Soil phosphorus fractions from toposequences of semi-arid Latosols and Luvisols in northeastern Brazil

Little information on P fractions and P lability exists for Brazilian semi-arid soils, particularly those under native vegetation. Soil P fractions were determined by a sequential extraction procedure for semi-arid Latosols and Luvisols under native vegetation for representative toposequences from Pernambuco. The P fractions, in order of decreasing lability, were: resin extracted inorganic P (resin-Pi), inorganic and organic P extracted by Na-bicarbonate (NaHCO3-Pi and NaHCO3-Po), inorganic and organic P extracted by Na-hydroxide (NaOH-Pi and NaOH-Po), sulfuric acid extracted inorganic P (H2SO4-Pi), and P measured by a sulfuric/peroxide digestion (residual-P). Total P was similar in the two soil types, with average values of 320 and 350 mg kg−1 in the Latosol and in the Luvisol A horizons, respectively. However, the total labile P amount (resin-Pi+NaHCO3-Pi+NaHCO3-Po) was significantly different with mean values of 22 mg kg−1 in the Latosols and 37 mg kg−1 in the Luvisols. All P pools, except organic P, were little affected by slope position for any of the horizons of either Latosols or Luvisols. Organic P, and also soil organic matter, tended to increase downslope. The P fractions, and particularly the labile P, tended to decline with soil depth, except residual and H2SO4-Pi pools. The residual-P was proportionally the largest pool (40–70% in Luvisols and 40–80% in Latosols) and the inorganic labile P (resin-Pi and NaHCO3-Pi) the smallest (1–7% in Latosols and 2–8% in Luvisols). The H2SO4-Pi proportion to total P of 6–12% in the Latosols and 8–20% in the Luvisols indicate a relatively large P reserve remaining in both soils.