered minerals are intimately mixed through erosion, limited leaching and high weathering intensities typical of semi-arid tropical environments. Only 5% of the total P was found in organic forms, mostly in stable forms of low availability with little contribution to P fertility.