PHOSPHORUS TRANSFORMATIONS IN A TOPOSEQUENCE OF LITHOSOLS AND CAMBISOLS FROM SEMIARID NORTHEASTERN BRAZIL

Concepts of phosphorus transformations in soils and landscapes have la rgely been developed in temperate regions on moderate slopes. Little i s known about the P dynamics and availability in semi-arid tropical so ils where rainfall extremes cause limited but periodically intensive l eaching and erosion. We therefore studied the different forms of inorg anic 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 w ere reduced in the weathering sequence from Lithosols to the older Cam bisols, 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 Cambisol s, indicating a maximum transformation of primary Ca-P in these lowerm ost horizons. Deposition of partly weathered materials from upper slop es explained some of the differences in the Ca-P contents between R an d C horizons and the overlying solum at the mid and lower slope. Simil arly, some Lithosols showed an exceptionally high Ca-P contents sugges ting that these soils have been replenished with unweathered material from rock outcrops above. Microprobe analysis revealed P-rich silt par ticles containing Fe, Al and Ca, which may be explained by the impregn ation of primary Ca-P with Fe-oxyhydroxides. This absorption of Fe by mineral particles would represent a short-cut in the transformations o f primary P to secondary, resistant P forms. The observation of Fe-oxy hydroxide coatings in feldspars from saprolite indicates that such tra nsformations did occur and may have general importance where primary a nd highly weathered minerals are intimately mixed through erosion, lim ited leaching and high weathering intensities typical of semi-arid tro pical environments. Only 5% of the total P was found in organic forms, mostly in stable forms of low availability with little contribution t o P fertility.