03118naa a2200277 a 450000100080000000500110000800800410001910000170006024500960007726000090017330000150018252022450019765000180244265000130246065000220247365000150249565000120251065000230252265000130254565000180255865000170257670000200259370000180261370000130263177301960264415454071999-07-16       1999    bl uuuu     u00u1 u    #d1 aNEUFELDT, H.  aPhosphorus fractions under different land-use systems in oxisols of the Brazilian cerrados.  c1999  ap.146-158.  aWe examined whole-soil samples and particle-size fractions to study the distribution of different phosphorus (P) fractions after land-use change from native savanna to crops, pastures, and reforestation on clayey and loamy Oxisols of the Brazilian savannas. Phosphorus was extracted sequentially, according to a modified hedley procedure, into inorganic and organic P (NaOH-extractable Pi and Po, respectively), and recalcitrant P (Phc1 and Pres). Under natural conditions of strong P deficiency, over 60% of NaOH-extractable P was organic, reflecting the high contribuint of Po to plant nutrition. Fertilization elevated inorganic Po. The increase of inorganic P forms from fertilizer P was greatest in the Pi and lowest in the Pres fractions. After fertilization, the reforested sites maintained high NaOH-extractable P through efficient recycling, whereas at the crop and pasture sites, P tended to accumulate in recalcitrant forms. Possibly, the adsorption of P to oxyhydroxides was reduced at the more acid reforested sites by complexation of Fe and Al oxides with organic acids. The ratio of NaOH-extractable Pi to Po appeared to effectively reflect the P status of the land-use systems and P deficiency increased in ascending order from native savanna &gt pasture &gt reforestation &gt crop, independently of soil type. The particle-size separates reflected P transformations along a biological and mineralogical gradient, which is discussed with respect to origin and distribution of natural and fertilizer P forms. That is, (1) P in particle-size separates was enriched in the clay and depleted in the sand fractions such that 70%-87% of total P was bound in the clay; (2) residual P increased relatively at the expense of HCl-extractable P with decreasing particle-size, indicating a continuously stronger adsorption to oxyhydroxides; (3) the proportions of organic P were generally lowest in the 20-50-um fraction because P in particulate organic matter was already depleted and transformed into microbially mediated Po, which was enriched in the clay and silt fractions; (4) fertilizer P accumulated in the 20-50-um fraction and was subsequently transferred to the silt and clay fractions but remained largely in inorganic form.  afractionation  aland use  aorganic compounds  aphosphorus  aCerrado  aComposto Orgânico  aFósforo  aFracionamento  aUso da Terra1 aSILVA, J. E. da1 aAYARZA, M. A.1 aZECH, W.  tIn: THOMAS, R.; AYARZA, M.A., ed. Sustainable land management for the oxisols of the Latin American savannas: dynamics of soil organic matter and indicators of soil quality. Cali: CIAT, 1999.