Organic and Inorganic Forms of Phosphorus in a Calcareous Soil Planted with Four Species of Eucalyptus in Southern Iran

The objectives of the present study were to evaluate the effect of four eucalyptus species on (i) selected surface soil properties and (ii) the distribution of inorganic and organic phosphorus (P_i and P_o) fractions. Soil samples were collected from soil 0–20 cm deep beneath and between trees. The P_i forms were determined by sequential extraction with sodium bicarbonate (NaHCO₃‐P), ammonium acetate (NH₄OAc) (OAc‐P), ammonium fluoride (NH₄F‐P), sodium hydroxide (NaOH)–sodium carbonate (Na₂CO₃) (HC‐P), citrate dithionite (CD‐P), and sulfuric acid (H₂SO₄) (H₂SO₄‐P). The P_o forms were sequentially extracted with NaHCO₃ (NaHCO₃‐P_o), NaOH (NaOH‐P_o), and H₂SO₄ (H₂SO4‐P_o). The NaOH‐P_o was subdivided into moderately stable (NaOH‐P_om) and highly stable P_o (NaOH‐P_os). Organic matter, clay and silt contents, total nitrogen, and available potassium of the soil beneath the trees increased. The OAc‐P and HC‐P forms beneath the trees were less than of that between them, which shows that these fractions probably are labile inorganic P pools. The NaHCO₃‐P_o and NaOH‐P_os forms were greater beneath the trees than those of interspaces, whereas NaOH‐P_om and H₂SO₄‐P_o were not affected by plantation.