Distribution of inorganic phosphorus fractions in some soil profiles of Zagros forest of Iran

The aim of the present study was to evaluate the status of inorganic phosphorus (P) fractions in ten virgin soil profiles, and to study the relationships between P fractions and soil properties from Kohgyloyeh and Boir Ahmad province, Iran. The soils belonged to Alfisols, Entisols, Mollisols, and Inceptisols. Mean annual precipitation and temperature ranges were 450-850 mm and 12.2-22.4 degrees C, respectively. Inorganic P was fractionated by sequential extraction with NaHCO3, NH4OAc, MgCl2, NH4F, NaOH, citrate-dithionite-bicarbonate (CBD), and H2SO4, which are referred to as NaHCO3-P, NH4OAc-P, MgCl2-P, NH4F-P, NaOH-P, CBD-P, and H2SO4-P, respectively. Generally, the NaHCO3-P, NH4OAc-P, MgCl2-P, and NH4F-P were higher in surface horizons, especially wherever there was higher annual precipitation. Results showed that H2SO4-P constituted the largest inorganic P fractions in all profiles. For all the soils studied the distribution of P fractions was similar to that of weakly weathered soils under arid and semi-arid regions which represent an early stage of development. Pedogenic P compounds were higher where weathering was higher, i.e., in surface horizons and at higher annual precipitation.