Phosphorus fractions and sorption processes in soil samples taken in a forest-savanna sequence of the Gran Sabana in southern Venezuela

P fractions and sorption processes were studied in samples taken from the organic surface layer and in the underlying mineral soil of a forest-savanna sequence consisting of: (1) tall primary forest (TPF), (2) tall secondary forest (MSF), (3) low secondary forest (LSF), and (4) open savanna (S) in la Gran Sabana, South Venezuela. The organic surface layer in the TPF and MSF showed the highest P concentrations in all analysed P fractions. P in this organic layer was mainly associated with inorganic forms, suggesting that this layer is an important source of bio-available P. The organic surface layer was not present in LSF and S probably because of the occurrence of recurrent surface fires. The conversion of forest to savanna influenced the distribution of the different forms of P in the soil. While non-occluded (resin-+NaOH-P extractable) and organic (NaHCO3-+NaOH-+HCl-Po) P declined from the forest to savanna, occluded (concentrated HCl-extractable+residual P) forms increased. The correlation between sorption maxima and soil organic C was not significant; however, organically bound forms of Al were the main component that explained the adsorption capacity of these soils. The above findings suggest that the organic surface layer and the soil organic matter are important for maintaining P fertility in the undisturbed and little disturbed forests. However, when the system is heavily perturbed by fire the organic surface layer, the main P source, disappears and the patterns of P cycling change.