Heating effect on inorganic phosphorus fractions and sorption of highly calcareous soils

The objectives of the present study were to evaluate the effect of heating on various inorganic phosphorus (P) fractions and sorption characteristics of highly calcareous soils of Kohgyloyeh and Boier-Ahmad province. Treatments were consisted of 5 soils and heating treatment (untreated, 200, 350 and 550 C) for two hour. P fractionation included successive extraction with NaHCO3 (NaHCO3-P); pedogenic Ca-P (NH4OAc-P+ MgCl2-P); NH4F (NH4F-P); NaOH-Na2CO3 (HC-P), citrate-dithionite-bicarbonate (CBD-P), and H2SO4 (H2SO4-P) carried on 1 g sample in duplicate. P sorption was studied by equilibrating 2 g of samples with 20 ml solution containing 5 to 400 mg P kg-1 soil with 0.01 M KCl as background solution in duplicate. Results indicated that generally high temperatures (350 and 550 C) significantly increased NaHCO3-P, pedogenic Ca-P and NH4F-P. H2SO4-P significantly decreased in 350 C but significantly increased by 550C heating. These changes indicate the transformation of inorganic P fractions to each other in studied temperatures in addition to transformation of organic P to inorganic P. Quantity of P sorption in the unity of P concentration (KF) decreased by 550 C in soils with lower CCE and higher Fe oxides, while it increased in soils with higher CCE and lower Fe oxides content. This effect is probably due to the transformation of crystalline Fe oxides to forms with lower specific surface area, which regualte P retention in the low concentration of P.