2.4 Sequentially extracted phosphorus forms
Given that the soils are derived from the calcareous parent material of lacustrine sediment, and the development of soils is weak in this study, we assumed that most of soil P was combined with primary minerals, existing in the form of “Calcium (Ca)-bound P”. Furthermore, the forms of Ca-bound P in the calcareous soils generally have a series of calcium phosphates with the complex transformation as follows: from dicalcium phosphate (i e. CaHPO4·2H2O, Ca2-P) to octacalcium phosphate (i e. Ca8H2(PO4)6·5H2O, Ca8-P) and apatite (i e. Ca10(PO4)6F2, Ca10-P) (Jiang and Gu, 1989). Therefore, a sequential extraction procedure, provided by Jiang and Gu (1989) and modified by Adhami et al. (2006), was applied to determine P forms instead of the sequential extraction schemes according to Hedley et al. (1982) for distinguishing the series of Ca-bound P. Briefly, the NaHCO3-soluble, CH3COONH4-soluble, NH4F-soluble, NaOH-Na2CO3-soluble, citrate dithionite-soluble, and H2SO4-soluble phosphate was successively extracted with 0.25 mol L−1NaHCO3 (pH = 7.5), 0.5 mol L−1CH3COONH4 (pH = 4.2), 0.5 mol L−1 NH4F (pH = 8.2), 0.1 mol L−1 NaOH-Na2CO3, 0.3 mol L−1C6H5Na3O7plus 1.0 g Na2S2O4 plus 1.0 mol L−1 NaOH (citrate dithionite) and 0.5 mol L−1 H2SO4, respectively. The above soluble phosphate was mainly composed of the form of Ca2-P, Ca8-P, aluminium oxide surfaces adsorbed P (Al-P), iron oxide surfaces adsorbed P (Fe-P), occluded P in iron oxide minerals (occluded P), and Ca10-P, respectively (Jiang and Gu, 1989; Adhami et al., 2006), and was termed as Ca2-P, Ca8-P, Al-P, Fe-P, occluded P, and Ca10-P respectively. Among these forms, Ca2-P, Ca10-P and occluded P are equivalent to NaHCO3-extracted P, diluted HCl-extracted P and concentrated HCl-extracted P, respectively, in Hedley et al. (1982) scheme. The Ca8-P, Al-P and Fe-P are inorganic P non-occluded in iron oxide minerals, and named as non-occluded P.
According to the results of Jiang and Gu (1989) and Adhami et al. (2006), the solubility and bioavailability of Ca2-P are higher than those of non-occluded P and thus Ca2-P and non-occluded P was named as labile P and moderate labile P, respectively, whereas occluded P and Ca10-P with a lowest solubility and bioavailability was termed as slowly cycling P (recalcitrant P or stable P). Total organic P was obtained as the difference in P through 0.5 mol L−1H2SO4 extraction between ignition and non-ignition soils (Saunders and Williams, 1955). The correlation coefficient (r = 0.990) between total P determined by H2SO4-HClO4 digestion and the sum of various P forms (including organic P) was significant (p < 0.001), which showed that the values of various P forms were reliable in our study.